Hello, I have just recently rebuilt my 1976 2.0 Litre, dual carby engine, it also incorporates a new Pertronix ignition system. The carbie I bought are Roger Penrose Racing similar to the Weber IDF-XE. Engine works fine, except a back firing problem, which I can isolate down to cylinder #1 only. The reason I think it is only cylinder #1, is that when I remove the spark plug lead from #1, back firing problem goes away and I run on 3 cylinders no problems. Other symptoms are as follows: - When I put my timing gun on #1 spark plug lead, the light does not consistently go off, indicating that the spark is not being sent every time per stroke. When I move the timing gun to all other spark plug leads, I get a very consistent light which is what I would expect to see. - When I disconnect the spark plug lead from #1 spark plug and hold it close to ground, I now get a consistent spark with no noticeable interruptions and also the timing gun light is also firing, missing no sparks. - I have changed spark plugs from other cylinders, no difference. - I have changed spark plug leads, no difference. - I have re-adjusted the tappets, no difference. - I have checked compression and all cylinders are around 125 psi. - I have not made any adjustments to the carbies, they have been installed "out of the box". Can anyone please help me with some more troubleshooting ideas, or solutions? Thanks in advance for your help.