[Ooops!] thanks to TimAT for the 'head's up' on my goof 
compression test question
Thread Starter
Instructor
Joined: Dec 2007
Posts: 245
Likes: 2
From: Jersey shore NJ
compression test question
I'm about to do a compression check on my motor and I had a question. ( I tried to search but did not see the answer).
When cranking the motor you should wire the butterflies open on the carb.
How do you keep gas from pumping into the intake?
Do you disconnect the fuel line and pump the fuel into a container?
Thanks for the replies
When cranking the motor you should wire the butterflies open on the carb.
How do you keep gas from pumping into the intake?
Do you disconnect the fuel line and pump the fuel into a container?
Thanks for the replies
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
You should remove the positive cable from the battery [or turn OFF your battery disconnect] and {this is overkill} pull the central distributor wire from the cap. As you will be wiring the throttle plates open, you will only get the first shot of fuel from the accelerator pump but nothing else during the testing (no vacuum is created with throttle plates open and engine not running, so no fuel is used).
Le Mans Master
Joined: Dec 2007
Posts: 6,422
Likes: 591
From: McHenry Illinois
There are basicly 2 things that make your carb deliver fuel. 1, you move the accelerator to deliver a squirt which subsides as you stop the accelerator motion. 2, Air velocity through the carb. There will be insignificant air velocity through the carb because the spark plugs will be out therefore the cylinders will suck air through that hole rather than from the intake. The fuel that the fuel pump supplies will serve to fill the carb till the float level is correct, then the needle and seat valve will stop the fuel flow from the pump. With a HEI ign., the easiest way to stop the spark is to unplug the heavy gauge red wire at the distributor, that way your plug wires won't be sparking.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
When you do the compression test, look at some different things- it will help with your evaluation later. 1st, what is the compression on the first stroke? 2nd, what is the final compression at 5 or 6 strokes (but try to do them all the same). 3rd, what is the difference between the high and low cylinders? Should be no more than 10%. 4th, do any adjacent cylinders have EXACTLY the same readings.
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