Vacuum advance misfire

[Mudguardandy]

So today I removed the electronic module and checked the wires thoroughly. Considering this unit must be 4 yrs old it still looks as good as new. I replaced, checked the baseplate was moving freely of all wires and fired up to 3000 revs. For a minute I thought the problem had gone. But gradually as I held the revs the misfire started creeping back in.
I’ll try changing to points next.

[Vic Butler]

It could be a fault in the wiring under the insulation. Is there an earth wire in the distributor?

[bogmoth]

It could be well worth considering whether there is a leak in the 'diaphram' that pulls the base plate back and forth in the distributor. This would cause air to enter the inlet manifold and cause symptoms similar to the ones you describe.
A quick and easy check would be to detach the hose at the manifold end and suck on it, then kink the hose and check that the contact breaker base plate does not return to to its resting place.
The fix would be simple - replace the diaphram unit - or the carburettor if feeling rich. I have never known anyone repair a diaphram unit.
They tend to be so very reliable, but obviously will fail eventually with age.
A simple cheap fix - if you are lucky. Hope this makes sense and might help. Alan.

[steve00136]

I found some good info about vac advance and added a link to it on my blog at https://mgbtips.wordpress.com/2019/11/0 ... uum-gauge/
Hope this helps.