Following much correspondence on the lack of compression on my newly assembled 1959 Model 50 - I never really got to the bottom of the problem. I completed the assembly with the rebore, new piston, gaskets, lapped valve seats etc. and got the engine running. Now, when cold and having not been run for a day or so the compression seems poor. However, once I've run the engine, just ticking over, for a few minutes a normal expected level of compression is evident. I stop the engine and then attempt to restart the warm engine - now I can stand on the kickstart ! Leave it again for a couple of days, limited compression, run & then re-check, full compression !
Is this just down to oil in the barrel sealing any air leaks past the rings ? Will things continue to improve as the rings bed in ?
Do a leak test.
... not worry and leave well alone!
It can take a while to bed the rings in.
A friend's 19S suffered occasional loss of compression very similar to what you have described ie plenty of compression when hot, sometimes none at all when cold. Eventually we found a worn exhaust valve guide, allowing enough movement of the valve to not always seat properly. Changing the guide sorted the problem. Many years ago a valve guide came loose in my 19S head (Alfin), giving similar symptoms. I know your engine has just undergone a top end overhaul so you probably checked the state of the guides at that time.
Yes, Mike, as others have said the compression will improve with time, run in carefully, and if possible run with about a thou clearance on the tappers. My M50 has nowhere the compression as my Venom, but it doesn't seem to affect the performance, after all it is only a 350cc, with quite a low compression ratio...........Don't worry about it, just enjoy.
Regards John O
Yes, new guides fitted. Pretty sure they are not thge issue.
I relise this would, if I had the kit, identify a leak, but would it identify where that leak is from ? Never tried this before.
How many miles will I need to do for the rings to bed in ? i.e. how long before I may need to look elsewhere ?