compression test

[robert bowen]

i have a ford 9n.today i did a compression test cyl.1 is 80 cyl.2 is 65 cyl.3 is 80 cyl.4 is 70.when i give it gas it's skipping and popping.is this due to low compression readings.any suggestions?

 

[souNdguy]

popping and skipping is RARElY specifically due to low compression.

leanness will cause popcorn.. skipping can be ignition or valves.

( valve can be a cause of low compression )

your numbers do not mention wet / dry.. you really need a wet and dry test to tell you something. single set bythemselves is not very usefull.

soundguy

 

[Royse]

SoundGuy gave you good advise here.
Check the fuel flow.
To do a wet test, dump a tablespoon or two of oil in each cylinder and see if the compression readings change.
Those numbers seem low to me anyway, but mine is running fine with lower numbers.
I assume Dell or Bruce(VA) will chime in soon to explain how to check the quality of your spark.
Since they've been helping me, that seems to be the #1 culprit.
You can also do a search in the archives for similar problems if you're in a hurry.

 

[Jerry/MT]

When you run a compression test the engine should be at operating temperature, the throttle and choke plate should be wide open and the air cleaner should be disconnected. Remove the spark plugs to make it easy on the starter. Run the test dry and then add several ounces of lube oil to the cylender and repeat, recording both numbers.

You should also adjust the reading for the barometric pressure. This is important if you are at higher elevation or the barometer is significantly different from 29.92 in Hg. the adjustment factor is pamb(inHg)/29.92 inHg.
So if you are at 3000 ft and the barometer is
26.82 inHg, 26.82/29.92 = 0.8963. If you compression spec says 125 psig new, 90 psig minimum, these number should be 112 psig and 80.7 psig.
you can get the local barometric pressure off of a home barometer or by calling the local airport or from the National Weather Service website for your area.