what would cause a burnt valve?
Thread Starter
Safety Car
Joined: Jun 2001
Posts: 4,004
Likes: 2
From: Severna Park Md
what would cause a burnt valve?
I Pulled the heads off a 1970 smallblock 355 and found that some critter had cut a slice a pie outa the #4cyl exaust valve. Before pulling the heads I didnt know what I might find. Im glad I found it since it makes all the work worth while. On to the question...what pre existing circumstances could lead to the burnt valve? After fixing the heads, I wanna make sure it doesnt happen again. The engine has a mild cam, headers and a Q-jet thats desperatly in need of a rebuild.
Thread Starter
Safety Car
Joined: Jun 2001
Posts: 4,004
Likes: 2
From: Severna Park Md
Re: what would cause a burnt valve? (nastee383)
Thanks for the input nastee...I went through a few header gaskets with this engine. I might try the soft aluminum type gaskets this next time.
Burning Brakes
Joined: Aug 2000
Posts: 852
Likes: 0
From: Katy TX
Re: what would cause a burnt valve? (Jvette73)
You should also consider the engine timing and fuel type/mixture. Was there considerable carbon build up on the valve? Also are the coolant passages in the head clean?
Just some ideas of other things to look at.
Just some ideas of other things to look at.
Racer
Joined: Dec 2001
Posts: 498
Likes: 0
Re: what would cause a burnt valve? (nastee383)
I agree on the exhaust leak at the head. I have always heard that a blown header gasket can lead to a toasted valve. Might as well do a little grinding on the heads while you are in there!
Racer
Joined: Dec 1999
Posts: 267
Likes: 0
From: Seattle, WA USA
Re: what would cause a burnt valve? (Jvette73)
Number one cause is badly adjusted valves. If a hydraulic lifter is adjusted too far down the bore or a solid is just plain too tight, the valve can be held open slightly when the engine warms up. On an intake valve, it's easily noticed because it typically causes backfiring in the carb but on an exhaust valve, it might go unnoticed. Unfortunately, an open valve will let hot exhaust go screaming past the edge of the valve and will act like a cutting torch, especially if you are running lean and there's unburned oxygen in the hot gas stream.
Another common cause is a bent valve caused by valve spring float. In rare cases, a chunk of carbon can break loose and get stuck on the valve seat long enough to cause the cutting-torch effect.
Another common cause is a bent valve caused by valve spring float. In rare cases, a chunk of carbon can break loose and get stuck on the valve seat long enough to cause the cutting-torch effect.
Burning Brakes
Joined: Dec 2000
Posts: 1,183
Likes: 2
From: Des Moines Iowa
Re: what would cause a burnt valve? (Jvette73)
Soundguy nailed it!!
Leaking exhaust manifold gaskets or running engine with very short or no exhaust pipes can cause exhaust valves to warp, as they may cool too quickly, once a hot engine is shut off and some of the exhaust valves will stop in the open position. I suppose it's possible that a leaking exhaust manifold gasket could cause this same condition, but believe such occurance to be very rare or highly unlikely.
Leaking exhaust manifold gaskets or running engine with very short or no exhaust pipes can cause exhaust valves to warp, as they may cool too quickly, once a hot engine is shut off and some of the exhaust valves will stop in the open position. I suppose it's possible that a leaking exhaust manifold gasket could cause this same condition, but believe such occurance to be very rare or highly unlikely.
