Unexplained loss of compression

[Andy51]

Hi All,

I have just put my '54 18S back together and there is no compression - it had plenty before I started work (8.5 to 1 piston). I have done the following work:

1. Removed and replaced Lucas ATD and pinions (the root of the problem - bike wouldn't start as ATD jammed in advancd position, took it to pieces, cleaned up rust, put it back on).
2. While in there, I replaced scrambles cams (too noisy and unnecessary with less sporting driving accompanying advancing years!) with road ones, using 350 marks on cam wheel to get improved performance over standard set-up.
3. Put it all back together with timing 1/2 in before TDC with ATD in proper setting and a squirt of oil down pushrod tubes to lubricate cams.

Tried to start, and a few 'coughs' (bike, not me) then noticed lack of compression. I have checked valve clearance (pushrods rotate), spark plug in properly, exhaust valve lifter has slack in cable. The only thing that happened during timing set-up was TDC finding tool (spark plug body with sliding rod) locked up once or twice as it enteres head at a slight angle, but only under hand pressure turning rear wheel (slackened off wheel, rod freed, carried on) but hardly the sort of force that would hole a piston.

Before I start taking off the cylinder head to investigate further, any ideas? Grateful for your thoughts, Andy

[Mick D]

Hi

Stuck valve? Can you feel/hear it blowing out of the exhaust or carb when it is on the compression stroke?

In the unlikely event that your TDC tool has damaged the piston crown you will be able to see it through the spark plug hole.

Regards Mick

[REW]

I know you have checked valve clearances but it is surprisingly easy to get them just that bit tight and hence enough to lose compression - at least 3 singles I have known have suffered thus.

I prefer very slight but perceptible free play in the pushrod rather than just freedom to rotate. You can always tighten them up if it proves noisy.

[willowbedone]

Hi have you checked valve lifter lever is not stuck holding exhaust valve open
Ian

[Duncan]

Its unlikely you have done so but when the TBC tool gets stuck it is often hitting a valve excessive force turning the engine could chip or bend the valve, its more likely to be pushrods, valve lifter or a stuck valve as noted above.

[56G80S]

IKBA but I stopped using a TDC tool such as you describe (originally got it for the notch in the Triumph flywheel, I think?).

Better accuracy and no real risk by using a piece of bent coathanger wire marked with small notches with a file. This can be held perfectly vertical and provided you dont forget where you put your head when setting it up is consequently quite accurate and moves up and down easily.

Only other thing is the "marks" on cam wheels, maybe use the Ken de Groome (I could PM the article if you don't find it in a search) approach just in case they are rogue markings/cams and ignore the marks.

As you haven't had the barrel off, the possibility of not setting the ring gaps at 120 degrees isn't present, thankfully.

Spriddler posted this a while ago - http://archives.jampot.dk/technical/Gen ... _types.pdf wrong one, but root around in the archives and you'll find it.................................

Good luck. If you don't want the other cams I can't find my SH ones..............................................!

Johnny B

[SPRIDDLER]

Spriddler posted this a while ago - http://archives.jampot.dk/technical/Gen ... _types.pdf wrong one, but root around in the archives and you'll find it.................................

No, that's not the one.

This is the right one below, but I think Andy has already seen it as he mentioned using the dots for the 350 on a 500.

http://archives.jampot.dk/technical/Gen ... e_Dots.pdf

Maybe the softer cams are lifting the valve(s) differently which brings a previously unpolished part of the valve stem into the valve guide causing it to stick.
I can't get my head around it at the moment (it's late) but I think that my suppostion is unlikely as I assume the scrambles cams would have a greater lift than the softer ones so the valve stems would have a more extensive polished range than the stems when using softer cams.