Help needed, Fixed

Thanks for the replies,

I also rang Nymeyer Automotive at Belmont NSW, explained the situation and was told to check the idle jets, they also explained that a backfire would not damage the motor internals. which was a major relief.

I had added another O ring to the original ones on the idle jets to make a better seal, but my thinking was wrong making and this had caused the problem.
I checked and removed 2 and 4 first that made it drivable but not perfect but heaps better

Today I checked 1 and 3 idle jets and removed the additional O rings and as yet have not started the car as I didn't want to upset the neighbours today.
I am sure it will be fine.
Many thanks to Nymeyer Automotive for their excellent advice,
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I changed the plugs on a 1915 motor, without side body cut outs, (bugger of a job). twin 40mm carbs with new kits fitted, 4 new plugs

Checked the dizzy, and crossed 1 and 2 plug wires (and me thinking I knew what I was doing) anyway I fired it up than a big backfire, left it for a while and tried to restart and another backfire.

Checked all the plug wires and found 1 and 2 crossed (swore at myself for being so stupid.)
Pulled off plug wires at the dizzy for 1 and 2 and it made no difference still not working.
Tried 3 and 4 and motor slowed, conclusion they are working.

Now motor only running on 3 and 4,
Checked fuel spray on 1 and 2 and working.
Pulled out N0.2 plug and tested with the lead to No.2 on dizzy and it was sparking, didn't try No.1

Haven't pulled off rocker cover yet or a pressure test on 1 and 2, have to get a gauge tomorrow.

Any idea what has happened.

I think I may have damaged something internal, (I hope not)

Thanks in advance any feedback will be greatly accepted and appreciated

Does it go ok away from idle?

Carb sync or blocked idle jets

Check you havent dropped a push rod off

lots of things to check