Low compression on one cylinder

[wozzer1340]

Hello all, I've just had a freshly rebuilt sprint engine fitted into my Dolomite. It's fitted with twin Weber's and I've kept the points on it. It's running low on one cylinder 100psi and over 150 psi on the other three. Taking the lead off didn't effect the engine so it's running on three, which is really annoying. Any ideas on what could be causing this? Ive read a few bits on line about the shims in the head and possible gasket issues that might cause this. This is a complete new engine and the only re-used part was the crank.. any help or suggestions would be appreciated. Car is in Worcester

[dollyman]

Have you done a wet test on cyl compressions? Also have you checked for an air leak on the cyl that isnt firing?

Tony.

[cleverusername]

Did you rebuild it or was it rebuilt by someone else? When you rebuilt I assume you checked the bores for wear and used new piston rings? Did you check the valves were sealing properly before fitting the head?

I mean the obvious thing, if it isn't the valves and the rings is a blown gasket or something on the head block isn't flat. Which cylinder is low on compression? I have seen other posts which say cylinder four is most likely to blow because of cavitation form the water pump.

Still, since it is so easy to do, it wouldn't hurt to check the rings by squirting a bit of oil into the cylinder and seeing if that solves the compression problem.

[Carledo]

Whilst having one cylinder that much lower than the others is far from optimal, 100 psi is still enough to fire on, I've had old Morris Minors and the like where all 4 cyls were hovering around the 100 mark and still running.

So while you DO have a compression problem, id also check the plug, lead and cap!

Did you buy the engine freshly built, or was it your motor you had rebuilt? If the latter, i'd just get onto the builders, it's their problem to solve!

If the former, it's likely to be your problem to solve, I'd start with checking the valve clearances, a tight one could cause this. A wet and dry compression test could point up a cracked ring.

Beyond this however, you're pretty much looking at pulling the head off to find the problem.

Steve

[cliftyhanger]

Just to confirm, you did the test with the throttles wide open, and ideally warm?
Best to remove all plugs too so the starter has an easier time...
And as above, all 4 dry, then in turn a couple of squirts of oil and retest.

But on a new engine, I suspect something has been assembled incorrectly. Valves an obvious one, then rings. I suppose the head gasket could be at fault....
And yes, usual checks that the non-firing cylinder is getting fuel and sparks.