Living with a Honda C90MT 12V electric start: maintenance and restoration: Starter clutch overhaul

I found that although the bike ran nicely with its replacement motor- there was an obvious whine. This could be a very loose starter or cam-chain (although these often rattle) or it could be a failure of the starter motor to disengage on motor firing. At about the same time I also noticed that the starter had become inoperative- the starter turned but there was no cranking. These 2 symptoms are potentially incompatible, but both suggest a starter cutch problem so I decided to begin there.

Ive always been puzzled by starter clutches- its easy to test them - they engage if you turn them in one direction in this case clockwise, and disengage if you turn them in the other. For years I thought this must imply some sort of change in rotation direction as the motor started- which always seemed impossible- but realization came in a flash today. There is no change in direction- that is simply a way of testing the clutch- but what does occur is a change in relative rotational speed. When the starter is first used it turns the motor in one way, clockwise. At this time the crankshaft isnt turning at all, so the clutch moves round the crank in a clockwise direction and grasps the crank collar- cranking the motor clockwise as well.

When the motor fires, the crankshaft continues to turn in a clockwise direction by the engine, but at a much higher rotational speed than that of the starter clutch. The crank then overtakes the starter clutch (which will slow and stop anyway once the starter button is released). This means that the starter clutch is now effectively going backwards relative to the crank, and so it disengages. Its likely something has gone wrong with mine. Luckily its a fairly easy strip and I ordered a rebuild kit anyway.

Start by removing the gearchange pedal. Here the clamp bolt has been removed.

Remove the flywheel and engine sprocket covers- noting where the different length bolts are fitted. I took advantage of the strip and repainted both of these.

Detach the multiclips and remove the terminals.

This reveals the flywheel behind which is the starter clutch and the driving chain mechanism. I dont need to remove the starter motor itself so Im leaving the chain and its sprockets in place. I'll clean them thoroughly and regrease. The gasket on this bike is in poor condition- something else to order!

The flywheel is held by a large nut and washer- in my case sadly a little mullered.

Use a ratchet strap to hold the rotor and release the nut. It has a normal right-hand thread. Do not damage the ignition trigger magnet on the outside of the flywheel.

Use a correct puller to remove the flywheel. Refit the crank nut to protect the threads beforehand.

Remove the puller and the nut and washer.

The flywheel comes off revealing the crank collar behind on which the starter cutch grips. This was greasy but the oil seal looks to be in good shape. I cleaned up the crank collar but I don't need to remove it.

The starter clutch is screwed to the back of the flywheel by 3 staked, countersunk M6 screws that also use threadlocker. Blue marking is a check on rotation in service- they havent unscrewed at all.

Straight away a potential problem emerged as one roller was loose in the clutch

To proceed further I need to separate the clutch and flywheel, this means dealing with the staked screws.

Several reports speak of using an impact driver alone. This doesnt work and incredibly, I broke one of the driver's bits trying! The only way is to remove the staking with a drill or Dremel. I drilled the stakes out and then removed the stump of the screws with the impact driver.

That wasnt easy either and I broke 4 drill bits trying, but eventually all 3 screws were removed. Obviously they will not be reusable.

The clutch comes off and I removed the remaining 2 rollers releasing their springs and caps. Looking at the rear showed why one roller was loose. One spring cap was stuck in its housing and so couldnt press the roller against the clutch. In the pic below its the 12 O'clock spring cap- the others have simply come out when their roller was removed. Note also the tell-tale corrosion or wear mark that shows this one roller hasnt been moving properly- or at all.

I removed the clutch parts using a punch to grab the stuck cap.

I cleaned up the crank collar- note blueing from excess heat here. This is good- perhaps Im right that the clutch wasnt disengaging.

I selected Allen countersunks for the replacement.

All parts were cleaned in sonicator. I cleaned off the tell-tale and smoothed the spring caps with emery cloth.

There is a shim between flywheel and starter clutch so I removed that for cleaning also.

Note that there is a location pin that ensures the clutch and shim are positioned correctly- presumably to maintain crank balance. It can only be fitted one way round but dont forget it.

Once I had cleaned the springcaps and rollers- and smoothed off the burr caused by the spring loaded punch, all the parts reassembled easily and the clutch rollers were able to move as inetnded. It would probably be fine just to reassemble and rebuild as is, and use the bike. However, Ive ordered a rebuild kit as well as a new gasket so Ill probably wait until these arrive so I dont have to do this job again....kit shown below

...and unpacked.

The old springs are slightly compressed compared with the new.

fitting the kit was straightforward. I did put a drop of light lubricating oil into each spring socket and a light film of auminium grease on the cutch inner surface and those areas of the shim on which the rollers slide.

I secured the starter clutch to the flywheel using Loctite threadlocker and tightened the screws with a blow from the impact hammer.

I staked each screw once using a centre-punch. Before fitting the clutch I removed remnants of the old gasket and cleaned the mating surface. I treated the chain to a dose of dry chain lubricant (PTFE Based) and fitted a new gasket- keeps muck out rather than oil in!

I could then refit the flywheel making sure the key was still in place. The rollers prevent the clutch for sliding straight backwards and directly over the collar so rotate the flywheel slightly anticlockwise as you push it in. This retracts the rollers. You can then align the keyway and push the flywheel fully home.