Low Compression 312


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By Will56 - 16 Years Ago
I bought a 56 Mercury Montclair that started. It wouldn't go past about 15 or 20 mph so I did a compression test and found cylinder number 4 had 0. I found out that the exhaust valve push rod had fallen out of place. I put it back on and now it gets only about 30 on the tester. At this point should I take of the head and check further?? Any advice would be much appreciated.



Will
By oldcarmark - 16 Years Ago
What are the readings on the other cylinders?
By Will56 - 16 Years Ago
The other cylinders were around 140 except number 7 at 120
By 56 big window - 16 Years Ago
30 psi is 30 psi . You will have to start with removing the heads to see what is going on . Dnt waste your time with oil testing the cylinders
By speedpro56 - 16 Years Ago
Just hope it's a blown head gasket on that cylinder so to make it an easy fix.
By jepito - 16 Years Ago
one other easy test is a cylinder leak test. get the piston to TDC on the compression stroke and plug compressed air to the cylinder.  You will be able to hear the air blowing out of the exhaust,carb,crankcase breather or bubble out of the radiator. This will narrow your search and only takes a couple of minutes to do. considering the push rod, you probably have and stuck/bent exhaust valve.
By Hoosier Hurricane - 16 Years Ago
It still should run faster than 20 mph on 7 cylinders, so you have more than one problem.
By speedpro56 - 16 Years Ago
Johns right on that one. My 56 tbird 312 + broke an exhaust valve spring on take off going down the 1/4 drag strip and still pulled a 13.92 on 7 cylinders. Sounds like timing, carburation or  ignition not working. I'd start with setting the timing on 12 degrees at idle for starters and hoping the balancer has not slipped and showing incorrect timing. A bad carburetor can cause it to run badly as well and make sure there's no stopped up fuel filters. 
By BIGREDTODD - 16 Years Ago
In my opinion, and as others have noted, you very likely have bent a valve to have 0 lbs. compression...I had a similar situation on the 312 in my 'Bird...I had 130-140 lbs. except for the #2 cylinder which had 80 lbs.

I did a leak-down and found that I had major airflow back through the intake port (I had removed the intake manifold previously). After Tim (mctim64) pulled the heads down he found a crack on the #2 chamber...

I would definitely do a leak-down, quick & easy way to find out where the air is going...should be really easy if you aren't mmaking any pressure.

You definitely have something else going on if it won't run on 7...I suspect fuel...as mine ran decent with ignition that was completely screwed...

Good luck, let us know what you find...

BIGREDTODD