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Originally Posted by Alpinestarhero
Apparently, the problem was due to the camshaft bearings...I don;t know the technical terms so someone will have to help me out here, but the part you bolt down ontop of the camshaft was made in a completly differant place to the bottom part, so the two parts where never perfeclty (as much as perfect can get on a mass-produced head) mated.
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Most are line bored, ie you bolt everthing together, then bore (ie drill) it through in one sweep, ensures that everything is inline. The VF750 apparently had too small holders, the VF500 had larger ones and didn't suffer as much.
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There was enough free play / clearance that the cam shafts would move around and pretty much wear themselves out after a few thousand miles; a typical owner started his/her bike on choke, probably let it run at a fast idle for about 20 seconds beore knocking it off and going for their ride. I suppose with these cold parts, the tolerances where even much worse than when warm and further accelerated the wear.
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IIRC the problem was a multiple one, not that simple but was essentially up to 6 things in combination.
1) Forked followers - they tilt when adjusting valve clearances (fix was to use two feeler gauges)
2) variation in cam to bearing clearances - needs a special tool to hold the camshaft[1]
3) Poor camchain tensioner
4) Soft cams
5) Heat - the V4 gets very very hot, rear cams apparently go first.
6) Poor lubrication.
Druid
[1] To combat this, Honda developed a special tool to hold the cam in place. Any bike shop tuning a V4 had better have this tool. It doesn't cost much ($15) and isn't hard to use. Get one; even though it doesn't completely solve the problem, it is a requirement. Part number is 07979-MK30000, name is "V4 tappet adjust". The service bulletin issued with this tool recommends valve clearance of .006 inches instead of .005 that the Sabre/Magna originally specified, as well as use of premium motorcycle oil, changed frequently, and avoidance of prolonged idling.