So now my Naughtistrada has new injectors and the starter looks good but the sprag clutch has gone awol. For those of you that don't know what a sprag clutch is - it's part of the starting mechanism, basically a one way clutch that disengages the starter once the engine starts. In a car you have a thing called a bendix drive - bikes run smaller, faster starter motors than you find on cars. The sprag attaches to the flywheel and has a series of precise rollers that bite in to a sort of drum that is part of the flywheel. If you try and start the bike with a weak battery or a backfiring engine the clutch is subjected to enormous forces that break either the rollers or the carrier that holds them at a precise distance from the drum. Part of the assembly is a gear that engages with an intermediate gear that in turn engages with the starter.
It sounded on mine for all the world like the starter gear had stripped but when I got the side panel off and tried it on the starter - yes i had removed the plug leads first to make sure it didn't fire with no oil in it - I could clearly see the sprag slipping.
In a break with Ducati tradition the sprag on these is incredibly easy to remove - there is usually a taper on the crank that the flywheel grips on to and a puller is required to remove it, in the Duc it's just a spline so the center nut is removed with a decent impact driver and the whole assembly slides off. It was when I did that I spotted what I believe to be the root cause for the initial starter motor failure and the subsequent sprag clutch failure and it was all down to a 10p part being incorrectly installed. Behind the sprag assembly is a thin shim - maybe about 15 thou thickness. Somebody had obviously been in there before because instead of there being a nice flat shim of exact dimensions there was a badly distorted and damaged shim that had been trapped between the sprag clutch's center bearing and the crank journal. It must have been causing a very large amount of drag and was certainly stopping the sprag from working correctly - those things have to be accurate to half a thou or so. In fairness with it all apart and on the bench the sprag itself doesn't look too bad but I think it's bad enough to need replacement. so one is on order and I fully expect that once it is assembled correctly it will all work a hell of a lot better.
The only thing left to consider now is the battery - the standard item is a max 210 cca - cold cranking amps - the best lithium one I can find is a massive 350 cca. A weak battery is really bad for a sprag clutch - the forces of it repeatedly letting go and grabbing are huge on a big vee twin. A lead acid battery is a quarter of the price of the heftier lithium but then again sprags are not cheap and are a pain to fit. Anyway, watch this space, I'll let you know how I get on.
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