What is your Method to do compression test?

[Guest btate]

Again I am not a true mechanic and I rely on you guys in the forum plus doing a lot of goggle. I will list what I think is the proper method of compression testing but I stand to be corrected. 1st --remove all plugs, 2nd connect a charger to battery to keep starter rpm's consistence, throttle valve wide open and since my 55' buick starter is in the gas pedal and to start must be pushed wide open, that opens the throttle. 3rd screw compression line in place of plug and being sure it is all the way in, 4th depress the gas pedal until it makes 5 to 6 rotations and proceed thru the other 7 plugs. 5 th remove the coil wire to prevent the engine from starting. 6th should I disconnect the fuel line because each time I test a cylinder I am washing the oil off the cylinder wall with fuel. I should have made this 1 st----warm engine to normal operating temp. If I get a low reading such as 90psi and should be 155 psi. After all cylinders are checked come back to the low cylinders and add a tablespoon of 30 wt oil and hit starter for two rotations to proper distribute the oil. Now screw in the compression gauge for 5 or 6 rotations. If I get something like a 45 psi rise in pressure, then I can conclude air is leaking past the rings. If I only get 5psi then it would be a valve. Not sure if that would be a intake or exhaust valve or could be both. Should I somehow disconnect the gas line, because each time I depress the gas pedal it squirts gas into the cylinders, washing" oil" off the walls of the cylinders. Am I leaving anything out or doing anything wrong. I am on vacation but will be back to do the test Monday. This is a new re build and I am anxious.

[Aaron65]

It all looks good, but I would avoid using the throttle to operate the starter. First, my starter switch activates with just a slight application of throttle, but if I were you, I may disconnect the wires going to the carb switch, and just touch them together to activate the starter as you do the compression test. That way, you aren't squirting any fuel into the cylinders from the accelerator pump. You can probably skip removing the coil wire, because the engine's not going to start without plugs, but it certainly won't hurt anything to ground the coil wire to the engine. Doing the test with a warm engine is probably more accurate, but a cold engine will show any low cylinders. Just make sure the battery is charged and you probably don't need to keep the charger connected during the test, because there's less of a load on it when there aren't any spark plugs in. Good luck! Write down the results.

[Guest NikeAjax]

Been a while since I did this, but, a really swell trick is to get a push-button switch, plunger-type, like you'd use for a starter and connect it to the posts on the carburetor. Have some robust wires too, 'bout 14-gauge, and wrap some tape on the end of the switch where said wires connect; you don't need to know amps that are that high from personal experience/exposure... What this is going to give you is a "remote starter", meaning you don't have to get in-and-out all the time: holding the button down is going to activate the starter, and once you let go,the power is cut... Does this make sense to you, I hope? An old timer taught me this one, he had a garage in the 1940's...

Jaybird

[JohnD1956]

I would not disconnect the fuel line. Even if you did the carb has a bowl full of gas to start with. So each plunge of the gas pedal will still send raw fuel into the intake manifold. If you disconnect the line do it before the fuel pump, or else you'll need a something to catch any fuel pumped from the open fitting.

As for disconnecting the coil I would do so at the positive wire connection. Do not just remove the center coil wire.

[60FlatTop]

Fuel is not much of a problem when compression testing. The small amount will usually pool in the manifold and evaporate while you work.

You can cause damage to the coil by pulling off plug wires or unplugging the coil wire. I usually put an alligator clip on the end of the coil wire and ground it to the engine. This reduces the resistance from infinity to zero and doesn't tax the coil capacity. Its the old E=IR thing again. You don't need that rare occasion when a coil might fail when you are testing some other problem.

I like the screw in hoses with a quick disconnect attachment for the tester.

Bernie

[buick5563]

I don't remember exactly the steps that I took the last time I checked compression on my 55, but I believe I just followed the Shop Manual directions.

I know that I disconnected the coil wire at the distributor so that the spark would not jump across the engine and spark anything unsavory.

I always use a trigger activated remote starter switch (Sears) and a screw in compression gauge (also Craftsperson ).

I really don't think there is a problem with your engine from your other posts, Bill.

Your brakes are not working correctly (catching on fire generally is your first clue), thus causing the engine to work harder against applied brake shoes, and overheating your engine.

Compression checks, while good, are not what I would be doing first...

[old-tank]

The only thing I would add is if you disconnect wires, tape the ends before turning on the ignition. The (+) wire to the coil and the wires to the carb switch are not fuse protected and if grounded will fry your wiring harness instantly! I agree at this point, ignore vacuum and compression and drive it when you can stop it.

Willie

[carbking]

I would add that you are looking for these items of data before adding oil:

(1) the compression reading on the FIRST revolution (should be 90 percent of the final reading for that cylinder)

(2) the compression reading after 5 or 6 revolutions

(3) the average compression (add the compression readings from all cylinders and divide by the number of cylinders)

(4) the deviation of any cylinder from the average (should not exceed 10 percent of the average)

Jon.

[Guest btate]

Jon, I had read the first revolution should be about 45 psi and 35psi thereafter per revolution. You are saying 90

[Guest btate]

Bernie, are you grounding the wire that goes to the distributor or the one of the small wires?

[carbking]

Jon, I had read the first revolution should be about 45 psi and 35psi thereafter per revolution. You are saying 90

Not raw 90; 90 percent of total on the first revolution. If total were 125 then 90 percent of 125 would be about 112.

Jon.

[Guest btate]

I meant to say 90 per cent

[60FlatTop]

I ground the coil wire. Just a jumper wire with two alligator clips will do it. Then you don't have to disconnect the primary wire at the distributor.

Bernie

[Guest btate]

Bernie, I hate to be so dumb but which is the coil wire and which is the primary wire?

[Guest NikeAjax]

That would be the negative (-) wire: you can put a "kill switch" in your car by running a toggle-switch, hidden, of course, to same post on the coil, and just attaching the other wire to anywhere on the chassis. I have one on mine: it's a really "cute trick" for some chimp who's wanting your car worse than you! What this does is ground the coil: you can try all you want to start the car, an' if that thing's grounded--NOTHING WILL MAKE IT START! This is a good feature when you have to leave your car unattended--SNORT!

Jaybird