1966 4.2 backfires and wont idle

The engine was running fine and suddenly we have difficulty starting, and backfire when we try to take the revs to 150. Points are right, timing is right, coil checks out at 3 ohms,(same as the coil in a V8 I have in my LR), The one test I was not able to do successfully was to put a tube onto the vacuum advance and suck.I did this and the distributor plate did not move.
I assume there is a diaphram in the vac advance and it may have split.

Any advice greatfully received.

Backfire through the carbs or exhaust?


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It can definitely be the condenser. I had brand new one that did this. Other possibility is a very lean condition, but that’s more common under load, not just sitting there.

Through the exhaust.

I like the condenser as a culprit. We recently accidently left the ignition on for an extended period and i thought we may have cooked the coil, but maybe the condenser took the hit.

Many thanks for your responses.

If the condenser doesn’t fix it look for a vacuum/air leak; hose fallen off somewhere etc.

I might expect backfire though the carbs from an overly lean mixture.

I agree with Nick.
Maybe fuel pump is faulty

Any update on this?

Pet peeve: backfiring is through induction system: afterfiring is out exhaust.

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Surely it must be frontfiring as the opposite of backfiring.

As a member of the medical profession I feel it my duty to ask if “frontfiring” is analogous to “frontbottom”?

Me too. Air getting in somewhere.

Hi, Thank you all for your input. A new condenser fixed the backfire and cleaning the plugs made it run much better. All that remains is an intermittent power loss which may be the coil (which I cooked) or a faulty wire. New coil ordered…we shall see.

Thanks again for your interest.