On the face of it, if only one cylinder isn't firing it can't be the rear
carb as that would affect a pair of cylinders. But the MGB has a large bore
balance tube joining all four cylinders so it's possible the 3rd cylinder is
getting just enough mixture from the front carb but the end cylinder isn't.
If it's only firing on three then you wouldn't really know if the 3rd
cylinder were firing fully or not.
It's a simple check to tell whether it's fuel or ignition, after running for
a few moments remove plug 4. If it's wet or smells strongly of fuel then
it's ignition. If it's dry and no fuel smell then it's fuel.
A plug can fire when laying on the block but not when in the cylinder under
compression pressure if the cap (or other HT component) is marginal and
breaking down under the higher HT voltage that is required to fire a plug
under compression.
After having the ignition on for a few moments it's another moments job to
remove the lid of the float chamber to check you have fuel in there, i.e. it
isn't a blocked float valve. If that's OK remove the air cleaners and blow
gently into the float chamber overflow port you *should* see fuel bubbling
up the jet in the intake to check the jet pipe and jet aren't blocked.
PaulH.
----- Original Message -----
> ... After re-inspecting the plugs, it appears that the 4th
> cylinder was not firing, but the plug was not gas or oil fouled.
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