273 compression loss MYSTERY...

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rickdick78

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Ive got a 67' 273 and i had two cylinders on the drivers bank that had no compression (50 psi and 90. I did a leakdown test, and to the best of my knowledge i heard air coming from the exhaust pipe. Now since i havent been adding lead to my gas for some years now, i figured i had burned out my exh valves.
I pulled the head, pulled the valves out and they look perfect. No chips no burrs they dont appear to be bent and there is no corrosion.I dont think i had a stuck valve because the engine is VERY clean and the guides appear to be perfectly smooth without much play.
I am really stumped. Ideas? Experiences? thoughts?
 
Ok thats something ill check. But how would that mess with my compression? (im still learning)
 
Did you check valve lash before you tore it down?
 
rickdick78 said:
Ive got a 67' 273 and i had two cylinders on the drivers bank that had no compression (50 psi and 90. I did a leakdown test, and to the best of my knowledge i heard air coming from the exhaust pipe. Now since i havent been adding lead to my gas for some years now, i figured i had burned out my exh valves.
I pulled the head, pulled the valves out and they look perfect. No chips no burrs they dont appear to be bent and there is no corrosion.I dont think i had a stuck valve because the engine is VERY clean and the guides appear to be perfectly smooth without much play.
I am really stumped. Ideas? Experiences? thoughts?
Click to expand...

If the cam is mechanical, the valve lash may not be adjusted correctly so the valves do not close all the way? Could just be a bad valve job. Lap them with valve grinding compound. That will tell you if they are sealing.
 
It's hard to diagnose over the phone isn't it.
 
Yea its hard to diagnose period ha ha ha. And my inexperience at this makes it harder.
I checked the pushrods and rockers before teardown and they didnt seem over tight. If the rockers were hung up due to a bad adjustment, they would be very tight, and they were not.
I have the valves out, and now that im messing around, i feel like the two bad valves have quite a bit more play and wiggle than the two good valves.Could excessive guide play lead to unevenly worn seats? That would def be where im losing comp.
 
One thing you can do is stand the head up and fill the exhaust/ intake ports with gas or even water

Or lay them "head side up" and fill the combustion chambers with water

See how badly the valves leak past
 
It is hard to diagnose....because you didn't do it. You just tore it apart. There are proper procedures to diagnosing an engine before you ever turn a wrench tearing it apart. Unless you find something blantently obvious, you'll just end up guessing.
 
StrokerScamp said:
It is hard to diagnose....because you didn't do it. You just tore it apart. There are proper procedures to diagnosing an engine before you ever turn a wrench tearing it apart. Unless you find something blantently obvious, you'll just end up guessing.
Click to expand...

Agree SS.
 
Well you are right to a point.I didnt tear it open stupidly, i DID do a compression and a leakdown test. And the leakdown told me it was valve seats.
I just realized that when i saw the seats i really dont know what a bad seat looks like.I know there can be thousandths of an inch imperfections that can lose comp, i just need to learn what that looks like. Ill put the valves and springs back and try the water test.Thanks for that.....

And to 65val, what should i look for on the headgasket? The two cyls ARE right next to each other!!
 
If not running leaded gas was the main cause of compression failure via burned exhaust valve, then wouldn't more than just 2 valves be leaking? The compression failure is in 2 cylinders that are next to eachother. I'm voting head gasket. Arn't most cam lobe failures only 1 lobe or all due to oil failure? Have you looked at the cam yet?
 
I'm not gonna get into the leaded/unleaded arguement....BUT bad valve seats look concave on the seat. they curve inward instead of being flat. That's called valve seat recession. It is mostly caused by a dirty air filter or very dusty conditions and not keeping the air filter changed often enough. How much pressure did the leakdown test hold for how long? What was the percentage drop? Did it pass or fail the compression test? If it failed, did you add oil to the offending cylinders to see if it came back up?
 
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