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Do you drill through the top of the "mixture screw cap" to get at the mixture screw? Or can I pull it off if I pull the carby out (again)?

There's a bigger story... basic amateur carby overhaul; now have backfiring in exhaust when changing gears up unless I'm very gentle. Played with timing and distributor is now at full rotation.

Seem to be using more fuel, have a fuel smell, and what seems like fuel smoke. I've put the hose onto the carby while running and nothing happened, so I'm taking that to confirm that I sealed it back up ok.

Maybe there are other things I need to check (new spark plugs, leads, ...), but this all happened after I cleaned the carby out, and really looks like too much fuel going in to me (but, then, I'm pretty amateur even on my good days!!)

cheers,
Francis

Drilling out the cap to get at the mixture screw wont help by the sound of it, you have a problem somewhere else in the carby. The mixture screw only changes things at idle.
Sounds like its runnung too rich. You may have a thing in the carby thats called a power valve in the bottom of the float bowl. It is usually closed and a drop in manifold vaccum, ie full throttle, is when it is opened and it may be not shutting properly now and therefore letting in more fuel than what is required.