Hehe ok this is one of those, always been too scared to ask sorta questions as it seems to be general knowlage.... But I never did find out what Burnt valves are exactly? and what causes them?
All I know is that it stops them from sealing... But uh yea. as Miss Hanson would say... Please explain.
Burnt Valves
Started by
_Viper_
, Aug 23 2010 08:10 PM
6 replies to this topic
#2
Litre8
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Thrillseeker
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Posted 23 August 2010 - 08:21 PM
I think its almost always the exhaust valve and the most likely culprits are too lean a mixture and very retarded ignition timing.
Both of these result in the combustion mixture still burning/reacting while exiting the chanber, torching the exhaust valve in the process. Others can probably confirm/clarify my thinking.
Both of these result in the combustion mixture still burning/reacting while exiting the chanber, torching the exhaust valve in the process. Others can probably confirm/clarify my thinking.
#3
TerrA LX
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Posted 23 August 2010 - 08:24 PM
Burnt, as it would suggest is when the valve is overheated, or cracked due to localised overheating.
Overheating is usually caused by engine tune etc and localised problems are usually caused by the guy not doing the head (valves and seats) correctly.
Causes are usually either over grinding the valve head or not getting the valve guide to stem clearance right.
And yes it is usually the exhaust valve that suffers the most because it sees most of the higher combustion temps on the exhaust stroke.
Overheating is usually caused by engine tune etc and localised problems are usually caused by the guy not doing the head (valves and seats) correctly.
Causes are usually either over grinding the valve head or not getting the valve guide to stem clearance right.
And yes it is usually the exhaust valve that suffers the most because it sees most of the higher combustion temps on the exhaust stroke.
#4
_Viper_
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Posted 23 August 2010 - 09:09 PM
So when they overheat they warp or something?
Oh and ive heard running no exhaust can cause it also?
Oh and ive heard running no exhaust can cause it also?
#5
TerrA LX
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Fulcrum Fixture
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Posted 23 August 2010 - 09:20 PM
This usually leads to valve recession.So when they overheat they warp or something?
Also it can crack the valves if as above the face has been ground too far or the seat has been sunk too far into the head or the seat has been faced incorrectly.
Oh and ive heard running no exhaust can cause it also?
This can run the engine lean.
Any lean condition can overheat the valve.
#6
rodomo
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Posted 24 August 2010 - 01:15 AM
Valve recession also leads to little or no valve clearance on mechanically adjusted valve trains.This usually leads to valve recession.
The result can be less time to tranfer heat from the valve head to the seat and in extreme cases, the valve never seats correctly causing a blast furnace past the valve face.
#7
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Posted 24 August 2010 - 08:30 PM
^ yep.
Other problems caused by re manufacturing can include incorrect valve and/or seat width and/or angle.
These can also lead to sealing problems as well as reducing heat transfer as alot of heat from the valve head is dissipated thru the valve seat and valve guide.
Other problems caused by re manufacturing can include incorrect valve and/or seat width and/or angle.
These can also lead to sealing problems as well as reducing heat transfer as alot of heat from the valve head is dissipated thru the valve seat and valve guide.
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