Compression check

FergySGM

Member
Conducted a compression check on my 55 TO 35. Dry test
showed 30 lbs psi and wet 90 lbs psi. I believe it should be
around 120 to 130. Any suggestions on what I should do
next.
 

Did you disconnect the air filter , open the throttle and make sure the battery was fully charged ? All these will lead to false or lower readings if not done .
 
Yes, the only thing I didn't do was the air filter. I took the bottom off the oil reserve on the air filter. Should I disconnect the air filter at the carb or where it meets the canister?
 

What you have done is probably enough unless the air filter is really bad . Won't take much to disconnect it completely, right at the carburettor , just to make sure . Look at the choke plate while you are there and make sure it is wide open .
The reason I question the air intake is that your readings are so low it is a wonder the engine runs at all . Next thing to do is suspect the compression tester of malfunctioning .
 
The funny thing is that it runs great. I believe the carb may need some adjustments. The reason I started all this was that I have gas in my oil. It is very thin and has a gas smell.
 
The gas in the oil can cause low compression readings. The diluted oil will cause the cylinder walls to be dry, rings need some oil to seal.

The gas got in the oil from running rich, or the carb is flooding. If the fuel valve was left on, gas can fill all the way up into the intake and into the engine.

Once the fuel problems are solved, the oil is changed, and the engine has been run, then recheck the compression. Chances are it will be good. If it idles good, it has good compression.

One other thing that will help keep gas out of the oil is the correct, working thermostat.
 
Thanks good info. I appreciate the feedback. I will do what you have suggested. I put a new thermostat that I ordered from YT. Temp stay in normal range.
 
Going to change oil and filter tomorrow. I'll check it again after that. Going to use same oil as you suggested.
 
I"d rerun the test if I were you. 30 psi is like a stuck open valve reading. Even 90 psi wet is marginal. That Big of a change from dry to wet indicates severely worn rings. What were all four readings, dry and wet?
 
Yea worry about the carb before you fret about the valves. I rebuilt the standard engine in my TEA 20, and it read even less compression then when I bought it... didn't make me feel too smart... it started anyway- instantly and loud- and been starting and running with as much power as any other fergie its size, and still the compression gauge reads zero.
In my experience, too low a compression will cause the intake manifold to set the carb on fire... then you know you have low compression...
 

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