[1bad69]

I just did a running compression test on 2 cylinders. they were completely different.

If you have been following my posts, I have been having an issue with #2 plug fouling all of the time. I've tried 2 ignition systems, reset valves, all new plugs, all new wires, new cap and rotor on both systems, leakdown test, etc.

So I decided to do a running compression test.

#2 cylinder reads pretty close to the cranking compression at 175lbs then when you hold down the guage, it goes down to 60-70 lbs

#1 cylinder reads 120-130 lbs, and when you hold it down, it reads 30 lbs pretty consistently.

Can anyone tell me what these readings mean?

 

[RB69SS396Conv]

What about the other 6?

(although, what you've got so far, doesn't sound too encouraging)

You did block the throttle wide open, use a GOOD HOT battery, take out the other 7 plugs besides the one you were testing, and let the cyl under test pump up until the gauge quit increasing, right?

 

[Tom Mobley]

What is a running compression test anyway? What does that mean "hold down the gauge?"

Run a regular compression test as noted by RB69. If no major discrepancies noted, run a leakdown test.

If compression and leakdown tests are OK get ready to pull it and inspect the rings for upside-down rings on those cylinders. While the heads are off you can pull the valve springs and check for any damaged guides and bad/missing seals. This would also be a great time to inspect the manifold gaskets for evidence that they're being compressed properly along the bottom edge. Check the thickness with a dial caliper or micrometer.

 

[1bad69]

What is a running compression test anyway? What does that mean "hold down the gauge?"

Run a regular compression test as noted by RB69. If no major discrepancies noted, run a leakdown test.

If compression and leakdown tests are OK get ready to pull it and inspect the rings for upside-down rings on those cylinders. While the heads are off you can pull the valve springs and check for any damaged guides and bad/missing seals. This would also be a great time to inspect the manifold gaskets for evidence that they're being compressed properly along the bottom edge. Check the thickness with a dial caliper or micrometer.

I have done all of the other tests... that is why I am at my wits end, so I am trying anything... all of the cranking compression was around 175-180 lbs each cylinder. all leak down tests were within 10 percent leak down. It keeps fuel fouling #2. that is why i'm baffled. I have done some research on doing the running compression test at http://www.theautomotiveengineer.com/2013/04/engine-running-compression-test.html

I just don't know what it means when I have so much running compression on one cylinder and not the other. It runs absolutely beautiful when it runs good with new plugs, but then it fouls it, and runs miserably. I'm baffled. So are all of my car buddies/mechanics...

I have also installed all new valve springs from Straub and all of the seals were new a few years ago, but we did check them, and they were all good. if that was the case, I would think it would be oil fouled instead of fuel fouled, but I don't know that is why I am coming here to get some advice. I need to know if it is in the heads, cam or bottom end...:beers:

 

[Blownchevelle68]

Did you pull intake yet to see if its leaking past the intake gasket from valley?


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[1bad69]

Did you pull intake yet to see if its leaking past the intake gasket from valley?


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Hi Jason... no I haven't... im trying to eliminate every possibility before I do that. But that is in the plans if we can't figure this out...

 

[sschevellefan]

without knowing what the other 6 cylinders read, a 40-50psi difference generally is not good. Could be bad rings, damaged cylinder wall or damaged piston or all of the above. Personally I`d check the rest of them and then prepare for a tear down to insoect the low cylinder. I know you said the leak down was 10% from the others but I would still look at the cylinder head. Maybe the valves aren`t seating right or they are hung up or bent slightly. As others have said, inspect all gaskets upon removal for signs of failure.

 

[Blownchevelle68]

Hi Jason... no I haven't... im trying to eliminate every possibility before I do that. But that is in the plans if we can't figure this out...

I can't remember all that you have tried-But...

Have you tried a different carb yet?

Tried a new plug wire just on that
cylinder?

New cap - rotor?

With everything else that you've checked, it still makes me think you've got something up with that intake gasket or valve guides for that cylinder.

If your leakdown test shows 10% or less than the cylinder should be ok unless you've got a ring upside down like Tom mentioned. (By the way-if that was the case it would have ran like this since day 1...has it or is this something that occurred recently?)



Post some pics of the fouled plug if you can.




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[blue66]

Running compression test are done when other test show normal. THey can find sticky valves, slightly burnt valves, broken valve springs etc. Sometimes a broken spring will not show up at cranking speed. Have done this several times myself. Do you have the running compression #'s correct for #1 and #2? The one fouling is giving you higher running compression?

Are you sure it's fuel fouling? Could be a bad intake valve seal/guide sucking oil fouling the plug, then after oil fouled it becomes fuel saturated??

 

[putput]

Your results seem odd. Running compression test should result in approx 1/2 of your cranking compression, and for #2 to be that high is either a testing error, or a valve problem. When you have the compression tester installed, engine running at idle, depress the shroreder valve to empty guage and then release it again. in a couple of seconds you will have your running compression. If your normal cranking compression is 180lbs, running should be around 90lbs roughly. If you have a cylinder really high, it would point to valves not opening correctly, not bleeding off cylinder pressure.
If for instance you have a cam going flat, intake lobe half worn out, the valve instead of closing 50' BTDC, may be closing 5' BTDC, allowing more swept volume during running test.

 

[1bad69]

I can't remember all that you have tried-But...

Have you tried a different carb yet?

Tried a new plug wire just on that
cylinder?

New cap - rotor?

With everything else that you've checked, it still makes me think you've got something up with that intake gasket or valve guides for that cylinder.

If your leakdown test shows 10% or less than the cylinder should be ok unless you've got a ring upside down like Tom mentioned. (By the way-if that was the case it would have ran like this since day 1...has it or is this something that occurred recently?)



Post some pics of the fouled plug if you can.




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I changed both cap and rotor on both ignitions and it has always acted this way.. This new (barely used) Mallory ignition system is new with this motor, so that is why I thought it was the ignition... but it acts the same way with HEI. I'll post a pic of the plug. If it is the heads or intake gasket it will be no big deal, but I am going to pull the intake today. I will take photos. If it is the heads, I will just get a new set. There is no way I am putting more money into these 781's. I have over $1200 in them now. I'll just order a new set of aluminums and call it a day.. I don't care what it is, I want it fixed. If I have to pull the motor to fix it... it will get an LS1 then I will add a procharger later.

 

[1bad69]

Your results seem odd. Running compression test should result in approx 1/2 of your cranking compression, and for #2 to be that high is either a testing error, or a valve problem. When you have the compression tester installed, engine running at idle, depress the shroreder valve to empty guage and then release it again. in a couple of seconds you will have your running compression. If your normal cranking compression is 180lbs, running should be around 90lbs roughly. If you have a cylinder really high, it would point to valves not opening correctly, not bleeding off cylinder pressure.
If for instance you have a cam going flat, intake lobe half worn out, the valve instead of closing 50' BTDC, may be closing 5' BTDC, allowing more swept volume during running test.

That is how I tested it... But if I had a cam going flat, which would be okay... i'm ready for a Straub magic stick anyway or back to a solid roller... would it fuel foul a plug every time? It runs absolutely great when it runs well. With a new plug and my Mallory ignition system. you can stomp it anywhere from 0-40 mph and it will light up the tires and pull very hard past 6000 rpm. from a dead stop, forget it, rubber all over the quarters.

 

[1bad69]

Do you have the running compression #'s correct for #1 and #2? The one fouling is giving you higher running compression?

#2 cylinder reads pretty close to the cranking compression at 175lbs then when you hold down the guage, it goes down to 60-70 lbs

#1 cylinder reads 120-130 lbs, and when you hold it down, it reads 30 lbs pretty consistently

Yes. #2 is the one that keeps fouling. I have been reading about this online and it says it should not be that high in compression while running, but it doesn't really say what could be wrong..