TB42s missing, wont reve & back firing though carby

[threedogs]

Yeah, I just copied it all out of my Haynes manual. So an air leak may be the go. Spray some wd40 around the inlet manifold and outside of the carby, and see if the revs change at all.

That was my guess
Ah ha Dual fuel I think you are running too lean causing it to back fire on the condenser there is a small screw with a spring on it turn that out 2 flats of the hex to make the mixture richer, remember where it was. 99% of all gas problems are electrical, be it bad leads[dirty] bad plugs ,electrical system needs to be on top of its game to function correctly
Does it do the same on gas as it does on fuel, gas is a PITB it drys out carbies allows for hotter running temps.
will melt an alloy carby to a puddle in 4seconds flat if not set right

[GQ TANK]

Tree dogs, thanks for the feed back - but its doing it on petrol and gas. I had a simalar thought - swpaed in my spare mixer - still the same problem.

[threedogs]

you'll need to hook up some vacume gauges, they'll tell you exactely what you don't want to hear
might be your valve seats are shot from the gas and recessed

[Petrolling]

Did you find the problem mate?
I've been have some trouble with my carby lately, they are sh*ts of a thing

[GQ TANK]

Not yet - i havepulled the head - notong sstood out as the issue - will be fitting a spare head his weekend.

[Bushy674]

Not to be presumptuous, but firing back through carb speaks to a timing issue, how's all your connections for your ignition system, I am bad for the KISS theory, it's usually the simplest of problems that cause the drama, or in this case I hope it's the simplest thing that's easy and cheap to fix! Best of luck!!!!