Burnt out valve

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Datsolow
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Burnt out valve

Post by Datsolow »

Hi all , can anyone advise is it possible to burn out an inlet valve ? :dontknow:
Scott V
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Re: Burnt out valve

Post by Scott V »

its possible to burn out an inlet valve.

-Scott V.
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terribleted
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Re: Burnt out valve

Post by terribleted »

More likely in a Corvair to have a dropped or loose valve seat that causes compression loss rather than a burned valve, but , burned valve is possible particularly if run hard with low octane fuel.
Corvair guy since 1982. I have personally restored at least 20 Vairs, many of them restored ground up.
Currently working full time repairing Corvairs and restoring old cars.
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bbodie52
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Re: Burnt out valve

Post by bbodie52 »

I found this answer on the Web, and it contains a pretty good general summary (some more applicable to air cooled Corvairs than others)...
What exactly is a “burnt valve” and what causes it?

Valves can bend, break and/or burn. If the valves don't provide a complete seal, for whatever reason, the hot gases are forced past the valve which eat away or burn away the edge of the valve due to concentration of heat and pressure.

Image

Causes that I can think of:
  • Anything that causes incorrect valve seal
  • Cooling issues (improper cylinder head cooling)?
  • Quality of fuel? (carbon deposit on the valves)?
  • Concentration of heat at a particular point on the valve during opening (for exhaust valve) due to improper design.
ADDITION

This is only expanding on the above mentioned points. Mostly cause I can't think of any more
  • Incorrect Valve Seal - for whatever reason (wear, weak spring, valve lash etc) - reduces the quality of valve-seat contact and hence impedes heat dissipation (through conduction to the head). Improper guide and/or loose seat can contribute to this. Also, as the seats wear (choice of materials) and the valves recede into the head, valve lash is lost , which leads to the same, improper contact.
  • Choice of alloy used, width of the stem (for conduction), etc.
  • Deposit build up can hinder conduction.
  • Valves can also run hot because of elevated combustion temperatures (for n number of reasons).
A burnt valve can happen for one of many reasons, but the underlying issue is, the valve is not sealing correctly and allows hot engine gases during the combustion phase to leak past the seal which is formed by the valve and seat. When the seal is lost, the hot exhaust gases escape past the valve (can happen to either the intake or the exhaust valve, but is much more common on the exhaust) and wreak havoc. The burnt valve is usually not the cause of the issue, but more so the outcome of a different issue.

A normal running 4-stroke engine has valves which allow for the incoming air/fuel mixture into the cylinder (or just air on direct injected engines) and for the outgoing exhaust fumes to escape out the tail pipe. Valves in and of themselves could not stand up to the extreme environment of the combustion process. When a valve closes and is sealed against the seat (which is embedded into the head), one of the salvation factors for the valve is heat transference. The head absorbs some of the heat which is collected by the valves. This heat is then transferred out to the coolant system, which takes it away from the area and allows things to run as we consider normal. If a valve, for whatever reason, is not allowed to close completely, a leak in both air/fuel during the compression cycle and exhaust during the combustion cycle occurs. This causes several different problems within the engine:
  • Cylinder power imbalance
  • If leak occurs on the intake side, it causes intake flow issues with other cylinders
  • Burnt valves (due to escaping exhaust gases during the combustion cycle)
A valve can get into non-sealing state for several reasons. These might include, but aren't limited to:
  • Carbon buildup
  • Physical valve damage (broke valve seat, bent valve, etc)
  • Sticking at the valve guide
  • Broke, damaged, or weak valve spring
Any one of these can cause a valve to stay open during the combustion event, which allows very hot gases to escape past the valve. Since it is not allowed to fully seat, there is nowhere for the excess heat to go. It stays at the valve head and damage soon occurs.

It simply means a half burnt valve thus preventing it from sealing against its seat

When there is no sufficient cooling in the engine it may cause the seal of the valves to give way(exhaust valve generally), gradually over the course of time the exhaust gases escape even without the valve being fully open thus increasing the risk of further damage. A partially burnt valve will be difficult to notice but will produce anomalies in the fuel economy.

Diagnosis:
  • Exhaust valves are generally hotter than intake so they are more susceptible to burning , as stated above , if your valve is burnt then the exhaust gases will escape even though valve is not open, you can do a compression test to adjudicate if the valves good.
  • Cylinder Misfire, if your head gasket is good and still cylinders misfire then there is a high chance that your valves might be burned since it causes compression loss.
  • Slight power loss, depends on the vehicle whether it will be noticeable or not.
  • Smoke in the exhaust.
Causes:
  • Engine overheating due to insufficient cooling.
  • Engine running lean (one of the common causes).
  • Having LPG kit fitted which on prolonged use causes less lubrication to the valves.
:link: https://mechanics.stackexchange.com/que ... -causes-it
Brad Bodie
Lake Chatuge, North Carolina
Image 1966 Corvair Corsa Convertible
Datsolow
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Re: Burnt out valve

Post by Datsolow »

Image
Head off inlet valve seat
Should I get all seats redone?


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miniman82
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Re: Burnt out valve

Post by miniman82 »

Yes.
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Click it!
Datsolow
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Re: Burnt out valve

Post by Datsolow »

Hi heads are in the shop , new seats @ £35 each x12 , new stainless valves, guides, etc gonna cost a few £££ ,
one question would you hone the jugs and fit new rings ? If so how do I measure correctly for ring sizes? .
All the best Paul🇬🇧
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terribleted
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Re: Burnt out valve

Post by terribleted »

If the jugs are still within measurements after honing then yes. Piston to cylinder clearances must be within factory tolerance for a proper job. This is the critical measurement. If there is too much clearance between piton and cylinder the rings will not seal properly. Rings are purchased based on the piston and cylinder diameter being used as in standard +20, +30. +40 etc. Are the pistons standard diameter or are they oversized from some previous overbore done on the engine. Oversized piston are usually marked standard piston are usually not marked, but, you can easily measure the pistons to see if their diameter is the same as the spec in the shop manual, or if they are some number of thousandths oversize...like +20 or +30 or something.
Corvair guy since 1982. I have personally restored at least 20 Vairs, many of them restored ground up.
Currently working full time repairing Corvairs and restoring old cars.
https://www.facebook.com/tedsautorestoration/

Located in Snellville, Georgia
Datsolow
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Re: Burnt out valve

Post by Datsolow »

Do you know size of standard pistons/bore? Cheers 🇬🇧
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