Did you re-seat the valves when they were replaced? I would do a few compression tests and also check that the valves are not sticking.

Location: Pembrokeshire

The valves were reseated. Compression and leakdown test are all normal. We even removed the spark plug in #2 while doing the leakdown test on #1 to see if possibly a leak across the two.

Location: US

George,

Plugs?

Tony.

Location: Australia

Yep. NGK and
Champions all different heat ranges.

You would think there must be an air leak somewhere that is only manifest at higher temp or press. I'm wondering about a crack at the base of a head stud that only opens up at times. Not sure if you could detect that with carb cleaner?

Regards, Stuart

George, I'm with Stuart on this one there has to be an air leek somewhere, can't think of any other explanation.
J

Location: As far east in Kent as you can get

After a test drive and removing the plugs 2,3,4 are normal but the #1 plug is blue on the lower end of the threads meaning that it is either getting too much air or not enough fuel. We've tried a manifold with 2 SU's and 2 different single carb manifolds. Always the same problem. The #1 cylinder pressurized with 100lbs of pressure remains constant and does't leak down.

Location: US

George,
What cylinder head are you using?
J

Location: As far east in Kent as you can get

It's a standard high compression head that has been cleaned up some inside. I had been in use with no problems before rebuild.

Location: US

George,
The fact that the threads on No 1 plug have "Blued" can only indicate excessive heat. Have you eliminated the problem which caused the engine to overheat in the first place, necessitating the need for a rebuild.
I know it's a long shot, but is there a possibility that the waterways might be blocked in either the head or block around No 1 Cylinder?
J

Location: As far east in Kent as you can get

Going back to the possibilities of an air leak. Have you checked the.compression washer on no 1 plug. Also to check out the plug have you swapped the plugs about and see if the problem moves itself to another cylinder that no 1 plug has been moved too..Over heating and your blue end plug could be caused by leen mixture caused by air entering through the plug hole if the washer is not doing its job.

John Mason

Location: Nottinghamshire

On the off chance. On two occasions a tappet cracked due to the taper on the lock nut. This caused the exhaust valve (both times) to stick but we couldn't detect it when we turned the engine over to test the compression.Try looking at the colour of the valve heads first. If one is sticking it will appear whiter than the others.

We've tried several different brands of plugs--all with new compression washers. We've swapped the plugs and even plug wires and the problem stays on #1.

Location: US

We took the side plate off and watched the valves while the engine was running. All normal.

Location: US

Sounds like you haven't solved your original problem of overheating prior to rebuild.
Not sure of an answer from me but the block can silt up badly especially between the cylinder wall and the tappet chest wall,effectively losing all the cooling around that area.
You can't see around that there,I only know this having sectioned blocks.You may get an idea if this could be the problem by removing the front block core plug.

Example, albeit on the water branch side of the block:

Location: North Herts

Hi George,
For the bottom threads of No1 plug to turn blue, it must mean that it is getting seriously hot,and has, I fear little or nothing to do with carburation and mixture.
Having satisfactorily pressurised No1 cylinder to 100 lbs it's safe to assume that there isn't an air leak, so now it has to be a cooling problem. Don't forget that the water outlet is as close to No1 plug as you can get!so any restriction in this area will allow the plug to overheat, have you checked the water flow through the head and block by removing the top and bottom hoses ? It should flow through faster than you can fill it,
and the radiator to!
Nick's photo above is all too common.
J

Location: As far east in Kent as you can get

We did the same but could not see it either. I think the valve shuts but does it more slowly than it should.The only clue was the valve colour.