Burnt Valves, what caused it?

[cheers731]

Hi All,

1992 Four Winns, OMC 5.8L Cobra outdrive.
I posted a while ago about low compression on cylinder 3 and 7. After removal of heads, the valves on cylinder 3 and 7 are badly burnt, almost have no beveled edge and one has edge chipped a little. The machine shop will go replace them and do a complete check up.

My question is what can cause the valve burn and how to prevent it from happening again? I have not experienced engine overheat in the past.

Your input is appreciate greatly.

Thanks,

Frank

 

[Bondo]

Re: Burnt Valves, what caused it?

Hi All,

1992 Four Winns, OMC 5.8L Cobra outdrive.
I posted a while ago about low compression on cylinder 3 and 7. After removal of heads, the valves on cylinder 3 and 7 are badly burnt, almost have no beveled edge and one has edge chipped a little. The machine shop will go replace them and do a complete check up.

My question is what can cause the valve burn and how to prevent it from happening again? I have not experienced engine overheat in the past.

Your input is appreciate greatly.

Thanks,

Frank

Ayuh,..... 9 times outa 10, it's from Detonation, usually induced by a Lean condition,....

Water, 'n crud restrictin' the filter is cause #1,...

 

[MikDee]

Re: Burnt Valves, what caused it?

Lean running in those cylinders, causing high temps. It could be due to water in the fuel?

 

[cheers731]

Re: Burnt Valves, what caused it?

Thank you folks, really appreciate the responses.
I will pay more attention on fuel mixture adjustment and make sure no vacuum leaks.

Frank

 

[harleyman1975]

Re: Burnt Valves, what caused it?

That was what I was gonna say intake leak.

 

[Lou C]

Re: Burnt Valves, what caused it?

After you get the valves done make sure that the fuel mix is right, timing is right (including total timing advance), there is no water in the fuel (dump out the water separating fuel filter) and the heat range on the plugs is right. Anything that can cause a lean condition or over advanced spark timing can cause this. Also another factor often missed that over-heats valves is over propping. If the engine does not reach its specified max RPM properly trimmed, the engine may labor (like starting a manual shift car in 2nd gear) and that causes combustion chamber temps to skyrocket. I also always use 93 octane gas to be on the safe side. Older engines build up carbon which increases the chance of detonation.

 

[cheers731]

Re: Burnt Valves, what caused it?

Many thanks, those are good advices. Really appreciate it.