The Stovebolt Site Forums The Shop Area Engine Shop 1 low compression cylinder

On my '56 gmc 270 six,I'm trying to figure out why it is running rough, popping and hesitating on acceleration, and seeming to have less power. I checked the compression when I got access to an assistant, and the readings were #1- 153, #2- 74/72, #3 155, #4 152, #5 158, and #6 155. Something is up with cylinder #2. Vacuum running was 20" with a steady 1/2" pulse. As noted in another thread, I recently blew up the muffler with an overly rich idle, and that is when this rough running started. I set the timing with the vacuum advance connected, and I think I need to try timing it with the advance disconnected and vacuum plugged, as common wisdom seems to be to disconnect it while setting timing. Also, when timing, the ball was below the pointer, and when I moved the distributor to line the ball up with the pointer, the engine would almost die. So I left it below the pointer, still visible in the flywheel window. It still often almost dies when you hit the gas, but not always. The #2 plug had fluffy black deposits. The heat riser was stuck on, now I got it stuck off. I replaced the coil and condenser, (points are new) and replaced the ballast to coil and coil to distributor wires, which seemed funky. It runs a little smoother now. I'm particularly interested in the next steps to figure out what is up with Cylinder #2, and then I should know if driving it will damage things, and how to fix it. It seems I should pull the valve cover and watch what happens, but I am not sure what to look for, and whether to look while cranking or running. I also would like to know what those compression readings mean in terms of performance- for example if I get the carb and timing right, should it be driveable?

Put some air pressure to the #2 cylinder with the piston on top of the stroke with both valves closed. Be sure the valves have the proper clearance first. Put the transmission in high gear and either set the parking brake or chock the wheels front and rear. Listen at the carburetor, tailipipe, and oil filler to see where the pressure is escaping. Bottom line- - - -you're going to need either a re-ring or a valve job, or maybe both. Compression that low is virtually always the result of broken rings or burnt or sticky valves. No amount of tuning is going to fix a low compression situation.
Jerry

Might also be a push rod that has bent because of a stuck valve.

My truck would have an intake valve stick over night raight after I put it back on the road. The push rod would pop out of place while cranking.

A little fogging the motor with an upper cylinder lubricant while it was running and the sticky valve hasn't stuck since.

But sure would make it run bad.

I would strongly suspect a valve with compression that low. It could be a burned valve, but there is a very good chance that it is a stuck valve. Stale gas will cause valves to stick. A good dose of Marvel Mystery oil or other UCL and fresh gas will fix that. If its an intake valve leaking, I would suspect a sticky valve. If its an exhaust valve, it is probably burned and you are up for a valve job.

Sorry for hijacking your thread Dave, but I'm in a similar boat. My 235 (when hot) shows PSI readings of 140,155,125,160,140,155. Squirting oil in the cylinders only uped the pressure by ~3lbs.

I don't know the history of the engine. When I bought it it had been sitting for a few years and several valves were stuck so I pulled the head and freed the valves. While I was in their I did a quick cleaning on the valves and lapped them. I should have sent the head out to be redone, but didn't since I didn't know the status of the bottom end I didn't. The engine runs great, it pulls hard and idles relatively smooth. I have put 1,800 miles on it so far. As far as I can tell the rings are fine, should I be concerned with that much variation between cylinders?

If the valves are adjusted right, a variation of up to 10% between cylinders on a warmed-up engine is considered to be acceptable. Your readings are right on the ragged edge of that specification, so unless you've got an obvious miss or backfire, don't worry about it. If it gets to the point that it's running bad, it's probably time for a rebuild or at least a valve job.
Jerry

don't worry about hijacking- it is the same topic....I'm about to air up that low cylinder and hear where it is leaking.....