Compression testing for n00bs

[jogirob]

I previously came across a thread that had some good pointers but I can't find it anymore. I was going to revive it with one of my n00b questions but I guess it's best to start a new thread anyways. There are plenty of resources online but I like the custom tailored to this car help I get from this forum

So what I've gathered so far:
-Run the compression test with the carb wide open.(*)
-Warm up the engine first
-While cranking wait for 3-4 gallops to complete your readings
-Do it with all spark plugs out, and wires disconnected (*)
-Pinch off the fuel line (*)
-While it's good to have 140-150 psi range it's more important to have continuity among all cylinders.
-Do a second run with a teaspoon of oil in the cylinder. If compression increases then the loss is in the piston ring seals.

* Questions:
-Any special way to open the carb butterfly valves while bypassing fuel?
-Do I need to worry about restricting the fuel flow into the engine that uses a mechanical fuel pump? If so where's the best place to pinch the fuel line at? I read that steeping on the gas pedal all the way to the floor cuts off fuel but am not sure if that's only for electronic fuel injected vehicles.
-Do I just let the plug wires chill in the engine bay while the engine cranks? Do I need to keep em separated from each other? Should I keep em away from metal by wrapping em up somehow?
-I'd be by my own so I don't know if cranking from the drivers seat will work for me. I learned I can activate the starter by shorting the starter relay. So pretty much I'd be trying to hold the carb open while shorting the relay...good idea or bad? I know these pressure gauges hold onto the pressure readings momentarily so maybe I'm just over thinking it.

That's all for now. Thanks!

 

[Don Frelier]

You've got everything correct.
1. For throttle you can either put a weight on the pedal or pull back the throttle under the hood and wire it back or insert something to keep the blades (and choke plate open)
2. As long as you just open the throttle once you really don't have to pinch off the fuel line. With all the plugs out there will not be any vacuum
so it won't suck in a bunch of fuel. But it certainly would not hurt. Just put small vice grips on a piece of rubber line wrap the line with a piece of cloth first. The pump is a diaphragm and only makes about 6 psi so it does not take much to stop it on either side, inlet or outlet.
3. Just remove the coil wire from the coil.
4. Yes most testers hold the highest pressure, but I like to activate from inside the engine compartment you can control things better.
Just take your time, it's not real difficult.

 

[oldbee]

Was told yrs ago that 2nd jump on gauge kinda shows valve condition & then 4th jump is rings. If everything is pretty even, then no need for adding oil. Big screwdriver down thru carb to hold throttle.

 

[65-440]

I've done conpression test both ways, throttle open and throttle left alone and the numbers were the same ... I typically don't hold the throttle open .. but definitely take coil wire off.

 

[MoparLeo]

Safest is to get a remote starter switch. Very in expensive. Connect to starter relay. Always have a fire extinguisher handy when working under the hood. Always best to have a partner when possible. You only have 2 hands. one set of eyes. Remember that things will happen. You are correct that in everything that you do or test. Consistency is the key.

 

[khryslerkid]

I've done conpression test both ways, throttle open and throttle left alone and the numbers were the same ... I typically don't hold the throttle open .. but definitely take coil wire off.

And I've seen the difference with closed verses open.

 

[BSB67]

I've done conpression test both ways, throttle open and throttle left alone and the numbers were the same ... I typically don't hold the throttle open .. but definitely take coil wire off.

Same. Don't really see a big difference between hot and cold either. But with that said, I still try to do it with a warm engine, and propping the carb open.

 

[coloradodave]

No need to pinch the fuel line. As said, wire the throttle open. Needle and seat in the carburetor will prevent more fuel from entering the engine while cranking. I start the engine and let it run for a couple of minutes before pulling the plugs and coil wire, then use a remote start switch and crank for 5 revolutions on each cylinder. Ideally, you won't have more than a 10% variation between them.

 

[vintage chromoly]

I do mine cold. Not a fan of burning myself, installing and removing the tester.
I’m looking for a large variance between holes, or one hole way out of wack.

 

[Kern Dog]

You also need to keep the battery voltage consistent. If it drops from testing the first cylinder to the last, your starter will spin slower and your readings will differ as a result. Maybe use a battery charger attached or jumper cables to a second charged battery.

 

[coloradodave]

I do mine cold. Not a fan of burning myself, installing and removing the tester.
I’m looking for a large variance between holes, or one hole way out of wack.

I'm with you. I didn't mean to imply that you get the engine that hot...just put some heat in it.

 

[Kern Dog]

If I am dealing with a running engine, I do the test on a warm engine. All the clearances tighten up a bit and the numbers are often higher. Small block cars are the easiest with the clear access to the plugs. I have 2 inch headers in my 440/493 so working around glowing header tubes is a real pisser! I let it cool down to where the headers can be touched without losing skin in the process.