E: Engine No compression

[Martyn Goodwin]

Check that the top of the valves stems themselves are not hitting the inside of the valve screw caps - it's a not uncommon problem. To check, All you need do is untighten the them just a turn or 2 - with the inlet this can be done with the UFM in place, its easy to do the exhaust. If compression returns after you do this the quick fix is to use 2 fibre gaskets under the offending cap.

 

[davidd]

Yes, I have noticed that singles often have no compression when kicking them over. It is usually cured by opening the throttle. When the throttle is closed the bike has trouble gulping enough air to produce a lot of compression. You can try kicking it with the throttle wide open if you pull the plug wire off.

David

 

[kettlrj]

As David says, it is likely that the carb slide is sealing well and when you turn over the engine slowly, enough air is dragged through to fill the cylinder and create compression.
I have recently helped a friend with his Comet rebuild and he had the same problem. Opening the throttle while kickstarting restored the compression. However the bike would only start with the throttle completely closed, so the drill was to turn over the engine with the throttle open a couple of times, bring it up to compression, pull in the decompressor and then kick with the throttle closed to guarantee a first time start. This only works if the pilot setting on the carb has been set correctly.
Forget about sticking vales etc. If that was the case you would get even less compression when turning the engine over slowly. The same applies to vales not sealing or cylinder bore leakage. Look for simple causes before exploring the difficult ones.
Regards Richard.

 

[Alan Ball]

Thanks all for your help, it is much appreciated.
David, Richard, you are absolutely right. When I open the throttle the compression returns.....I am sure it didn’t used to do that before I did the mini restoration. Maybe the carb strip down and clean had an effect? It still won’t start, so a quick check showed that I have no spark at the plug, so just need to sort that and hopefully will be ready for spring.

thanks again for the help
Happy riding
Alan

 

[Colin]

In any event Alan, I thought you played a blinder in the 1966 World Cup!!!!!!

 

[Alan Ball]

Its been downhill from there

 

[Alan Ball]

Just wanted to give a final update. Yesterday I sorted out the ignition troubles, fired her up second kick and she is running beautifully. I just need the rain to stop and get out for a proper shakedown run.

Thanks again all for your help in pointing me in the right direction