Burnt Valves

[Webb Sledge]

General question for anyone: what exactly is, and what causes a burned valve? I was riding my friend's four wheeler the other day and the muffler clamp fell off, and then my friend got on to ride it, but his stepdad yelled to him to stop, and that riding it like that would burn a valve. Why?

 

[Banjo]

Low exhaust backpressure (especially in 2 stroke and small engines) or other conditions like incorrect timing that lead to high combustion chamber tempratures or situations where the gasses are still burning while exiting the cylinder, can actually work on the edges of the valves (usually exhaust valves) like a cutting torch.
It actually eats part of the edge of the valve away. Then you lose compression in that cylinder because the valve no longer seals to the valve seat.
That's an informal explination
(note) 2 strokes don't have valves like a 4 stroke . In that case it actually burns the piston.

 

[Guest]

I've heard that also, maybe has something to do with not having sufficient backpressure on the exhaust side I would imagine. I really have no idea what the actual reason is, or if this is a real phenomenon or just urban myth. I've run cars on open exhaust before (no mufflers) and never burned any valves, but there was always at least a few feet of pipe between the header and where the end of the pipe was. Back in the day my dad used to run cutouts on some of his Stangs, where you could climb under the car and turn a butterfly valve right after the header collectors and bypass the entire exhaust system on both sides. These were popular with drag racers back in the 50's and 60's from what I understand. He never burned any valves doing this to my knowledge, and he ran many races that way.

Edit - Just read Banjo's explaination, makes sense. Guess this is a real thing. Ever see this happen to a 4 stroke car engine?

 

[Banjo]

Yes, I had a 91 Honda Civic that I burnt #2 exhaust valve out of. High RPM and incorrect ignition timing caused it in that case.
Translation: I was beating on a high milage car with a known issue in the ignition. It was my own fault

 

[Dennis]

When I first bought my spit back in 83, the kid I bought it from had the timing setup wrong and the exhaust valves were burned. I was looking for a project car at the time, I should have picked up the gt6 I was looking at but made a poor choice and only got the spit) That was when I did the first rebuild on the engine, second rebuild was 10 years later when I broke a valve spring and dropped the valve into the cylinder.

 

[jessebogan]

I would think that tight valve clearances would cause the valves to burn long before any other cause. The valves are actually cooled by the contact with the valve seats, and tight clearances reduce the amount of cooling available. As far as the exhaust, or lack there of, look at an average radial aircraft engine. At altitude, it is often 40 degrees below 0. The radials usually have short, unmuffled exhaust stacks, and burning valves is fairly uncommon, even over a very wide temperature range. LBCs, before the catylist era did not have hardened valve seats, and relied on the lead in gasolene to protect and lubricate the valve seats. In the absence of lead, under mostly higher RPM conditions, the valve seats slowly erode, causing the clearences to close up, finally leading to burned exhaust valves.

 

[Dennis]

Very true, which is also why I decided to have hardened valve seats put in when I had the head rebuilt. Wasn't sure how long the new valves would last running unleaded fuel, even with having the ignition and carb setup correctly.
Dennis /ubbthreads/images/graemlins/driving.gif

 

[Guest]

I don't know about spending money on hardened seats for low mileage cars. I know running unleaded fuel is bad for unhardened seats but I've talked to a few people who build motors as a living and they say unless you're putting 20K miles on the car annually it's not going to make a big difference. If you've got a collector car and you only put a few thousand on it per year it'll take 20 years for that unleaded fuel to really start to impact your valve seats. Thats what I've heard anyway, I'm sure there are plenty of people around on both sides of the fence in this arguement.

 

[Dennis]

I went through that discussion back in 83 with a friend who was a mechanic. The car at that time was a daily driver, I drove it all year round including during a few snow storms when I shouldn't have. You know you might be in trouble when you can feel the snow under the car through the seat....
I bought the car when it was 9 years old, body was in great shape at the time. The only thing that needed work was the engine and the brakes.
Anyway, I agree with you 100%. If it's not a daily driver then it's not worth the expense.

 

[vettedog72]

It is strange how some engines have this problem. Before "unleded" gas was the norm and back in the '50's (possible before); American Oil, aks Ammaco Oil had its "high test" gas as a no lead (frequenly call white gas)and many cars only had the nolead high test in them with out burnig the valves up. This was even before harden valve seats. It hard to belive that a sodium filled valve (74 and up TR6) with a harden valve seat ever burns unless the cylinder is too lean and or the valve is not adjusted correctly.

 

[RomanH]

Hey vettedog,
Are you sure about the valves being sodium filled? Can you please verify!
Thanks

 

[vettedog72]

Gads! In my mind it is true, if I am in space, tell me as I am in pursuit of printed, published proof./ubbthreads/images/graemlins/crazyeyes.gif