Low Compression
 

I have low compression on my #2 cylinder(35 lb's). I found it by pulling the plug wires while the motor was running and there was no rpm change on that cylinder. The motor was warmed up at that point. I ran the motor with the spark plug removed to see if the valves were moving and they were. It looked to me like the intake valve wasn't traveling as much as the exhaust valve.
The motor is a 1946. Compression on 6 of the cylinders is 110 lb's and one at 85 lb's. When the motor is running, it shows 20 in of vacuum with very little needle shake. From what I was told, it's a low mileage, stock motor that's never been apart. I pulled the drain plug and ran my finger around the inside of the pan and there was no sludge built up on the bottom.

Is there a way to tell if it's my rings as opposed to my valves?

Is there a way to check the lift on the intake and exhaust valves?

Anything else to check or try?

Thank you,
Joe

Re: Low Compression
 

I would say do a leak down test and see if it is your rings or valves?
OR this ; put a tea spoon of oil in the cylinder and do your compression test again , if it comes up it will be the rings if not a bad valve[s]

Re: Low Compression
 

Leak down test.
Basically you put that cyl at tdc compression stroke, valves closed and pump a bit of air in through the spark plug hole.
Then you listen for where the air leak is. Intake, exhaust, or crankcase.
Harbor Freight has a reasonably cheap tester/ not sure of quality tho...

Re: Low Compression
 

Hi Joe, correct info above, squirt a little oil in and redo the comp test! Disconnect the coil and keep the throttle-blades wide open.

Another option, bring that cyl to TDC (firing position only), put about 100# of air in at the spark plug hole and listen for leaks. Could be at carb, exhaust, or in the crankcase! This should pretty much tell you what's up.

Thanks, Gary in N.Y.

P.S. Hope all is well? Your parts are still "safe & sound"? Ya' just never know!

Re: Low Compression
 

poor mans leakage tester, weld a pipe nipple on a broken off spark plug to hook up your air hose. you dont say if this is a new problem, or a motor that has been sitting awhile. if sitting, an allen wrench and a light tap on the valve (if found to be stuck open) will some times bring it back to life. hit the center of the valve, not the edge

Re: Low Compression
 

I wish my compression was that good 65 lbs on seven cylinders and 35 lbs on one.
Starts and runs ok . 17 in of vacuum, needle is steady. Hope it holds up till the new engine is ready

Re: Low Compression
 

Thanks everyone! I will try this tonight and post the results.

Hey Gary! All good here! Talk to you soon.

Re: Low Compression
 

Could be a stick valve, try some Marvel Mystery oil. G.M.

Re: Low Compression
 

To loosen a stuck valve is the MM oil added to the gas tank or crankcase?

Re: Low Compression
 

(Per G.M.) Trickle a little down the carb until it starts smoking while engine running. And, about 1/2 pint per tank.

Re: Low Compression
 

So here's what I did.
Engine cold, throttle wide open, all plugs out:

#1-100
#2-40
#3-100
#4-120
#5-120
#6-115
#7-110
#8-110

I added a little oil to #2 cylinder and it went up to 45.
Does this mean the rings are bad?
Should I do a warm test?

Thanks to all!

(G.M., Curt in AZ & 39topless) I run MMO with every tankful. I also poured it down the carb with no results.

Thank you but what's next!!!!

Re: Low Compression
 

If the oil in cylinder #2 didn't help then it is more than likely a valve.

Re: Low Compression
 

That' what i like to hear!

Should I do with a leakdown test?

Re: Low Compression
 

I did a leakdown test and it showed about 55% leakage!

I can hear the air coming through the exhaust.

I know that valve isn't stuck because I can see it traveling with the motor running and the plug out.

Re: Low Compression
 

There is a problem with the exhaust valve seating or maybe a crack.

Re: Low Compression
 

Hi Joe, next step will pull the intake and make certain you have (somewhat normal) lash on that valve.

Thanks, Gary in N.Y.

P.S. It's all going to come down how your "luck's" been lately, if it's not a lash issue it all points to a "burned" or "hanging" valve. With 40#/45# it has all the earmarks of a slightly burned valve??

Re: Low Compression
 

Thanks guys! I guess I have my work cut out for me.

I'll keep you posted!

Thanks again to everyone.

Re: Low Compression
 

I pulled the intake and pass. head. It looks like a factory relieved block. It's a 59L out of a 1946 COE. Here's some pictures.
Picture 1 is cylinders 1 & 2
Picture 2 & 4 are close ups of cylinder 2 where I think the problem is. When I did a leakdown test, the exhaust valve was leaking and I could hear it in the tailpipe. I'll try to take out the valve tomorrow to get a closer look.
I would like everyone's input on this please!
Thanks,
Joe

Re: Low Compression
 

Looks like that valve and or seat may be the problem.

Re: Low Compression
 

Hi Joe, has all the earmarks of water damage, the rust/corrosion on the valves and around the seats?? Couple domes look as though they've been "washed" by hot water! We're you having any gradual water usage?

Hard to tell from the photo's though! It does appear that the valve is not sealing?

With the cylinder "sealed" up for the test I would have suspected air leaking up through the carb though?

Thanks, Gary in N.Y.

P.S. Repairing that OEM valve setup in the car may prove to be somewhat difficult with the non-adjustable valves. Also MAY be a seat issue there! It has to come apart further!