Author Topic: What causes low compression in one cylinder when engine is warm  (Read 7531 times)

Offline Norm

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What causes low compression in one cylinder when engine is warm
« on: October 22, 2011, 09:54:22 PM »
I just got done rebuilding a 1976 340 Olympic on Dec 2010. I've replaced the crank seals (both), one piston and rings on the side that fouled plugs (blown head gasket and cracked rings). Compression was at 80 and the other 150. Once the repairs were made I had 150 in both, started first pull and idled perfect. Test driven for the first 20 minutes, then lost power, did not idle anymore and fouled the plug. Compression dropped to 120 on the new piston while the other remained at 150. I left it get cold, retested compression test back up to 150, clean the plug then it started back up perfect then after driving for 20 to 30 minutes same problem.  Does anybody have any idea of the cause? Is it possible wrong size piston rings or maybe the cylinder was honed to much?

Offline Up Late

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Re: What causes low compression in one cylinder when engine is warm
« Reply #1 on: October 23, 2011, 11:00:20 PM »
Did you use a OEM piston or an aftermarket one?
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Offline Norm

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Re: What causes low compression in one cylinder when engine is warm
« Reply #2 on: October 24, 2011, 06:33:01 PM »
They sold me an aftermarket piston.

Offline Up Late

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Re: What causes low compression in one cylinder when engine is warm
« Reply #3 on: October 24, 2011, 06:51:26 PM »
They sold me an aftermarket piston.

It's hard to say if that's the problem (I'd probably suspect that last - as aftermarket pistons can expand a little differently than the OEM.), but the compression getting out somehow or the side is running hot and that issue hasn't been fixed.

The rings could not be seated well (cylinder should have been honed).

Also check the head gasket and torque.

If it's a dual carb, check the carb on the side losing compression to make sure it isn't leaning out and getting hot. See how the plugs look between the 2 sides.
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SatanClaw

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Re: What causes low compression in one cylinder when engine is warm
« Reply #4 on: January 09, 2012, 01:48:06 AM »
Sounds to me like lean mixture...

I suppose it's the cylinder furthest from the fan that's the problem ?

I blew two pistons due to a faulty carburettor...  it had enough cooling on the cylinder closest to the fan, but failed, and burned the piston twice on the clutch side.

Check if it runs ok @ full throttle when you turn the choke on... If that is the case, I would check the fuel pump\carburettor..