Ran across this years ago and it has served me well
The volumetric efficiency test or running compression test
is a very good test to use any time you have a engine miss
that can't be traced to ignition or other basic systems or
when there is a lack of power.
Step 1. Perform a cranking compression test with all plugs
out and the throttle open.
Step 2. Record your cranking compression readings. Make
sure the charging system was up to snuff so all cylinders
were tested under the same conditions.
Step 3. Install all plugs but one. Install a standard
compression gauge in the remaining hole with the shrader
valve in place.
Step 4. Start the engine. Yes, it will run but will be
missing on the cylinder with the compression gauge. Burp
the gauge and allow the reading to stabilize. It will be
bouncing around at about 50 to 60 psi. (the throttle is
causing a restriction and the piston is moving fast
compared to cranking speed causing only a partial fill of
the combustion chamber).
Step 5. Snap the throttle wide open and return to idle. The
gauge will hold at the peak reading. This reading is higher
than idle because the peak comes at the instant the
throttle restriction is removed and the piston speed is
relatively slow.
Step 6. Record your readings for running snap compression
for all cylinders. The running compression reading should
be approx. 80% of cranking compression. (example: cranking
compression = 150psi running compression should = about
120psi)
If your running reading is below 80% of cranking
compression, the intake system is causing a restriction. If
the running reading is above 80% of cranking compression,
the exhaust system is causing a restriction. If the problem
is in one cylinder you either have a problem with a worn
camshaft, broken spring/rocker, carbon build up, etc. If
the problem is on all cylinders look for a restricted
intake system or clogged cat/muffler. The 80% spec is a
ballpark spec and should be used as such. Remember that
cranking compression is a measure of cylinder seal while
running compression is a measure of volumetric efficiency
or the cylinders ability to draw the air/fuel mixture into
the combustion chamber.
This is the readings from a Flathead ford V8
cranking
Test Numbers, first hit/ fifth hit on gauge
1) 80/120
2) 50/125
3) 75/120
4) 75/110
5) 75/115
6) 65/100
7) 85/125
8) 75/120
Run'n
1st number running compression 2nd # blip throttle
1) 50/115
2) 50/115
2) 50/100
4) 50/100
5) 50/100
6) 50/100
7) 50-115
8) 50/115
No need to go farther with any cylinder efficient test,,, problem was (dual carb set up) one barrel was stopped up.. I would have bet money it was a cam are valve issue before the test...
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Today's Featured Article - Third Brush Generators - by Chris Pratt. While I love straightening sheet metal, cleaning, and painting old tractors, I use every excuse to avoid working on the on the electrics. I find the whole process sheer mystery. I have picked up and attempted to read every auto and farm electrics book with no improvement in the situation. They all seem to start with a chapter entitled "Theory of Electricity". After a few paragraphs I usually close the book and go back to banging out dents. A good friend and I were recently discussing our tractor electrical systems when he stated "I figure it all comes back to applying Ohms Law". At this point
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