Poor Compression and leakdown in #3 cynlinder

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sstcobra
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Poor Compression and leakdown in #3 cynlinder

Post by sstcobra » Sun Feb 25, 2018 7:30 pm

Need some advice please. My M422 has not been running well at low rpm's. Does not want to idle and has a miss. At higher rpm's runs pretty good. Did lots of troubleshooting to the carb, points, wires, distributor cap, rotor, changed spark plugs, looking for a vacuum leak and verified timing - all without any improvement. Finally did a compression test and leak down test this weekend. All cylinders except #3 have 118 - 124 PSI compression and less than 5 % leak down. #3 cylinder was only 90 PSI and 25 % leak down. I wanted to try to torque the head before doing an engine teardown. Has any tried torqueing the cylinder head on an older engine and can you offer any advice? Should I try this with the engine warm or cold? Any hope this may fix the problem?

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Radar4xfour
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Re: Poor Compression and leakdown in #3 cynlinder

Post by Radar4xfour » Wed Feb 28, 2018 6:59 pm

SSTCOBRA,

Was the condenser replaced? If not a bad one can exhibit some of the poor running conditions you described.

Checking the torque on the cylinder head should be done cold. The manual state a 3 stage torque sequence at 10, 15 & 18 ft. lbs. A torque wrench measuring in in/lbs. is the most accurate wrench to use. A 0-100 ft.lb. torque wrench is not the tool to use for this job due to its lack of accuracy at the low end of the scale.

Hope this helps a little.

Radar

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Re: Poor Compression and leakdown in #3 cynlinder

Post by sstcobra » Wed Feb 28, 2018 7:28 pm

Radar,

Thanks for the response. I did have a chance to torque the cylinder head as you suggest today when I was down to the garage. There were several loose nuts and I did have a 0 - 200 inch lb wrench for the job. And the engine runs much smoother! Not perfect - but noticeably better. Several nuts took 1/2 - 3/4 turn to hit the right torque. But now I can go back and play with the timing and carb adjustment (and I will try to replace the condenser) and see if I can get this running as smooth as I think it should.

Another question....at the slower idle I now hear a pretty frequent backfire in the exhaust system. Nothing hear in the intake. Do you think could also be a collapsed lifter or maybe the condenser or ???

I drove the M422 for a few minutes and did not hear the backfire at the higher rpm's.

Gary

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Radar4xfour
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Re: Poor Compression and leakdown in #3 cylinder

Post by Radar4xfour » Thu Mar 01, 2018 11:18 am

SSTCOBRA,

Now I know you are a good Mighty Mite owner / mechanic because you own an in/lb. torque wrench, way to go!

The backfire could be a result of valve(s) not seating or incorrect timing. I would focus on the condenser and the timing to see if that improves things. Use a vacuum gauge for setting the timing and not the timing mark. I'm convinced few Mighty Mite owners get their vehicles to run nicely using the timing marks.

Radar,

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Re: Poor Compression and leakdown in #3 cynlinder

Post by Marty, SoCal » Mon Mar 05, 2018 8:22 pm

If you put a little oil in the cylinder does the compression come up?

When doing the leakdown test, did you hear air escaping from the intake (intake valve sealing), exhaust (Exhaust valve sealing) or crankcase (Rings and/or cylinder chrome plating)?

Pretty common for these engines to have the chrome peel off the cylinder walls causing a loss of compression and oil burning
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