Compression - daft question?

[AndyD]

Hi,
Nearly finished main rebuild of the CJ250T engine and have a question I think I know the answer to but maybe don't want to!
Bores honed and new IMD piston rings fitted and all went together well.
Haven't fitted the valve cover yet so just has the cam, camchain and valves etc. so cam turns but valves just stay shut.
Engine turns over OK by hand but I put a finger over the plug hole and didn't get much compression and could hear a 'wheeze' sound which may be coming from the crankcase.
Fear is that this means there is very little compression in the bores unless I'm missing something obvious?

Cheers,
Andy

[K2-K6]

It'll feel odd like that as all the valves are closed and the piston going down the bore can't draw much air if you've got the plug hole covered with finger or thumb,  so in effect creating a vacuum in there. Not much to compress like that.

Should get realistic appraisal with cam cover and rockers fitted.

[McCabe-Thiele (Ted)]

You definitelly need the valves adjusting before you can make an assesment of compression. Adjust the clearances and try again.

[AndyD]

Thanks,
I guess my thinking was why there would be little compression as the valves are all fully closed.
I tried letting air in on the downstroke, then covering the plughole and compression still felt low but you're right I should try with the rockers fitted - I was just leaving this until later for no real good reason!
Will give that a go and hopefully will all be better.

Cheers for swift replies and giving me hope it might be OK!
Andy

[McCabe-Thiele (Ted)]

Doing a cold compression test will definitely not give you the best reading but it is a indication of what might come when the rings have bed in plus finally a hot engine test. That said I fully understand your anxiety to get some sort of handle on how good they will turn out to be. Back in the day when I was an avid BL A series engine tuner I remember changing to half race cams and finding compression sometimes  lower - I blamed the change in valve overlap. Once driven I would be surprised at how well the engine could perform when on cam were talking over 3,500 rpm on those old engines.

[AndyD]

Thanks,
Put the rocker cover on and compression feels more normal just covering the plug hole so hopefully worried about nothing.
Just seemed odd that with all valves closed there felt to be little compression.
Anyway, relieved that seems OK so can move on again.

Cheers,
Andy