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low compression excessive exhaust valve clearance

jellywrestler

Well Known and Active Forum Website User
Non-VOC Member
I've done 280 miles on a freshly rebuilt motor and she died, went down onto one cylinder then popped and farted then stopped altogether
I did a compression check and it's low on the rear cylinder and has quite a lot of exhaust valve clearance.
Adjusted this and re checked compression and is now even lower.

there are several possibilities including a naffed camshaft.
If this is the case I didn't notice any lack of power and was pretty cautious of my surroundings.
the last run I did was 100miles a day for two days.
I wouldn't have expected the cam to be so worn down 'off the cam' as it were.
other option is a burnt valve.
What advice have you folk got, or should I just have a few beers and forget about it?
 

passenger0_0

Well Known and Active Forum Website User
VOC Member
Certainly it's not likely to be the cam given you check clearance on the base circle which never wears. Take it back to the rebuilder as A-BCD is correct in that it's likely dropped a seat - although this is more common on the inlet the exhaust could go if installed poorly.
 

chankly bore

Well Known and Active Forum Website User
Non-VOC Member
I have known cases of the valve collar ET35 not being tapped home before assembly and then going where it should when mechanical progress ensued.Result,excessive tappet clearance,burnt valves and noise.Unlikely,but a cheaper fix than a loose valve seat!Let us know the result.
 

jellywrestler

Well Known and Active Forum Website User
Non-VOC Member
I have known cases of the valve collar ET35 not being tapped home before assembly and then going where it should when mechanical progress ensued.Result,excessive tappet clearance,burnt valves and noise.Unlikely,but a cheaper fix than a loose valve seat!Let us know the result.
that's interesting thanks,
won't be able to pull her down for a wee while, backlog in the shed, just sounding things out at the moment
 

davidd

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VOC Member
I do not think that a compression test tells you enough. I would do a leak down test, which will tell you where the leak is. I remove the adjusters when doing a leak down test so that it does not matter where the piston ends up, but put the bike in gear and hold the rear brake. When you remove the adjusters it might be useful to pull out the pushrod of the valve that you are constantly adjusting and check it for length. My intake on the racer needed several adjustments which I thought was a lobe or a follower going away. It was a pushrod collapsing, which is not uncommon. The pushrods were new from many years ago. I think six inches is the quoted length in KNB.

100_2890.jpg
 

Bracker1

Well Known and Active Forum Website User
VOC Member
I would check the compression release assembly first. If it was molested during the rebuild, may have gone out of adjustment. It would of course give you loss of compression. Sometimes it is a simple fix. Good luck, Dan
 

clevtrev

Well Known and Active Forum Website User
VOC Member
I do not think that a compression test tells you enough. I would do a leak down test, which will tell you where the leak is. I remove the adjusters when doing a leak down test so that it does not matter where the piston ends up, but put the bike in gear and hold the rear brake. When you remove the adjusters it might be useful to pull out the pushrod of the valve that you are constantly adjusting and check it for length. My intake on the racer needed several adjustments which I thought was a lobe or a follower going away. It was a pushrod collapsing, which is not uncommon. The pushrods were new from many years ago. I think six inches is the quoted length in KNB.

100_2890.jpg

5 15/16" is nominal.
 
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