Poor Compression

[rotorwrench]

In aviation we use a differential compression test set. The E2A is the type I've always used. You can see some in this link. http://www.aircraftspruce.com/search...TESTER&x=0&y=0
A person has to get the piston up to TDC on #1 then plug the tester into the spark plug adapter. The regulator, with a shop air hose attached, is then adjusted up the scale of the first indicator to reach 80 PSI. The second indicator will read the differential pressure. If it's 65 psi or better, the cylinder has a good enough leak down to go. If it is lower, a person has to listen at one of the three normal leakage areas, ie exhaust, intake, or crankcase breather. This is how you can tell where the leak is on either valves or cylinder/rings. After you do # 1, you follow firing order for each cylinder till all have been tested

These can be utilized on automotive applications but a person has to fabricate or find a spark plug air adapter suitable to the engine being tested.

To Add for safety, if the engine rotates when you introduce air into the cylinder then you have to stop and turn it back up to TDC opposite the direction it rotated. You can find exact TDC this way if you introduce about 5 PSI while you are turning the crankshaft. Coming up on compression, it will have resistance to turning until it is at TDC and the indicator #1 quits climbing to higher pressures. If you go too far, it has pressure rotating it the opposite direction so a person has to find the sweet spot where it no longer tries to rotate either way. If you put the car in gear and it tries to rotate then you have a problem. Best I can say is DON'T Do That! 80 PSI in a cylinder at TDC can get thing moving better than you might think.

[JSeery]

Originally Posted by rotorwrench

A person has to get the piston up to TDC on #1 then plug the tester into the spark plug adapter. The regulator, with a shop air hose attached, is then adjusted up the scale of the first indicator to reach 80 PSI. The second indicator will read the differential pressure. If it's 65 psi or better, the cylinder has a good enough leak down to go. If it is lower, a person has to listen at one of the three normal leakage areas, ie exhaust, intake, or crankcase breather. This is how you can tell where the leak is on either valves or cylinder/rings. After you do # 1, you follow firing order for each cylinder till all have been tested.

Sounds like a normal automotive leak down test to me, which part did you think was different?

"Performing the test is very simple. Bring the cylinder to be tested to top dead center on the compression stroke. Insert the appropriate adapter hose in the spark plug hole and just snug it against the o'ring. Do not over tighten. Plug the adapter hose into the right hand side of the tester. Turn the yellow knob on the regulator counter clockwise fully and plug in an air supply with at least 120 lbs pressure. Failure to turn the regulator down before hooking the air supply can result in damage to the regulator. We do not replace any damaged regulators. With the air supply hooked up, turn the yellow knob clockwise until the pressure on the left gauge reads 100 lbs. If the engine is in perfect condition, the right gauge will also read 100. Anything less is the amount of leakdown that cylinder has. Example. If the right gauge reads 90 psi, that cylinder has 10% leakage. 80 lbs, 20% leakage, etc. 2-3% is usually considered "very good" If the leakage is 10% or more than you should be able to hear it. Listen in the carbs or air box with the throttle open. If you hear air escaping, it is an intake valve. Listen in the exhaust pipe for bad exhaust valve. In the oil filler means bad rings, etc."

You can use different test pressures, the reason 100 psi is often used is because it provides you a percentage value directly without having to calculate it.

[Kruzn40]

[BUBBAS IGNITION]

Yep pretty normal old Sum electric cylinder leakage tester. The gauge was zeroes ay 100 psi so the leakage would show up as a percentage. 85 psi would read 15% on a backwards reading gauge etc.
These are still pretty common today and are used with hilborn fuel injection on the barrel valve adjusting the bypass or leakage amounts etc. I have a couple laying around here somewhere....good tool.....

[GOSFAST]

If this is your first time attempting this I would recommend keeping your hands away from the fan, if all isn't lined up correctly it MAY spin on it's own when pressurized!

Thanks, Gary in N.Y.

P.S. When you set it at TDC it must be on the "compression/firing" stroke for that cylinder. It's at TDC twice but only once for firing that specific cylinder!

[chap52]

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Bent Valve was my thought...Valve not bent, I even replaced it with another straight one.

[chap52]

I appreciate all the information on a "leak down test". I plan on doing it after I get some compression in #1. Don't think it will hold any air in the state it is now.

[JSeery]

Have you checked closely for any cracks around the valve seats or anywhere else in the combustion area?

[chap52]

I gave it a pretty close look and didn't notice any but will be looking even closer when I get it opened up again. I may be able to tinker on it tomorrow. Despite the head scratching I really enjoy chasing after these gremlins. Guess even at 70 I still enjoy the adventure.
Thanks again to all.

[JSeery]

Might want to use a dye on it, magnafluxe would be best if possible. The air has to be going somewhere.

[cas3]

broken spark plug with a pipe nipple welded on to fit your air hose is a fine cylinder leak test. make sure its on top dead center, regulated low air is all thats needed, put a ball valve on to shut it off, and listen to where the air is going. out the exaust? out the carb? out the breather pipe?

[chap52]

Thanks again...All responses are appreciated...

[Bluebell]

Your better off sweating an air line tail to the spark plug. Then it is easy to connect an air chuck to it after you have screwed in the plug (or when you want to remove it)

[chap52]

Thanks cas3 & Bluebell. That was my initial approach to determine where the leak was. I was going to do the "leak down test" in the future just to determine if any other cylinders had any issues. Shucks I am old enough to remember us poor teen drivers back in the early 60's created all kinds of "tools" and "thin-a-ma-jigs" to diagnose problems and keep our cars running.
Appreciate the input.

[chap52]

It was a cool morning, only in the 80's and I made a couple od discoveries with my truck's compression issue. I was able to determine it is the exh. valve leaking as suspected.
I also found that an air hose fitting has the same threads as my spark plug and was able to put it in place of the spark plug, assure that my #1 was TDC with both valves in the closed position. Attached air hose to the fitting and set pressure at about 20 psi.. Air flowed out the exhaust pipe. Poor man's leak check.
I will be finish the job after my iced tea. It's now in the 90's.

[Bob C]

The threads may be close but they are not the same. The plug is
14-1.25 MM and the fitting is 1/4-18 NPT.

Bob

[Ronnie]

Originally Posted by Bob C

The threads may be close but they are not the same. The plug is
14-1.25 MM and the fitting is 1/4-18 NPT.

Bob

Absolutely not the same plugs are metric and the air fitting is npt thread.
Hate to say it but get away from the bubba stuff.That is a good way to fubar up the threads in the head.
Just use the hose off your compression tester.

R

[chap52]

Thanks, Got it done without damaging threads. Found a bad valve seat. Gotta let it sit for a couple of weeks before I can get it taken care of.
Thanks for all the advice and walking me through this one.

[Capt Kirk]

Can you post a picture of what you found?

[chap52]

Didn't take a picture when I found the problem. The upper portion of the seat was carboned up pretty badly. I then attempted to "lap" a new valve to see the extent of the carbon build up and pitting. There seems to be a lot of seat material remaining and I believe it will clean up by grinding with the proper stones. I have the correct set available but it will be a couple of weeks before I will have access to use them.
Here is a picture after "lapping".