quite simply, if you're getting a lean backfire, you've put the choke in too soon.

That is, of course, assuming all other aspects are in order.. carb adjustment, timing, etc.

Yep, it's totally trial and error.. every one of these is different. I generally gradually push the choke in, not getting it all the way in until the car is warmed up. And if it's really cold, sometimes I'll yank the choke for a second when leaving taking off from a stop, just for a second..

Tweaks in ignition timing can help too. Both with running and starting. Different settings for warm and cold weather.

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-Matt I ♥ my ♂