Jeffs71

I have a quick question about what a good number range I should see during a compression test on my engine. I found out its currently only running on 7 cylinders, mech says it's a burnt valve. I plan to have a valve job done in a few weeks, and wondering what the comp.should read after.
The engine is bone stock.

Thanks for any input

MDchanic

What's your compression ratio?

- Eric

oldcutlass

And what were the readings of each cylinder vs the bad one? Was a leak down test performed?

don71

Jeff, its been stated before that compression readings should be within 10% of each cylinder.

If I were you, I would do a test now. Record those numbers.

If your mechanic did this, he should have told you. Plus one, for a leak down test. That can really narrow down a valve problem. Maybe he did that, he should have told you.

An aftermarket cam can effect these numbers. If the engine has been rebuilt prior, all bets off.

To answer your question...I would be happy to see 135 on a stock broken in worn 1971 Olds engine. I'd still be ok at 125. let it ride.

Don't open up your wallet too far if you don't have to. This is only my opinion.

olds 307 and 403

Those numbers sound about right. I would bet your timing chain has shed it's nylon gear, further retarding timing. My 8 to 1 stock bottom end 76 Olds 350 with a compression robbing Performer cam had 140-142 psi all eight cylinders. Your motor has similar compression, just does it in a different way. If you need head work, make sure the heads are milled .020", if the stock huge dish pistons are in there, unless you like 7.6 to 1 compression. The best thing is a total rebuild with 6cc dish LF2321 pistons, will give low 9 to 1 compression.

Jeffs71

Thanks for the quick response.!

The rest of the cylinders were reading around 125-130 range, with number 8 reading zero. We pumped air into the cylinder, and is vented from the tail pipe...exhaust valve . We did remove the valve cover and verify its not a bent or sticking rod, or rocker.my buddy is gonna do a valve job and install the new carb in a couple weeks...waiting for the salt to be washed from roads before removing the vert from storage

MDchanic

125-130 is right for a stock 8:1 compression motor.

Just out of curiosity, which cylinder was it?

- Eric

Jeffs71

Hey Eric , the bad cylinder is number 8..right rear.

MDchanic

Yeah, if I recall, when I had a burn exhaust valve, it was #8 as well.

I'm curious as to whether #8 is the one that tends to burn, or whether it's just a coincidence.

Anyone else have a burned valve in a stock engine?

- Eric

Jeffs71

Interesting!

Ok just so I'm understanding this...a burnt valve isn't closing into the seat due to carbon buildup around the edge right? I'm new to the classics, learning more everyday!

I'll definitely mention the timing gear to my mech, would be a great time to check and or replace it .

MDchanic

Burned valve:










- Eric

Jeffs71

Thanks for the pics, I can see how the valve gets damaged..and the leak by!

1BOSS83

Are there certain conditions that would make a a valve more vulnerable to burn up aside from normal wear and tear?

MDchanic

Lean mixture and inadequate closing time (due to a bad valve adjustment).

Other than that, just wear and bad luck.

By the way, you will note that the exhaust valve seat was not recessed into that head, at 106,000 miles, in a high compression 350.

- Eric