I recently bought a 1983 Honda VT750 Shadow that looked like an interesting and worthwhile project. It quickly became apparent that things were not as one would have wished in the starter motor department. I had given the project to my friend, James as a bit of a learning exercise and so he could make a few bob on it, having recently had the dirty done on him by a Skegness motorcycle shop, which shall remain nameless. Anyhoo James had gone through the carbs, put them in an ultrasonic cleaner and put them back on the bike but was having trouble getting it to turn over reliably. We had a chat via messenger and he sent me a video of the sprag clutch being very naughty indeed. I thought this would give me a great opportunity to talk about sprags and how they work. I hope you are sitting comfortably as I shall begin anyway:
There are generally speaking two types of starter motors - bendix drive starters that are usually used on cars and pre-engaged starter motors that are usually used on bikes. Bendix, also called inertia, starters are fine on cars - loads of space, weight isn't an issue and you can have a thumping great battery. No such luxury on a bike - they have to be small and run off a much smaller battery - they therefore usually run pre-engaged starters. Such starters are powerful little beasts, they spin very fast so stand being geared down a lot to produce torque but there needs to be some way of disconnecting the starter motor from the engine once the bike is running. This is where the starter clutch - or to give it it's correct name - the sprag clutch comes in to play, It's just a one way clutch that engages with the engine's flywheel when the starter turns but automatically disengages once the engine is running. They are made to incredibly fine tolerances and will fail if they wear beyond their limits. In the vt it has a hard life as it's a high compression twin that is prone to spitting back.
The one in our example was absolutely monstered and could sometimes be turned almost a full revolution before it would grab, sometimes it wouldn't grab at all. This turned out to be a bit of an issue on such an old bike as getting a replacement on an imported bike that celebrates it's 40th birthday this year is like trying to find a left handed Unicorn. I found one after much searching, my wallet still hasn't forgiven me but I thought the bike deserved it because they are an uber cool machine.
so anyway, if your bike suffers from the starter continuously disengaging or refuses to engage at all when attempting a start it's most likely the sprag having a hissy fit, it needs replacing. Good news is that most are pretty cheap - a Honda CBR600 one we would sell used for about 30 quid, tested and ready to go, most 80's bikes would be a similar price. If the starter doesn't spin at all it's not the sprag - it's either the starter, solenoid, side stand switch, clutch switch or starter button - basic electrical testing is needed rather than mechanical know how .
Just as a foot note we also had another problem in the same area on our bike - the magnetts had at some time become unstuck from the flywheel. They were showing signs of distress and it looked like the stator coils had already been replaced. You can see in the photo how chewed up the original flywheel is. Because we could not guarantee the quality of the repair we decided to replace the flywheel as well. There seemed no point in putting it all back together ny to have a problem 3 months down the line. The sprag assembly bolts on to the back of the flywheel so we replaced the entire assembly as a unit - you can see the 6 torx headed bolts that fix the sprag to the flywheel. Rather expensive but we can both sleep at night knowing the job has been done right.
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