Overheating.

[Vic Butler]

My 1977 MGB GT has a strange overheating problem. I can leave in running stationary at about 2,000 rpm, the temperature goes up and then the electric fan cuts in and down goes the temperature. On the road though as soon as I put my foot down the temperature gauge shoots up into the red and I can feel the engine is overheating. There's no loss of water and no oil in the water. The cylinder head is modified with MGC inlet valves , carried out by Brian Slark, ex Downton cylinder head wizard. Because it doesn't overheat when stationary I've rulled out the water pump and radiator but I would have thought that a cracked head or blown gasket would have resulted in water loss. Compressions are within limits, number 4 cylinder is down on the others bit within tolerance. I'm convinced it's not the timing as I have experimented with various settings to no avail.
Any ideas? It looks like I'll have to remove the head.

[Paul Hollingworth]

Are you sure your timing isn't too retarded. Seems you have done the other obvious thing and flushed the radiator. Have you checked the thermostat?

[Vic Butler]

I've fiddled around with the timing but it didn't make a difference Paul. I thought had it been the thermostat, water pump or radiator it would have overheated when I ran it stationary. With number 4 cylinder being down slightly I wonder if the problem is there. The 1.75" inlet valves have very little clearance to the exhaust valves.
The overheating occurs immediately when the engine is under load.

[Ian F]

Vic,
I think we have discussed this previously, and I recall you saying that you timed the engine "by ear"? This may be recommended for Land Rover engines but I would strongly recommend you using a timing light for your MGB!
Overheating under load strongly indicates (to me at least) that some engine parameters are not set correctly - too weak a mixture or too advanced/retarded. You should be aiming for circa 30 degrees advance maximum.

Ian F

[Vic Butler]

Ian, I didn't time the MG by ear. It was running ok as was then it started overheating. I hadn't altered the ignition timing. It suddenly happened. It displays no problems when off load.
I suspect a cracked head with the valves being extremely close to each other.
It's very strange though that there's no water loss or oil in the water or vice versa.

[Ian F]

Thanks Vic. Let's hope it isn't a cracked head...

Ian F