1936 4F/6 OHC Square Four - spark plug caps

[orabanda]

Hi everyone,
Almost finished a friend's bike.
It has been fitted with NGK spark plug caps, and the engine backfired from 1/3rd throttle. Putting the choke on reduced the backfiring, but it still occurred. I richened the pilot mixture and lifted the needle. This was an improvement - the backfiring  firing has reduced but it is not eliminated.

The NGK plug caps have a suppresor fitted (5 KOhm).

before I try a larger main jet, am considering replacing the suppressed caps; opinions sought.

I have also ordered four new plugs (B7HS).

Cheers,

Richard

[groily]

My cent's worth: If it's still running on the magneto, I'd replace them Richard. Might make no difference at all to this particular problem, but as a general rule suppressors and/or resistor plugs aren't ideal and make the mag work harder than it needs to. If the ignition system has been upgraded to something more modern, then I'd leave them.
I know suppression is a legal requirement in some places, but with digital radio and all that I doubt it really matters any more.

[Triton Thrasher]

As a test, you could remove the plug caps and secure the wire cores to the plugs with the little nuts on the plugs.

Failing under throttle load can be an ignition HT problem: insulation cracks, tracking through dirty surfaces, distributor cap, faulty ignition coil.

Can also be restricted fuel feed, water in carb, main jet fouled.

[muskrat]

G'day Richard.
TT's suggestion of HT wires direct to the plugs should eliminate it if that is the problem.
I found it hard to find non suppressed plug caps.
Cheers

[Adler]

Hello Richard,
is the backfiring through the carburettor or through the exhaust? Or have you already found the root cause for the problem? Would be interesting.
Best regards, Manfred