Intermittent misfire

[12jslater]

Right now I'm stumped, my car has an intermittent misfire "pop pop pop" when idling and when being revved.

First I found the carburettor was pulling air where the float bowl meets the body, I then trued the surfaces with a file and sandpaper and made a paper gasket to ensure there is not even a whisper of air getting through, I've checked timing, coil, I've even borrowed a friend's electronic ignition from his distributor, there are no air leaks on the manifold, no blocked jets, I've tried gravity feeding the carburettor with some fresh fuel, tried raising the float height, replaced the distributor cap and rotor arm they were looking tired, same goes for spark plugs, and new HT leads (the cars been stood for years so needed those anyway) even checked in the dark for stray sparks and yet it STILL misfires.

I'm pulling my hair out, any ideas as to what it could be?

[Reckless Rat]

You haven't said what kind of distributor you have, but the Lucas types do need to be properly earthed. Normally the screws that go from the top half through the bakelite to the bottom part do the trick but sometimes a belt & braces approach can solve an annoying breakdown in continuity.

[12jslater]

Its a Lucas distributor on a big seven, I've ran a lead from the bolt holding it to the block to the positive side of the battery and it didn't change anything.

[Tiger]

Compression check may help.
Was it previously running ok.
Could be a sticking valve

[John Mason]

You have said that you have changed the ht leads and rotor arm. Have you replaced the condenser which if starting to fail will cause miss fires.

John Mason.

[Tony Griffiths]

Did you clean or replace the points? Left standing for a long time the points, if open or closed, can cause problems. One one motorcycle engine I had, after standing through the winter, there was no spark at all. A pull through the points of some almost talcum-powder-smooth emery cloth and it fired up.

[12jslater]

Yes, points and condenser are new, tried putting the old ones back on, and no change, even tried borrowing a friend's distributor with eletronic ignition,  getting a nice fat blue spark with all set ups (points and electronic)

[12jslater]

Went out today and have double checked everything, tried different petrol, attempted to run on just starting fluid sprayed into the intake, checked exhaust to manifold gasket, checked all other exhaust joints, checked compression, all high and within 6-7psi of each other, no plug is cleaner or dirtier than any other, all a nice tan colour, so no steam cleaning from a head gasket leak...

Attempted running coil straight from battery too, I'm absolutely stumped with this, I can't think of anything else, it runs nice and quietly but then "pop pop pop" despite everything appearing absolutely fine, valve gaps spot on too, no vacuum leaks either as I used an unlit blow lamp gas around throttle spindle, gaskets, manifold etc, I have even tried an Austin A30 carburettor zenith 26vme that I have in the garage that was working absolutely fine until last summer, and the EXACT same.

Anyone have any idea what it could be.

It's driving me mad haha

[Ivor Hawkins]

It’s not unknown for some brand new condensers to be a bit dicky, can you get your hands on one which is known to be okay and give it a try?

Ten to one that sorts it!

[12jslater]

I've tried a condenser from a chap down the road from his Morris minor with the early sidevalve engine and it still misses, as I said I have even borrowed a friend's distributor base plate with electronic ignition.

It's very confusing