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E: Engine No compression

Alan Ball

Website User
VOC Member
Hi all

I have a '52 comet and have been working on it for the last 6 or so months on and off, so still learning my way around it. It was a runner (for a short time anyway) when I bought it and have fixed many minor issues so it has not been out for a run for a while

It has now developed a problem in that when I crank the engine slowly there is full compression (and the valve lifter works as it should), but when I kick the engine over to start...there is none. I assume this is a valve 'sticking' but before I start to investigate I thought I would ask if anyone else has seen and fixed a similar problem.
Ideas and advice gratefully received
 

Bill Thomas

Well Known and Active Website User
VOC Member
This is hard to put into words, With a Comet, If it's just over TDC of the firing stroke,
You can kick it full swing without using the valve lifter and there is no Compression, It's all about gearing of the kickstart, In fact that is the way I start mine !.
The other thing it could be, My Comet is old and a bit worn, And after a winter lay off,
One of the valves does stick a little sometimes, Maybe a bit of coke on the valve stem ??,
But I take the spark plug out, Squirt a bit of WD40 in the plug hole, Kick it over a few times,
And it's OK for the rest of the Summer.
Make sure your tappets are not too tight before doing anything, Cheers Bill.
 

Alan Ball

Website User
VOC Member
Thanks for the reply Bill. I usually kick it over by getting it to compression, then using the valve lifter to go just over TDC and then giving it a good kick. Now it just continually turns over without compression unless I turn it very slowly, when it returns....weird. I will try your tip with the WD40 and see how it goes.

Thanks!
 

timetraveller

Well Known and Active Website User
VOC Member
Take out the spark plug. Take off the two screw caps which cover the tappets. Do this carefully and if you feel some resistance, screw the cap down a little, turn the engine over half a stroke and then take the cap off. Turn the engine over so that both valves are fully shut, that is the adjusters at their minimum height and see if there is any up and down play. Ideally there should be none but when you are learning then make sure there is a thou or two up and down movement. Put the spark plug back in and then test for compression. Turn over so that there is compression and then do whatever it is that gets rid of the compression. If it is a sticky valve then you will now find that you have lots of slop in one or other of the tappets, If you have then you know which valve is sticking. If not then it is not a sticky valve problem and you might be looking for something to do with the valve lifter. If that is the case then before you take off the timing cover to delve in there, check the cable going into the rear of the timing cover which operated the valve lifter and make sure it is not hanging up on something. If nothing is obvious then come back on here with more symptoms or questions.
 
Last edited:

Martyn Goodwin

Well Known and Active Website User
VOC Member
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.
 
Last edited:

davidd

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Website User
VOC Member
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
 

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