compression test

[hamsterhats]

Can someone give me a quick lesson on how to check compression? I have a gauge, just not sure how to properly do it.

 

[1wild&crazyguy]

Here's how I do it.

Get a remote start switch/or screwdriver arced off the start switch on fender and with the carb return spring removed pin the carburetor open. Then crank the motor over, watching the gauge only crank it over about 4-5 revolutions.

 

[idrift]

You should also remove all the plugs before doing it to get a true value.

 

[hamsterhats]

doing it after dinner. what do i have to do with the vac advance on the distributor

 

[72bluNblu]

First, warm up the engine for about 5-10 minutes. This gets the oil hot and seals the rings as they should be.

Now that everything's hot :-D (do'h!), remove the all the spark plugs. This will let the engine spin over faster, save the battery and starter, and give you more accurate numbers.

Now set the throttle wide open, either by wiring open the throttle lever on the carb or by the proverbial "brick" on the gas pedal. The throttle should be wide open so the engine can pull all the air it needs, but you don't want to be repeatedly pushing the pedal to WOT every time you test a cylinder as you'll pump the engine full of gasoline that way, which will wash down the rings and give you crappy numbers as well as no oil in the cylinders (not good!).

Now screw in the gauge and turn the engine over 3-5 revolutions, this should get you to the max compression on the gauge. The needle should go up a ton on the first couple revolutions, then top out with the last couple. Try to do the same number of revolutions for all the cylinders.

Record your findings and move around to all the other cylinders. If you're quick about it the engine should stay warm and the #'s will stay consitent unless there's a problem. Ideally, all the cylinders should be within about 10% of each other.

 

[72bluNblu]

what do i have to do with the vac advance on the distributor

Nothing. I suppose if you wanted to maximize vacuum in the carb you could cap all the outgoing vacuum ports, but that really shouldn't make a big difference at cranking speeds.

 

[hamsterhats]

doing the test now

 

[hamsterhats]

all cylinders except #3 were at 120, #3 was at 115. Is this ok to start putting money into it? Plugs were nasty, as were the wires, changed them out. Couldn't start it, didn't want to anger the neighbors

 

[STR-14]

An average of 120 pounds is pretty decent. Five pounds in one cylinder is nothing to worry about at all. Spend away.
Daryl

 

[SGBARRACUDA]

As a rule to tell the seal of the rings and condition of the cylinders they should attain 80 % of there max on the first compression stroke and max out by the third.

 

[72bluNblu]

all cylinders except #3 were at 120, #3 was at 115. Is this ok to start putting money into it? Plugs were nasty, as were the wires, changed them out. Couldn't start it, didn't want to anger the neighbors

Seems pretty decent to me. 5 psi isn't anything to worry about, the rest of them being that equal is a good thing.

 

[hamsterhats]

318 or 360, that is the question! Will it be hard to find heads intake etc...