I have a problem - burnt out exhaust valves.

Some answers here.

I use additives which differ in Brand names from country to country. Also the 98 octane fuel this has cured the MG from all sorts of problems. We have 3 grades of fuel.

The valves should not have been affected but the valve inserts are the bits to wear. I have seen a head modified to unleaded, only to find that the valves stems and valve guides picked up and stopped the valves from seating, seemed from lack of lubrication, which leaded fuel did.

This could be your problem if the valves are burnt out.

Hi!

I have heard that different petrol suppliers are using different lead replacement additives and if you were to mix the different types then the overall effectiveness would be reduced, try sticking to one type of fuel or use an additive such as castrol valvemaster plus with any normal unleaded, I have run it for high mileages and heavy right foot for a long time with no problems although it does turn your spark plugs red due to the manganese content!

besides, you say that it was an unleaded head, are you certain that it does have hardened valve inserts?

other causes could be running too lean (detonation) too much timing advance (detonation) too much timing retard (increased exhaust gas temp overheating valves) valve seats too thin ( not enough heat transfer from valve)

hope that helps!

I personally use shell optimax and castrol valvemaster plus as my head is not converted and I get much better mileage out of it!

Miniman

My understanding was that the valve stem is lubricated by oil from the rocker area. If valve stem seals are installed with too little clearance the valve stems will seize in the guides. This is one of the main features of the teflon seals, they're harder to install but will maintain a small amount of clearance for lubrication. Obviously, this is a balancing act since too much lubrication will contaminate the intake charge.

Lubrication from leaded fuel is in the seat area itself. It keeps the valve from welding to the seat at elevated temperatures. Someone long ago decided that this was a lubrication phenomena although to me it sounds more like adhesion impairment.

Cheers,

Kelley

In addition to all of the good advice in Miniman's post, it is possible to run too lean with out having detonation. The exhaust temperature rises drastically and leads to burned exhaust valves.

In the U.S. we have been running A-series engines on unleaded fuel for more than 20 years. Few of these have been converted to lead-free heads and there have been few problems. I suspect our lack of problems compared to the U.K. is that we may be willing run richer since our fuel prices (read: fuel taxes) are lower. Richer running leads to cooler exhaust valves, leads to less valve seat recession. Just a theory though.

Cheers,

Kelley

I think you would have to sight the problem for a full understanding. It was not a valve stem oil seal problem as it had the o ring type seals on the valves hence it was was not a clearance question. Like you have indicated below the valve stem "welded" to the guide at the combustion area end.

Playing devil's advocate...

It's certainly not a valve stem seal problem with the o-ring seal. I've never really understood how they work to limit oil in the guides. I suspect they do very little. So, this means that you should have had good lubrication to the valve guide/stem.

If the engine was running lean the exhaust gas temperatures would have been very high. This tends to burn off any lubrication. In addition, overheating the valve stem and guide can cause sticking.

I'm not trying to specifically analyze what happened to your engine. I'm just pointing out that there are more likely or more common things than the lack of lead lubrication on the valve stem that could cause a valve to sieze.

Uncontrolled lean running is about the worst thing that can happen to an engine. It's always best to err toward the rich side of the mixture adjustment. It's usually more fun as well.

Cheers,

Kelley