BMW F650GS Dakar restoration

I've bought a 2001 BMW F650GS Dakar in fall of 2024. The bike was cheap and equipped with some tuning parts (ohlins forks, enduro stands, aluminum boxes, etc). The only drawback was the bike condition. It was quite beaten up from frequent falls during off-road usage, lack of maintenance was visible on few items, pain of the engine was partially eaten away by a battery acid, etc. But the engine was running well except for problems with starting when hot and the price was a 1k USD lower than these bikes usually cost in a stock configuration. And I've also got an aftermarket exhaust and OEM front forks with the bike. Well, a good project bike a though. I kind of changed the decision during the process and here's the whole story.

Initial state

From the beginning, I knew about few issues:

• Not starting well when hot
• Engine paint damaged by battery acid (bike fell and the overflow hose was not properly connected to the battery)
• Bodywork not in the best, the "fake fuel tank" had few cracks and the plastic covers below seat were crushed badly and few pieces were missing, the box behind the seat was cracked and part of the plastic was gone
• A long crack in the seat upholstery
• Bent rear brake lever
• Tachometer readings were erratic
• Fuel hoses were quite old and the top layer of the hose was visibly cracked
• Aftermarket handlebars, the original were bent in the past adventures
• Battery was not in a best shape

Nothing serious, mostly cosmetic issues, not hard to repair or to get second hand parts for few bucks, looked like worth the price. The initial idea was to take the engine out and repaint it, fix the cracked plastic covers and do a little bigger maintenance like changing all fluids, brake hoses, wheel bearings, swingarm bearings, timing chain, etc. Just the basic stuff you usually want to do on a 24 years old bike. When I took the swingarm and the engine off the frame, I got the brilliant idea to take the bike apart completely as there was not much left on the frame anyway and repaint the bike as there were signs of rust and quite damaged paint on few places. And here I've started to realize the deal was not one of the best ones I've made.

The list of issues I've found is endless, few examples of the more visible ones:

• Plastics covers were cracked here and there and someone tightened the screws with way too much force - some of the mounting points were badly damaged. The underseat plastics were KO, twisted, piece of material was missing,... There were screws with like 4 different head types used to secure the bodywork... The license plate holder plastic was cut away and replaced with a piece of steel, the license plate was drilled and secured by screws. The chain guard was cracked, the dashboard cover was cracked,... I guess there was not a single piece of a plastic part not damaged in a some way.
• Everything below battery tray was heavily damaged from the battery acid, there was a hole in the battery holder plastic, the steel piece below was quite rusty and the paint was eaten away, there were some discolorations on the main frame paint from the acid.
• The engine paint on the middle part was nonexistent or could be wiped away with a cloth, the oil pipe was rusty, the oil pressure sensor was partially eroded away, the connector was unusable too (it fell apart in my hand)
• Swinging arm pivot was rusted solid to the engine, it took the largest wrench I could find, impact wrench and a large hammer to get it out.
• No-one lubed side stand, center stand, swing-arm and angled lever since the bike was made I guess, the rust is everywhere. The swingarm/angled lever was not that bad, the rust of the bearings just started and was not deep, but the main stand inserts were quite rusted and it took a 2t press to get on of the pivots out of frame.
• Hot-fixes everywhere - don't have a proper cotter pin? Just use a nail. Don't have a snap ring? Put a piece of random wire in there. The weld-on nut was torn away? Use a hot glue to fix it...
• The front light was hanging on a single screw (out of 3) as all the other mount points were cracked.
• Neutral gear sensor was eroded away by acid, I was surprised it still worked while is was looking just like a piece of black goo
• The rear shock didn't see maintenance since manufacturing, the spring was rusty, the preload control hose was cracked on several places, the valve inside was stuck, the oil was pretty old..
• Some smaller pieces were missing - plug on drain hose from the airbox, cotter pin in the main footrest pivot, one of the sidestand springs, half of the cable holders, spacers on the engine guard, spacers on the luggage holder ...
• The engine bashplate had one mounting hole enlarged by vibrations.
• The engine internals were in a good shape with some notable exceptions:
  • the water pump shaft was grooved badly (a known issue of this engine), it's wonder it was not leaking, I guess someone must have replaced the seals and keep the already grooved shaft in there...
  • one of the oil pump shafts was scored badly, it looks like something like a piece of sand got into the oil
  • there were marks of the valves in the piston, it looks someone fucked up the timing in the past and the valves hit the piston. Fortunately the valves and valve guides were straight - I guess that someone changed them after collision.
  • the valves clearance were at or below the lower limit, one of the intake valves had a zero clearance
  • the drain plug was tightened by a horrible force, impossible to loosen, almost destroyed it completely to get it out of engine (wrench, impact wrench, bigger impact wrench, hammer and a chisel)
• The aftermarket exhaust I got with the bike was half-way cracked around the edge. I haven't noticed, sold it and had to return money later, feeling like a moron...
• Wrong bearings were used on the rear wheel, the rear wheel shall contain a standard bearings and the sprocket a C3 bearings, the actually fitted bearings were the other way around
• Few LEDs in the rear light were dead
• Some water got to the rear brake pump assembly, it was quite rusty, the duct cover had a nice hole in it. The front brake pump plastic dust cover was ruptured.
• The aftermarket clutch lever fitted to the bike was shorter than OEM and was not touching the clutch switch at all. Also the pivot hole was much bigger than OEM, si it was just wiggling around.
• The cable harness was solid as rock, the insulation/wrap turned to stone after all these years. Nothing really serious, it was working well, but not something I wanted on the bike...
• The electrical installation was just horrible:
  • the flashlight headlight switch (that button you press to flash the headlight shortly) contained a piece of aluminum foil instead of the actual contact pin.
  • the turn signal switch was cracked and piece of plastic was missing from it
  • few wires were cut for some reason and connected back (probably fixing a broken connector or something), but the connection was horrible, the wires were just twisted together or soldered by a drunk baby or something like that
  • some connector pins were soldered to wires instead of being crimped properly
• few threads on the bike were messed up by an excessive force

During the disassembly I've also found something rather interesting. The bike was supposed to be Dakar variant from 2001. But the engine was two spark that was made since 2004 and most of the parts were from 2004 according to the markings on the plastics, front forks,... After some digging, I've found that the VIN number corresponds to standard GS version from 2001. Wait, what, standard GS? This is Dakar! Well, it looks that someone bought a frame with the paperwork for given VIN and swapped most of the parts from Dakar to it.

I should have known better before buying, different engine (requiring a different wire harness, etc) is a huge red flag, on the other side, the two spark had a better ECU with some of the issues of the previous models fixed, also the OEM front forks I got with the bike were from 2004, which is nice, the old model had some failures reported.

Anyway, the bike was already torn to pieces and there was no way back, I couldn't return the bike in this state and I didn't want to give up and sell it for parts. It's time to fight!

Engine restoration

Paint removal

The engine paint was severely damaged by the spilled battery acid. It looked quite desperately after removing all covers around the engine and first looking on the actual surface under a good light. The idea was to remove the paint on a fully assembled engine first to avoid any pieces of paint or other particles getting inside of the delicate internals, then split it to pieces, degrease and repaint.

So I've taken the engine out of the frame and started grinding the paint and remains of the acid away with a brass wire brush in a cordless drill. That was not my brightest idea. The places with paint eroded by the acid were quite easy to clean, just few touches of the brush and the shiny aluminum surface appeared, however the undamaged paint was tough as hell. The brush was creating a deep scratches in the paint, but not removing it completely, it usually ended up by creating a soft scratches in the aluminum surface without actually removing all the paint.

After few deeper scratches and a bit of desperation, I've finally decided to use some paint removal chemicals. After testing the substance on a piece of a bare aluminum to make sure it won't damage it even after a prolonged use, I've started painting parts of the engine with the paint remover gel and removing the bubbled out paint with brush. It worked quite nice, although there were few stubborn places that needed several cycles of chemical and mechanical warfare to get rid of them. Event with the paint remover it took me like a whole weekend to get the job done. Also the smell of the paint remover was quite unpleasant and the bubbled paint attacked with a wire brush tends to fly right into your face, so I've done most of the work in the respirator with a proper filter, gloves and glasses.

Disassembly

Having some experience with the old czechoslovakian two stroke motorbikes from 70ties I've boldly started taking the cleaned engine apart, carefully measuring all the parts for wear (as mentioned in the BMW workshop manual). The first issues appeared quite soon. The valve clearances were all on or below the lower limit, one of the intake valves had no clearance at all. No wonder the engine was hard to start when hot. On the other side, the automatic decompression lever was in a pretty good shape, that's not that common after 60 thousands km.

After removing the cylinder head, I've found the first nasty surprise. The piston had markings from the intake valves on it. Oh crap I though, the engine is busted, I'll need to get new valves, new valve guides, regrind the seats, etc. After some cursing, I've taken the valves out of the head and started doing some measurements. Surprisingly the valve stems were not bent, everything was in the correct tolerance ranges. It seems someone messed with the timing in the past, caused collision between exhaust valves and the piston and then fixed the issue and replaced the valves. Ok, my heart just started beating again.

Some of the screws holding the engine together were rusty as hell, I had to clean them up and plate with zinc again.

I had some troubles trying to remove the alternator flywheel: I've made a puller with a M10 bolt in the center (the biggest one I had in my workshop), took the impact wrench and after few attempts to pull the flywheel from the cone, the M10 bolt snapped. After another round of nasty words, I've drilled out the bolt remains (the flywheel magnet collected all the chips and I've spent like an hour removing every last piece of metal from it), found an M12 bolt, made a new thread, heated the flywheel to like 90 degrees and after another attack with the impact wrench the flywheel finally came loose.

The water pump shaft was found dead, there were deep grooves from the seals, it was a miracle it was not leaking the water/oil like a hell. This was kind of expected, it's a standard issue of these bikes. Easy to fix with an aftermarket waterpump repair kit, but I've taken a bit different approach - the seals are dirt cheap when bought from anyone else than BMW, the impeller was in a perfect condition and I've got a friend with a machine shop at hand, so I've got him to make me a new shaft from a better material, that won't go bad as fast as the OEM one.

The last engine related surprise appeared when taking the oil pump apart. One of the pumps shaft was scored at the side that's turning in the aluminum engine body. The machinist friend was tasked again with making new shaft. The shaft housing in the engine was not that bad, I've checked it for any hard particles, cleaned it thoroughly and put everything together again. The oil pump body itself and the corresponding hole in the casing were perfect, without any scratches.

Once the engine was torn to pieces, I've degreased it thoroughly in a soapy water, a rag soaked in the acetone and finally with isopropyl alcohol over and over until the paper rag remained white and clean.

Btw, it's a good idea to use an impact wrench, it makes removing the problematic nuts (like the clutch one) a breeze even without specialized locking tools.

Paint

I've spent a long time googling for a proper hight temperature paint. I was not able to find any 2k system locally, so I've opted for the VHT Engine Enamel. After some discussions with a friend of mine, I've started the painting with etching primer from Maston with heat resistance of 120 degrees. Then two layers of the VHT Engine Enamel primer and finally two layers of the VHT Engine Enamel paint.

The result looked quite epic, nice, smooth surface. Oh crap, I've found few tiny spots in the corners where bits of the original paint remained, clearly visible under the new paing. The hell with it, no one will notice under coat of dirt and mud after few rides. So it was time for a bit of cooking, the paint is supposed to be cured under 93 degrees C for like 30 minutes. I've misused the old kitchen oven we are fortunately not using any longer for the curing.

The resulting paint is just epic and quite though, hard to scratch. Nice, exactly the way I wanted it. Or not. It's not very hard to crack the paint with a pressure or accidentally hitting it with a wrench, it just cracks and a small piece separates from the aluminum. I've tried a bit more complex test of the paint quality on the oil filter cover (easiest to repaint) by scoring it with a knife in 1x1mm pattern and peeling the paint using a strong sticky tape, it hold just perfectly. I guess the lower parts of the engine were not degreased perfectly, maybe the oil is too deep in the casting pores to remove all of it simply by soapy water and acetone. Next time, I'll try baking the engine after first round in soap water for an hour or so to get rid of all the oil traces. Hopefully the paint will hold and won't flake much in upcoming years, I would hate to redo it again.

Assembly

As I'm an idiot who had thought that the bearings are quite cheap (they are, but only on my 50 years old CZ 175/477), I've removed all 6 bearings from the engine casing before actually checking the prices, thinking it won't hurt to replace them when the engine is split. Well, I guess I could rather buy an used working engine in a better state. It was like 250 USD just for the bearings, some of them were locally available from SKF, some of them had to be ordered from OEM, which just doubled the real price of the stuff (thank you BMW...). Btw, the BMW ships fully sealed bearings for the counterbalance shaft and this happened to multiple people - come on, sealed bearing in the engine is a big no-no, I've pried the seals away and I hope it will work well. Add set of seals, new timing chain, new neutral sensor, engine paint and few other goodies and you are on like 600 USD just to get the engine back together in a hopefully better shape than it was before.

Removing the bearings in blind holes was pain in the ass, the specialized tools are expensive, so I've welded nuts to the bearings and used a reverse hammer to get them out. On the other side, installing new ones was pretty simple, I've heated the block to like 90 degrees in the oven, took bearings from the freezer and gently tapped them into the place...

I've cleaned everything thoroughly, blown all the oil passages with compressed air, put the gearbox and crank inside, assembled the engine halves, gently hit on few places with a rubber hammer to get it together nicely and noticed, the rubber of the hammer is not in the best condition. It kind of chipped and spread tons of small rubber particles all across the engine. Oh, just great. So I took everything apart again, cleaned, blown with compressed air and put together again, this time avoiding the rubber hammer...

The BMW service manual suggest locking the crank with the TDC bolt and few pages later tightening the gear for the timing chain and magneto flywheel with 180 Nm. Hey, I had to buy a new torque wrench just for these two nuts... Anyway, the problem I saw was that the TDC bold is M8 and the groove in the crank is not very deep. The 180 Nm required would probably just snap the screw and I would need to split the cases again... I've seen people locking the crank with a piece of wood between crank and balancing shaft, but that just didn't seem right, the wood could still split and piece of it would block some oil passage later. Nope. Both nuts are on the same crank, only on opposite sides, so I've tightened the nuts against each other - hold the nut on one side, tighten the one on the other side. The torque might be a tiny bit off on the second side, but the nuts are glued with loctite anyway, so that shouldn't be a problem.

In the meanwhile, I've also lapped the valves and measured the contact patter, just to make sure it fits the specs. Aside from the worn oil pump shafts, the engine seems to be in pretty good shape, all the measurements are much closer to the "new" value rather than "worn limit" and the cylinder still shows a nice honing marks. Well, with exception of the worn plastic gears of the oil pump, I guess this has something to do with the shot seals and worn shaft, so I've bought new ones...

Tightening the nut on the clutch basked and the output sprocket turned out to be a bit more complicated. The gearbox makes the shafts running opposite to each other, so I couldn't tighten these nuts against each other like the nuts on the crank, once I would have started tightening one nut, the other one would unscrew loose. I don't have a tool to lock the clutch basket, so I've decided to do the rest in the frame after locking the rear wheel.

Frame

There was some acid damage on the frame and few metal parts around the engine, also the frame paint was damaged around the stands and on the bottom side, so I've decided to do a full paint job on the bike.

I've sandblasted the smaller parts that fitted my small sanding cabinet (the largest part was the center stand) and hand sanded the rest (not perfectly, but enough to remove all rust and damaged paint and smooth the transitions between metal and remaining paint). There were some minor issues I needed to fix by welding - the brake lever stopper on the frame was bent and cracked and the sidestand handle was broken. I've also straightened the bent brake lever.

The metal parts were the primed with 2k epoxy primer and finalized with a high quality 2k polyurethane paint with a recommended higher percentage of hardener for best mechanical and chemical resistance. I've tried to stick with the original paint scheme, silver on the frame, black on the rear frame and other parts. It was my first paint job with a spray gun that I've done completely myself and the results were surprisingly not that bad. Only the inside of the main frame has a bit rough finish due to a bit denser pain and not the best order or painting - the paint on the inside of the frame dried quite fast and when spraying from outside, the paint droplets that reached insides did not fuse completely together with almost dry paint already in there. I've should have thinned the paint a bit more and started with the outside surfaces first.

Btw, even if you work in a closed garage with the air compressor in a different room, the pressure you create by painting in the enclosed space carries the paint mist surprisingly far away through all the holes and space around doors. This is especially annoying with the black paint, I've found a thin layer of black dust 2 rooms away from the garage...

Bodywork

As I mentioned in above, the plastic parts were quite damaged. The tank sides were cracked a bit and upper mount points were either cracked or completely missing, the underseat parts were so damaged I rather bought a good ones from some wrecked bike.

A friend of my father makes a living by welding plastic parts of cars (among other things) and welded the damaged parts quite nicely and very cheaply. He has even repaired pars with missing pieces of material.

Where needed I applied some thicker plastic filler to even out the dents after welding and spent some quality time with few pieces of a sandpaper.

The preparation for the final paint is always quite annoying. I had to remove all the stickers (razor blade worked perfectly for this operation), apply a filler where needed, sand the parts to remove any imperfections of the surface while trying to avoid sanding through all the paint layers down to the bare plastic, degrease, prep the bare surface with plastic adhesion agent and finally spray everything with a filler.

I've spent some time trying to come up with a nice paint scheme for the bike, but after downloading hundreds of photos from the internet and getting a severe case of a decision paralysis, I've came to a conclusion that a single color paint is the simplest and still looks good. The original idea to make the bike black was kind of replaced with a better idea to make it blue (mostly because that was the paint noted in the legal documents of the bike and I was too lazy to ask the official places to change it). I've use some 2k PUR paint as for the metal parts.

And I'm quite happy with the results. There are some minor imperfections, a bit of orange peel here and there, few dust particles trapped in the paint, but that's nothing to be worried about on an enduro bike, right? Btw, it's impossible to tell what was damaged before.

Shocks

As I've sold the Ohlins forks (after counting all the additional money I've put into this "cheap" bike), I needed to use the OEM ones I got with the bike. As I'm a bit crazy when it comes to "repairing" anything, I took the front forks completely apart. The teflon coating on the bushings was quite worn away and the seals were who knows how many years old, but the rest looked well, no bent tubes, no rust, no stripped threads. So the forks received some new bushings, new seals, new oil and a bit of polishing. Just to show you the difference, here's the old and new bushing...

The rear shock is a bit complex, I don't have the tools and knowledge to do the overhaul myself, so I've found a local company Moto Beran that did it for me. The shock haven't seen any maintenance since the manufacturing date, so the process was a bit more expensive that I expected (260 USD). It got a completely new paint, rubber parts, preload screw, hose and fresh oil. Just to illustrate the complexity of the rear shock, these are the internals.

And while I was at it, I've also changed the steering head bearings. I managed to find a nice set from AllBearings that contained a nice rubber "seals" instead of the original steel one that let the dust inside the bearings.

Radiator

The water cooling radiator has no real protection on this bike and it has seen some harsh terrain, some fins were bent, the side was hit by a rock or something and the bottom mount pin was torn away from the radiator body. Also I've found a quite large stone stuck between radiator and the frame.

The first thing to do was to was the radiator with some hot water to remove all the dirt. The amount of dirt and mud that left the radiator was pretty surprising, it's a wonder it was still working. Then I've spend a nice winter evening with a tweezers, straightening all the bent pins and getting the remaining dirt from all the corners.

Electronic repairs

The main wiring harness was hard as stone, the wrap tape was peeling on few places and it was full of dirt accumulated over years, I've also found few repairs, so I've decided to make a new cable harness from scratch. After checking availability of the connectors and the prices, I've changed my mind and decided to verify the repairs quality and just change the wrap tape of the original harness. After removing electrical tape from the repaired spots, I've almost screamed in horror. The repairs looked like they were made by some child. Some wires were pretty badly soldered, some just twisted together and wrapped in tape. And all the repairs were on strange places, e.g. all wires going to fuel pump were cut and connected again, like someone just tore the rear subframe away from the bike without disconnecting cables first.

I've removed the original wrap from the harness and redo it with automotive tesa tape for wiring harnesses. It has a soft surface which is probably not best for a muddy environment, but now the harness is at least flexible and as I've reworked all the repairs, I have gained some trust to the wiring again.

Another surprise was hidden in the light switch, it was not working properly. The original contact pad was missing and someone replaced it with a piece of aluminum foil. Oh really? I turned a new pad from a cooper instead of this aluminum "fix".

Instrument cluster

Another thing to focus on was the instrument cluster. The tachometer readings were quite erratic when I bought the bike. One of the causes could be metal chips near the sensor in the engine, another could be the wiring, or the dashboard PCB itself. I've kind of eliminated the possible former causes, so the last one was the PCB. The dash in F650GS is known to go bad after years due to a low quality capacitors, so I've decided not to take my chances and replaced them too. I've bought a high quality long life automotive capacitors with higher working voltage, so hopefully they will last for years to come. Btw, there's a nice guide online on how to do it.

Apart from the capacitors, I didn't really see anything that could cause the issues, so I put everything back together hoping the new capacitors and cleaned connectors would do the trick.

When I first started the bike after the rebuild, the erratic tachometer behavior remained and another problem appeared - the speedometer showed a non zero value when engine was running, this finally led me to a proper investigation on what's wrong, especially as the second hand dash costs over 200 $. I've tried rewiring the cables to reduce noise coming from the ignition coils, I've cleaned all the connectors related to dashboard, battery, grounding and speedo sensor, nothing really helped, therefore some measuring and bit of reverse engineering was needed. The crossroadz page provides some details on the connectors pinout, etc.

First of all, I've noticed the plastic holder of the time display was moving a bit and some fine dust appeared below the plastic, that's a know issue of this model, the plastic vibrates itself free and chafes through the paint and eventually also traces on the PCB. I've desoldered the display to find nothing is really wrong below, some chafing was visible, but nothing serious. I've added piece of a kapton tape as an additional protection and soldered the display back, making sure it's held firmly in place.

The first step in deeper debugging was the power path verification, the dash is powered through pins 1 (unswitched 12 V) and 2 (switched 12 V). The unswitched 12 V line is always connected and powers the clock when the bike is turned off, the switched is connected when key is turned to on position. The PCB wiring is pretty straightforward, Both power inputs are reverse-polarity protected by a diode, another diode on switched input separates both lines from each other so the switched part of the PCB is not powered when ignition is off. An electrolytic capacitor smooths the 12 V line and 5 V LDO is connected just next to it to provide 5 V for MCU. I've poked the board with multimeter and oscilloscope when the engine was running to check for noise on the power lines, noting really suspicious here, just a small ripple of few hundreds of mV which is probably to be expected.

Let's move to the tachometer. The RPM signal is connected directly to the ignition coil trigger running signal from ECU. The signal stays at 12 V (or whatever voltage is currently there, when engine is running something like 14 V is expected) until the ignition event - the voltage drops close to 0 V and a spike to somewhere around 30 V follows. Nothing really strange here, although I've kind of thought the spike when ignition coil is triggered could be filtered in the coil assembly, well, never mind. The signal conditioning circuit is pretty simple, just a resistor divider, filter capacitor and two diodes in a single package to cut out possible over/under voltage peaks, the result goes directly to interrupt pin of the MCU.

The RPM signal seemed to be a crystal clear, the pulses were nice and regular, I couldn't see any reason why the needed jumped erratically between 0 and 1500 (idle RPM) all the time, it looked like the MCU was missing the pulses once in a while. After some more poking the board with probes and browsing through the MCU datasheet (available through crossroadz), I've noticed the voltage levels at MCU interrupt pin are very close to the limits for H/L states defined in the datasheet, this would explain the MCU misses pulses once in a while. One resistor change later and the tachometer needle started to behave - I've just needed to change the 5k6 resistor in the divider to 6k8, the peak voltage level increased above the minimum stated in the datasheet and that fixed the issue.

Regarding the origin of the problem, I'm not sure what caused the problem, someone might change the resistors in the past, it could have been tuned this way from the factory and this just might have been a piece where all the tolerances summed together in a wrong direction (higher voltage on the 5 V LDO output, lower voltage on the divider output, lower peak from the ignition coil,...). Anyway, the problem is fixed for few cents and few hours of tearing my hair.

The speedometer was a whole different story. Someone in the internet had a similar issue and fixed it with a new sensor. However the sensor is surprisingly hard to source if you are lucky to find one, it's usually very expensive and probably taken from a wrecked bike, this justified a further debugging than just trying to swap parts.

First thought was that it worked before I've dismantled the bike and the issue only showed when the engine was running, when I've disconnected the sensor, the speed dropped to zero - electric noise? Ground was ok, ignition cables were as far as possible from the main harness. The scope showed nothing interesting on 12 and 5 V lines on the PCB, the 12 V input measured direclty on connector was a bit noisy, but nothing serious, just a standard noise of the voltage regulator output. I've tried swapping the steppers between speedo and tacho, just to be sure it's not caused by worn gears or something, the issue was still there. The speedo sensor is same as for the ABS (although my bike doesn't have ABS) - there's a perforated ring on the rear wheel with induction or hall sensor nearby, in this case, the sensor provides a current output - when connected to 12 V (over something like 100 ohm resistor), the sensor draws 7 or 14 mA depending on the ring position. The current loop is very resistant to electrical noise, so the problem must be somewhere in the processing.

The interrupt signal at the output of the conditioning circuit showed some 1-3 V peaks (yellow trace) with repetition frequency much higher than the RPM (blue trace - RPM signal from ECU). I've reverse engineered the circuit to get some background on what might go wrong. The sensor is powered thought 100 Ohm resistor (used to measure the current), PNP transistor and 39 Ohm resistor, probably used to protect the circuit from spikes. The voltage on the PNP base is \(12 - I_{sens} * 100 \Omega - 0.65\), this voltage is applied to base of another PNP transistor, to open this transistor, voltage on base has to drop below \(12 - 1.2 - 0.65\). Therefore this circuit behaves like a simple comparator, when current is low, the base voltage is high, close to 12 V, keeping the second PNP closed and the output voltage is 0, once the current reaches high enought, the voltage on base drops, opening the second PNP and moving the output high. the voltage differences necessary to open the transistor are quite low and the circuit is powered from switched 12 V line which has almost no filtration, there's only one small ceramic capacitor.

I've put a 100 nF capacitor parallel with the capacitor already present on the 12 V switched branch, which mostly cured the erratic needle behavior. Some small and very short spikes were still visible on the signal output. There was an unpopulated part between ground and signal output, just right size for 1 nF capacitor to smooth out the signal completely. So the speedo works well now!

As a conclusion, I'm not very sure what was the real cause of both of the issues. The instrument cluster seemed to be unmodified and at leas the speedo was working before. Maybe the new capacitors have a much lower ESR, changing the 5 V LDO behavior a bit, resulting in a voltage closed to higher limit, moving all the detection levels a bit. The signal conditioning circuits are not very well designed and there are very tigh tolerances between good and bad signal. Anyway, it works now and it hopefully stays working for years to come. And BTW, poking the SMD parts with oscilloscope probe on a bike with a running huge single cylinder engine is not an easy task, everything is shaking, including you hands.

Other repairs and notes

I've kind of touched every part of the bike, the only parts that were not taken apart completely are the water thermostat (quite prone to damage when taking apart and it was working before) and the fuel pump assembly (need a special tool to unscrew the lid on the fuel tank). I've changed bearings in both wheels, sprocket carrier, swingarm, swingarm linkage (or how is that thing between swingarm and rear shock called), steering head bearings, several rubber parts here and there, cleaned all the parts, replaced hoses, seals, straightened few fins of the water cooler radiator, repainted everything... Few of these things are worth mentioning in more details.

The swingarm bearings are "protected" with a fibre washers. I didn't like that solution, also the bushing was a bit rusty as some water got trapped inside, so I went for a better solution and used rubber seals in the same way as in the swingarm linkage. The only modification needed was pressing the new bearings around 2 mm deeper to make space for the bigger seals. There are two plastic dust caps on the swingarm, after the 20+ years, the plastic was cracked and not in a good shape, so I've made new set from nylon (OEM parts were not stocked at local suppliers and were quite expensive anyway).

I've changed all rubber parts in both brake pistons and pumps. Btw, I just hate Brembo, the rebuild kit costs 20 USD and contains a single O-ring and a bit of grease, the second seal and rubber boot are not there, the boot can be bought separately for another like 30 USD and the seal is not available anywhere. The whole brake pump can be bought brand new for 90 USD... I wish I knew before ordering the rebuild kits. During changing seals in the front brake pump, I've noticed there's a tiny washer missing, the BMW manual explicitly warned not to loose it during disassembly. The seals were in a quite good condition, therefore I guess some has done the breaks before but lost the washer. The washer cannot be bought separately and the whole assembly is quite expensive, so I had to order some bronze sheets and make it myself. It's actually quite thin, so it was not that straightforward, I had to clamp the sheet between two pieces of aluminum and turn it in a lathe...

The rear brake pump tends to get rusty, it's worth checking once in a while. The locking ring is quite hard to source locally and Brembo is not selling it as part of the rebuild kits...

I've also replaced the brake hoses, the original rubber hoses were at the end of the life and the front one was incorrect length due to longer ohlins forks that I've bought the bike with. I've ordered braided hoses from HEL Performance, these are supposed to be road legal and were cheaper than OEM rubber ones....

The Ohlins conversion used a different wheel spacers than the OEM variant, so I had to make new spacers:

I've zinc plated them for some rust protection and better look. It's actually quite simple process without nasty chemicals, although some personal protection (gloves, glasses) is still advised.

The results are surprisingly good:

I've also decided to polish the exhausts as they were not looking very good. During that I've noticed a dented and scratched surface on side of the exhaust, it must have been caused by a nasty fall on some rock. They said buy an old bike, they said it's cheaper than a new one. I guess I should have bought new CFMOTO 450 MT and be happy without spending so much time on this piece of crap... Anyway, I've polished the exhausts with some scotch-brite first and finished it with some polishing compound and a angle grinder with a polishing wheel.

The original frame protection stickers were heavily damaged and I had to remove them before before painting anyway, so I've made a template and cut out new ones from a thick plastic sheet with an adhesive layer.

The previous owner only had a single key (in a pretty bad shape), the OEM keys are insanely expensive (no electronic inside, just a piece of shaped metal and plastics), so I've bought a pack of blanks from Chinese supplier and asked a local locksmith to cut it for me. So for a price of two beers, I have nice set of 3 brand new keys...

The engine bashplate had one of the mount holes elongated from vibrations or maybe some accident, I had to cut the hole back into a circular shape and make a bigger insert than the original one.

The original windshield was a bit annoying during tthe test ride, it directed most of the air directly onto my head, making a loud buffeting sound. Also the bottom mounting point on the bike was damaged and someone replaced it with a nut and tons of glue. I was afraid the thin plastic of the front light assembly could get damaged with windshield mounted directly to it from the forces caused by wind, so I've made a steel bracket to spread the load from the windshield to a wider area and riveted a nut to it to provide a convenient way to secure the screen (much better than a nut glued to plastic). I've also made some spacers, to change the windscreen angle and make some space below the screen to reduce the buffeting.

Changing the tires was a pretty tough for me as I've never done it on a larger bike before, but after some cursing, I manged to get the old tire off the rim, putting a new tire on was much faster, but I can't image doing this with only very basic tools somewhere in the middle of nowhere after getting flat tire. I guess I'll switch to the tubeless rims in the future.

And btw, the "ecological" balancing weights from steel are annoying, I had to use like three times more weights than I would need with the lead ones.

Assembly

There's not much to tell to assembling everything, the BMW workshop manual provides most of the necessary details including the torques needed. I've managed to damage the thread of the front wheel axle clamp. I didn't reach the specified torque yet when the screw started slipping, tearing the threads. I had to drill remains of the thread away and put an insert there...

The manual states a procedure to fill the engine with oil, but some of the details were not clear to me. The first one - bleeding the oil circuit requires removing one of the valves below the oil filter, the name of the valve is stated in the manual, but there's no detailed image or anything to show which one it is - after some digging in oil circuit schematic, it's the one with the hole.

The second issue is the proper oil volume. The engine has a dry sump and to check the oil in a correct way, the engine has to run for an extended period of time - at leas until the fan goes one when idling, or better, ride it for few kilometers before checking the level. So, to fill the engine, you need fill the oil tank to the top, remove spark plugs, bleed the oil circuit and then keep the starter running until there's enough space for the remaining oil in the tank. The total capacity stated everywhere is 2.3 litres, but that's for an oil change, not for a completely dry engine - when you change the oil, some of it remains in the oil lines and oil passages inside the engine, therefore the actual volume needed will be higher. But how much higher? I was not able to find any details on this, so I've filled the prescribed 2.3 litres, started the engine and kept it running until most of the oil disappeared from the oil tank, filled enough oil to reach the very bottom of the dip stick, kept it running until the fan went on and finally filled it to the middle of the dipstick.

After first short ride, I've rechecked the oil level. It was way above the maximum, which is not very good for this bike (seal in the engine might go bad, oil returns to the airbox through the crankcase vent,...) so I've removed enough oil to be right in the middle of the dipstick, giving a total volume of 2.6 litres - therefore additional 0.3 litres was needed above the volume in the manual.

Conclusion

So, it's done. The bike is back on the wheel after half a year worth of weekends, it runs nicely and I finally have some trust to the bike to take it abroad. There are still some items on my TODO list left, like adding an USB-C charger, heated grips, mount for a smartphone, so I guess I'll update this page in the future with more details.

The bike is in "brand new' condition, the issues with warm starts are gone (fixed by fixing valve clearances), the instrument cluster fully works, the bodywork is almost perfect, so let's see how long it stays in this state, I guess the first fall in a terrain will change that soon :).

And the total cost?

• Bike: 2100 USD
• Sold Ohlins forks for: 700 USD
• Engine (bearings, paint, timing chain, misc): 700 USD
• Rear shock, front forks restoration: 370 USD
• Paint: 200 USD
• Other parts like bearings, seals, fluids, tires, hoses, seat cover, etc.: 1200 USD
• Total cost of bike and repairs (with ohlins price subtracted): 3870 USD
• Cost of work: priceless, could have spared some money and bought much better bike :D

A Dakar in a reasonably good shape and similar mileage is usually being sold around 2800 USD, I would probably spend another 1000 USD on a good bike for preventive maintenance (wheel and linkage bearings, timing chain, all the fluids, brake and fuel hoses, fuel filter, tires,...), so I would have ended with a bike in a same condition for much less work, but on the other side, I know my bike inside out now, so if anything goes wrong, I'll know where to look and who to blame and I'm pretty sure there are no hidden issues anywhere.

Resources

• crossroadz.com.au - Extremely helpful page with info about every part of the bike
• faq.f650.com - Detailed FAQ to all the service works and issues of the bike