compression testing two cylinder JD

WORNOUT

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a little confused re compression testing a two cylinder JD engine .Have been advised by an experienced JD mechanic that taking the engine compression does not indicate the actual cylinder compression he claims this is because of the valve firing sequence, could someone expand on this theory
 
(quoted from post at 21:30:53 04/03/18) a little confused re compression testing a two cylinder JD engine .Have been advised by an experienced JD mechanic that taking the engine compression does not indicate the actual cylinder compression he claims this is because of the valve firing sequence, could someone expand on this theory

"valve firing sequence", eh?

Call out your DEERE "experienced mechanic" buddy for "mansplaining"!
 
What theory? The valve firing theory? I was not aware there was any such thing. Making up words is something children do often because they need too. This is almost inexcusable abuse of my mind while I ponder on what in world could he have meant by a statement like that. Unless it's taken out of context or with your insertion of words into his words.

It finally made sense. No, he is doing the compression test in the wrong manner. You remove both spark plugs so #1 doesn't slow down #2 for an accurate reading of #2. You also open the throttle plate so air can flow freely thru the engine.
 
The compression would be measured the same on any gas engine. He is trying to BS you. The compression would be at its peak right at TOP dead center on the compression stroke. Valve timing would have nothing to do with this. Unless some thing was seriously wrong. Both the intake and exhaust valve are closed on the majority of the compression stoke.
 
the next thing he'll be saying is that your combubulator is fouled. Get a mechanic with "less" education. Less is "more" when working on real tractors.
 
He is correct, actual compression ratio is a combination of the amount of air that can be sucked in due to valve timing and the compression ratio due to differences in volume. Are you looking for a good bad number, or something more?
 
So, to reiterate, two engines with identical pistons and combustion chambers will have different readings due to cam differences. This generally shows up at higher RPMS's, but can show up in a compression test.
 
Perhaps we're overcomplicating things. As has been advised, do your test with the spark plug out of the opposite cylinder, and the throttle wide open. Easy. Don't worry about valve timing or camshaft action- that's for high compression, high speed engines. You will end up with a pretty good educated guess about the condition of your cylinders/pistons. If you still have concerns after a compression test, do a leak-down test. This is another SIMPLE test requiring a $30 gadget, and some compressed air, and will give you more information about your cylinders and valves. Both tests are easy to do. Most important, have fun. unc
 

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