# low compression



## johnny67 (Aug 28, 2018)

I have low compression on #5 cyl.60psi and on #8 30 psi 67 firebird 400 428 heads Crane rollers and cam. have valley pan off every thing is moving okay cam on one side is a bit worn but not bad, was running crappy at the end and backfiring , could it be a burnt valve, put new valve seats in 5 years ago etc. also engine was not warm at time, and I did not spin over 5 compression cycles as stated on site. appreciate any info thanks .


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## PontiacJim (Dec 29, 2012)

johnny67 said:


> I have low compression on #5 cyl.60psi and on #8 30 psi 67 firebird 400 428 heads Crane rollers and cam. have valley pan off every thing is moving okay cam on one side is a bit worn but not bad, was running crappy at the end and backfiring , could it be a burnt valve, put new valve seats in 5 years ago etc. also engine was not warm at time, and I did not spin over 5 compression cycles as stated on site. appreciate any info thanks .


Not good in my book. I would consider an electrical issue, but with compression numbers that low I would think it to be a mechanical issue.

I didn't think roller cams wore out? Just a "little worn on one side"? What does that mean, just a little flat instead of really flat?

Could indeed be a bad cam causing the problems if the valves were not fully opening, but at those numbers I would suspect they would be hardly opening - Battery needs to be fully charged, all spark plugs removed, throttle on carb held fully open. No intake or exhaust gasket leaks?

Did you check your rocker arm-to-valve adjustment. If the valves were too tight, then they can hold the valves open just enough to cause backfiring & poor running.

Weak valve springs could cause valve bounce and cause backfiring & crappy running.

Burned valve usually pops back through the carb if an intake or can backfire through the exhaust as some of the unburned gas is pushed out the exhaust on compression and is ignited when the spark plug fires.

Any gas smell in the oil? May have cracked a piston or even busted a ring land if ring gaps were too tight - common problem with the hypereutectic type pistons IF you do not follow manufacturers gap requirements.

Detonation/pinging sound associated with a high compression engines can do damage to pistons or the rest of the engine over time. Incorrect timing or timing advance can create eventual engine problems.

Next thing I would do is to get each cylinder at TDC with both intake and exhaust valve closed. Then pressurize each cylinder using a compressor and air fitting designed to go into the spark plug hole and listen for air escaping. If out the intake, then an intake valve. If out the exhaust, then exhaust valve. If into the engine block, could be bad/broken rings or piston. Could also be a combination of several.

So at this point, with the low compression as it is, I would first try the pressure test to narrow it down and then be looking at engine disassembly to thoroughly go through the entire engine and diagnose the problem as you may find other problem areas that you would not if you just pulled the heads. It may indeed be as simple as the heads, but you won't know until they come off. 

Just my thoughts on it and hopefully others will chime in with their opinion. :thumbsup:


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