TOPIC:

The ambient temperature may have expanded the metal to make it a tight fit - don't have that problem in Scotland, bolts dropping out all over the place.....

P

This is weird With the cam belt tensioner set a bit loose allowing the cam timing to be retarded slightly it turns over without touching with the tensioner set correctly it touches valve to piston, I have no way of knowing if the head or barrel has been skimmed reducing piston to head clearance or if there is a carbon build up on the piston crown, I just hope the latter, anyway looks like a head off job
Keep you posted
John

both cams are at the right place?
Somehow i can remember a problem you described.
Hmmm.... i think it was a Saturno engine with Nordie cams. The rocker arms are somehow different. Still using Saturno rockers the adjusting srews can get in touch with the covers ore something like this. But that is somehow differnt to your problem.

ciao Joachim

Hi Joachim,
I have not stripped the engine (yet)and the only bit of known history was that the Sprag clutch was slipping and the bike was abandoned as the owner could not be bother to replace it, or bump start it I have only removed the left side case to fit a new Sprag and the right hand cover to fit the kick start shaft bought from Achim last year, every thing looked tidy inside the side cases and I suspect had never been disturbed from the day it left the factory, just wish I had an endoscope to look inside from the spark plug hole, I've got all the top end gaskets so think best to take the head off for a look see and use a degree wheel to set the cams and adjust with offset keys if they are a bit out of proper setting.
Valve to piston clearance must be very small if a little slack in the cam belt can make the difference between touching and clearing the piston
Regards
John

Was it turning over OK before you took the belt off to remove the sidecover for the sprag clutch change?

Sounds like it could be the belt one tooth out on one cam wheel but you've obviously double checked that.

The clearance is probably more than on a Husqvarna 510. They would touch valve to head if over-revved with a slack camchain.

I've heard of cam wheels becoming loose on the cams and slightly rattly on very high mileage bialbero motors but I can't see how that would advance the timing as any lost movement to take up slack would retard it - I think

It's a mystery.

Hi 72,
I hooked up a battery to confirm the Sprag clutch slipped, but as it still had a plug in it didn't go over TDC.
What I have noticed is that the std timing marks are very slightly advanced when at tdc with the belt correctly tensioned, not up to 1 tooth out, maybe 1 degree, which made me wonder if the head or barrel may have been skimmed
If I had skimmed it I would have used a thicker gasket under the cylinder to recover the lost height and not disturb timing.
Piston to valve clearance must be small as they have valve cutouts in the piston crown as std not something you had to machine yourself when you have raised compression
Regards
John