Poor Compression
 

1952 8BA. New Pistons, rings, wrist pins, has adjustable lifters. Was running fine. Started having a power problem and tapping sound. Also had occasional backfire through exhaust (not through carb) Suspected a stuck valve and ran a compression check.95-110 for cylinders 2-8, #1 was only 35 lbs. Opened the top up and valve operation looked OK, removed head and pulled Exhaust valve assembly. Lapped valve in case it was something not allowing the valve to close. Re assembled, adjusted both exhaust & intake valve. After assembling, disassembling and assembling again I am still hearing the sound and have no compression in #1. I enjoying chasing and solving the problems but would appreciate your input. Thanks

Re: Poor Compression
 

I haven't heard it used for cars but my plane mechanic uses a special compression tester on my plane that allows compressed metered air into the cylinder when it's at TDC. If there's a problem cylinder, he can tell if it's leaking past the rings or either of the valves.
Would noise be of a frequency similar to a lifter tapping? Sounds like the exhaust valve on #1 and the only thing I can think of is a sticky guide.

Re: Poor Compression
 

Did you check for broken valve springs?....Look closely, they're sneaky little devils....

Re: Poor Compression
 

I tore #1 exh. valve apart, inspected "closely", even squeezed the spring in a vice to look for any hidden cracks, lubed and reassembled. both hand turned and starter cranked with it opened up to see if there was a snag somewhere. I am baffled.
Thanks for the thoughts...

Re: Poor Compression
 

You need to do a leak down test.

R

Re: Poor Compression
 

It sounds like a stuck open exhaust valve or something is under the valve.

Re: Poor Compression
 

That was my plan. Stepping back for a couple of days to let the "fog" clear. Two days in my 90+ degree shop really inspires a guy to grab a tall glass of iced tea and "reflect" before proceeding.
I am in a part of Arizona where a snow flake brings out National News Teams.

Re: Poor Compression
 

A 90+ degree shop inspires me to grab a different sort of tall glass to reflect!! Wonder if pouring some Marvel Mistry Oil down the carb when it's doing this might result in a change? You could enjoy you're "tea" during this quick little test. Engine assemblies act differently when their in your hands on the work bench than when they do with 1000* exhaust blowing past them.

Re: Poor Compression
 

Thanks Kirk, will try the MMO tomorrow.

Re: Poor Compression
 

Hard to do on a flathead! but I was shown a method for checking overhead valve heads for a leaky valve. Pour a small amount of kerosene into the ports and see if it leaks out. It is amazing how a valve and seat can look perfect and leak with this test. Know this won't work on an in-block valve and seat, but it shows you how difficult it is to visually tell if there is a leak.

Re: Poor Compression
 

Maybe slightly off topic but maybe not. When my plane was getting a compression test (horizontally opposed, air cooled) with the valve covers off. If he came across a low cylinder that was leaking through one of the valves and would have required a $1000 repair...he would take a rubber mallet and tap the rocker arm on the valve side and 100% of the time in the last 18 years, the leak stopped. It doesn't take much of anything to make a valve leak slightly. In the OP case...I don't think there's something tiny under the seat...I think there's something that's binding the valve.

Re: Poor Compression
 

Thanks, I will keep you posted.

Re: Poor Compression
 

You don't say if it was bored. If not, any of the cylinder's ridge that wasn't completely cut away it will cause the top ring to bind/break or break the next land in the piston.

Lonnie

Re: Poor Compression
 

Ronnie is correct. Easy way to check each cylinder. You will need to bring
each one to TDC.

https://mobiloil.com/en/article/car-...-leakdown-test


http://www.hotrod.com/articles/cylin...akdown-tester/

Re: Poor Compression
 

Not Bored, no ridges. The new pistons worked fine for about 300 miles now. It appears that the #1 exh. lifter is "lazy" and hangs up at the top of it's stroke not remaining seated on the cam. I will try some MMO on it for a couple of days and if that doesn't loosen it up I will pull the valve and lifter (again) and deal with whatever is hanging it up.
Thanks for the input. I am still going to fabricate a tire valve and an old spark plug for a leak down tool. Even if I never use it, it will look cool in my "What's that used for" custom tools box.
I will keep you posted.

Re: Poor Compression
 

You need more than a tire valve and a spark plug shell to do a leak down test.

R

Re: Poor Compression
 

OK, I will research some more. Thanks

Re: Poor Compression
 

OK I splurged and found one on Amazon Prime for less than $30 with free shipping. Saved me a couple of hours trying to make one. Thanks for your responses.

Re: Poor Compression
 

Bent valve?

Re: Poor Compression
 

That's the name of the nag I'm putting my money on.

Re: Poor Compression
 

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.

Re: Poor Compression
 

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.

Re: Poor Compression
 

Re: Poor Compression
 

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.....

Re: Poor Compression
 

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!

Re: Poor Compression
 

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

Re: Poor Compression
 

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.

Re: Poor Compression
 

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

Re: Poor Compression
 

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.

Re: Poor Compression
 

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

Re: Poor Compression
 

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?

Re: Poor Compression
 

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

Re: Poor Compression
 

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)

Re: Poor Compression
 

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.

Re: Poor Compression
 

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.

Re: Poor Compression
 

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

Re: Poor Compression
 

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

Re: Poor Compression
 

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.

Re: Poor Compression
 

Can you post a picture of what you found?

Re: Poor Compression
 

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".