Recurring burnt Exhaust Valve

[MGTF1250Dave]

Aloha All,

For the second time in a short period of time, probably around 1000 miles, number four exhaust valve has eroded resulting in little or no compression. This is on my 1250cc XPAG engine in my TF.

About five years or so I had the head rebuilt using new stelite faced exhaust valves, hardened valve seats and manganese bronze valve guides. The larger diameter valves for a TD MKII/TF head were installed in my head that was originally fitted on earlier TD XPAG engine. The head replacement was only discovered after I purchased the parts and delivered everything to the machine shop. Help from this forum confirmed the casting number on the head was not original to the engine, but compatable.

After reassembling the engine things seemed fine. After about 1000 miles, I noticed a lack of power and a compression check showed very low compression on number four cylinder. I replaced the exhaust valve that was badly burnt. The replaced valve again burnt after about 1000 miles. Examination of the other exhaust valve revealed that number two had some erosion and would probably burn out soon.

I would like to correct what ever is causing these valve to erode so quickly. Any suggestion as to the cause and how to fix it?

Mahalo.

 

[Bob Claffie]

The classic answer is that you are running much to lean and / or you have an intake leak.
Bob

 

[aeronca65t]

One of the things I've seen with manganese bronze valve guides is that they need to be a bit more loose than ordinary steel guides (I'm guessing your car did not come stock with manganese bronze guides).

If a bronze guide is reamed to the standard guide value, it may end up tight when the engine heats up (because it tends to expand a bit more). This can restrict full closing of the valves, causing burning.
Things can often feel normal when the engine is cold.

I've seen this with A series BMC engines. Not really sure if it applies to older MGs, but may be worth looking into.

 

[David_DuBois]

I would agree with both assessments. The lean running can be checked with a color tune unit used to view the color of the combustion when the engine is running. I have also experienced problems with bronze valve guides sticking in a Magnette that we had for a time. I had them all removed and went back to the cast iron guides.
Cheers,

 

[aeronca65t]

Yes....I said "steel" guides, but I think most (or all) of them are actually iron or PM (powdered metallurgy process).

 

[MGTF1250Dave]

Thanks for the suggestions. I don't think running lean is the cause. I have checked the mixture in the past and it was fine and I will do it again after the engine is rebuilt. I have read that the bronze valve guides need to be reamed to prevent sticky valve operation when at operating temperature. Would weak return springs contribute to the burn valve? I'll have a different machine shop check the head when I get the crankshaft and other engine internals checked.