The Stovebolt Site Forums The Shop Area Engine Shop compresssion test procedure

Just getting ready to do my first ever 56 Cameo 235 compression test with my handy dandy Craftsman compresssion tester Christmas gift. I'm reading the shop manual and it says to open the throttle first (no problem I have a hand throttle) but my question is -- why?? Doesn't that fill the cylinders with gas they don't need. Just a question - I'll do it - I'm learning, so dumb questions come easy to me.

Thanks, Dale

Having the throttle open and the spark plugs out, puts less load on the starter, cables and battery and, as the engine spins faster, the cylinders fill faster, and the test is faster too.

There are no dumb questions, and good work using the shop manual! We all started wondering the same stuff, I know I did!

Thanks Mr. Lang - That helps a lot I suspected those kinds of reasons, but -- you just don't always know exactly why the book says what it says do you? (at least I don't). It is meaningful (for me at least) to understand why as you go. I appreciate your experience and insight.

Thanks again, Dale

If you have the throttle wide open you can get air into the cylinders to get a true compression reading.

When you do your test, count the compression strokes as the engine spins. Watch the gauge and when it quits going up, thats the end of the test. Usually 5 or 6 compression strokes will fill the cylinders, now do all 6 just the same. You are looking for differences, the actual numbers don't mean all that much, its the difference from one to the other. What ever you do to one do to all six.

Joe

I remember seeing somewhere that its a good idea to clamp off the fuel line between the pump and carb to cut down on any fuel flowing into the cylinders during the test. I think it was to help prevent "cylinder burn." Has anyone hear of/done this??

...Larry, are you talking about when gas washes a cylinder dry?

If so, yes I've seen it. Back in 65 while working at the dealership I seen a rookie (I was to) keep patting the gas on an engine that he had already flooded, and the engine actually locked up and wouldn't turn. (A new car)
The top mechanics finally got it going but I did see it happen.

i would agree that u want to cut off the fuel supply while doing the test but would be looking at how much compression it has as well as how much differance there is between the clys.

It won't get much fuel in being wide open, I think it would be more trouble then its worth. You might as well pull the carb off the manifold since the engine will pull fuel out of the float bowl long after you unhook the fuel line. When I break in engines, I always shut off the electric fuel pump and let the engine run the lines dry, the last one was a 428 Pontiac with a Q-jet. It could run for 4 or 5 minutes at a fast idle before the engine started stalling!

Joe

If the plugs are out of the engine, not too much fuel is going to get sucked in. The only cylinder that will really have much vacuum on the intake would be the one with the gauge connected.

Ok. Got it.

Now, in the "no stupid question" category, do you pull all the spark plugs out before you start? I always did one at a time so I didn't accidently cross up the plug wires.

I plan on getting a compression tester from Harbor Freight this weekend and want to do the test right just like Dale.

I always pull all the plugs. I figure it's easier on the starter motor. I dunno if the manual says to or not.

Just use some masking tape to mark the wires !,2,3... etc. Then you won't get confused. That's what I always do!