The Stovebolt Site Forums The Shop Area Engine Shop leaking exhaust valve

some of you are familiar with my 1954 235 inline 6 in my 1951 chevy 3100. for those who dont, it was recently rebuilt, about 8 months ago, and after doing way too much tuning and adjusting (the guy who rebuilt it did nothing adjustment wise) ive discovered that i have a leaking exhaust valve somewhere. i did the ol' dollar bill test on the exhaust and sure enough it was being sucked right back in. im very positive a valve isnt burnt or broken, i believe there is most likely a bad valve seat. does anyone know how to fix this, or if it involves tearing down half the engine, will it hurt anything over time?

The first task is to identify which valve. A compression test will find any leaking valve - much lower CCP, and that spark plug will be visually different from the others.
If a valve is leaking, the head has to come off.

Cold Cranking Compression

ive already done a compression test a few times and every cylinder is the same... ok the head has to come off like i suspected. can any sort of damage result from a bad seat?

What are the compression readings? If the compression readings are all around 130 psi, and all within a 10% window, that would indicate no issue with a valve seat. Depending on the engine/head combo, the absolute number may vary a bit from that, and isn't as important as the consistency between cylinders. If there is a low reading in a cylinder, then it could be valve or seat, and the head would need to come off for further diagnosis (as Panic said). But if all the readings are good and consistent, then that doesn't indicate a reason to pull the head.

I've followed your earlier threads. There may still be a valve adjustment issue. Perhaps a bent pushrod. Pushrod check is easily done (perhaps each pushrod has already been checked?). Next is to verify that the valves are adjusted correctly. If all that is OK, are there any symptoms other than the "dollar bill test?"

If I recall, you don't have much run time on the engine since rebuild? Did the builder run the engine or have you run it appropriately to break in the cam (assuming the cam and lifters were all new)?

Hopefully, the questions will help the forum help you get this resolved.

Pull all of the plugs and look at them closely for even a slight difference.
Pull all of the plugs and look at them closely for even a slight difference.
Pull all of the plugs and look at them closely for even a slight difference.
I typed this thrice as it cannot be overstated.
You can often read a spark plug like a book to find an issue with an engine.
The one plug which looks different will likely be from the cylinder which has an issue.
To determine if a valve isn't seating tightly in that cylinder, put that piston at top dead center. If you have the type of compression gauge which has a quick disconnect, you can separate the hose from the gauge and add shop air to the male quick connect fitting. It should fit your shop air hose.
Leave the truck in gear to keep the piston from moving down from the air pressure. Listen for air escaping through the carburetor or the exhaust. Fold and pinch the air hose to reduce the air pressure from your compressor. Too much pressure will force air past the rings and give you a false reading.
Carl

If the only symptom is the paper test, I suggest driving it to detect a more significant symptom.
If none can be found, ignore the symptom.

This train went off the rails several miles back!
Jerry

If every cylinder has the exact same compression as you stated I wouldn't care if a raccoon could be sucked up the exhaust pipe. Do yourself a favor and do a search on "exhaust reversion". They sell anti reversion valves for around $50 if you insist on keeping your dollar bills wrinkle free. We did a lot of research on exhaust flow since it is free horsepower and legal. While you're wasting time on reversion go ahead and check out "scavaging" and "Coenda affect". I don't think you have a problem but first hold your dollar bill with the engine off idle as ANY engine has exhaust pulses if turning slow enough and second have it scoped as a good scope man can spot a valve or guide problem immediately. Does a vacuum gauge give a steady reading? Yes, vacuum gauges can also be used to detect an exhaust problem. Don't be in a hurry to pull the head.

definitely not in a hurry to rebuild a brand new engine... what i get from the forum here is that even if its a bad seat there wont be any damage caused, so i wont worry about it. thank yall for all the help!

ive done this, and the engine was running rich before, i cleaned them, pulled them after tuning it right, and theyre all still the same. i was just wondering if there was something to fear, which is seems there isnt, so i should be good to go!

they were all 130 as i can remember. certainly wont hurt to do it again, i mine as well, seem as im pretty paranoid about this whole ordeal. even if it is a bad valve seat ill just let it do its thing... like i said before, im in no hurry to rebuild a brand new engine.

If your brand new engine has problems, it isnt going to get better.sooner or later you will have to get to the bottom of the problem. If it were me ,sooner,maybe done already. Also,maybe it isnt a good rebuild and there short cuts under that nice paint.

We have wasted billions of electrons and hours of time discussing a sticky valve that only has one fix- - - - -PULL THE COTTON-PICKIN' HEAD AND GET THE VALVE STEM CLEARANCE RIGHT! The engine builder should have done his homework right in the first place. Setting up stovebolt engines to the microscopic clearances that modern engines require is a disaster waiting to happen, and today's machinists are too stubborn to read the service manual specifications that have been there for decades. What we considered to be "normal" clearances 50 years ago is "worn out" by today's standards.
Jerry