mgb runinng bad

Let me try to explain this again, when i start the car in the morning it starts just fine. Runs great for about 20 miles. after that the car starts to spit and sputter and tries to stall. when it does stall, i have to wait for it to cool down before i can start it again. the Carburetors are too hot to touch. it has an electric fuel pump so i don't think this has anything to do with the problem. it also has an electronic ignition which may be a problem or it may be a distributor problem. i don't know where to start or what to do to remedy this. After taking out the single Carburetors with the automatic choke and replacing it with duel SU carbs from a 1963 the problems still occurs. any opinions on where i should start would be helpful.

G'day,

Nothing like an old MGB to give problems :)

Has it still got the condenser and resistor based ignition and distributor, or has the electronic unit replaced it?

G'day,

Nothing like an old MGB to give problems :)

Has it still got the condenser and resistor based ignition and distributor, or has the electronic unit replaced it?

electronic

electronic

Assuming it's not over heating, then it sounds like the electronic ignition has failed.
As it warms up, there is very slight expansion in either the ignitor module, or the points module, which is enough to cause them to fail.
Once cool again, the problem goes away as the parts shrink slightly.

Do you have an old set of points and coil you can put on it to test it?

What kind of gas are you running ? If you can find some, try a full tank of gas without ethanol. Look at the web site www.pure-gas.org to see where it might be available in your area. I experienced the same symptoms you were with my 74 mgb. With the alcohol free gas, it runs just as well as it did when brand new.