Backfire on start-up

my 79 3Jota is very sensitive to battery load wth regards to backfiring on startup. That and ABSOLUTELY NO throttle before or during starting. The minute i even look at the throttle, i can bet my right hand for a cracking backfire out of the zorsts.  Choke at least 3/4, but full open works best.
 
Half, 3/4 or full choke doesn't mean much. Every bike is different because they won't all have the same free play in the choke cables. To find your optimal choke setting you need to experiment on your own bike. Try Lothar's recommendation of pulling the choke on a bit at a time. It might also help to check that the free play in the choke cables is the same for all carbs. It might only be one cylinder that's responsible for all the backfiring.

BTW, we all call it a choke, but we're all wrong.

Cheers,
Cam
 
I sure Keith has diagnosed it accurately - Low coil voltage.This I found very common on low charge BTZ units as fitted to many1200's (also served warning when battery voltage was getting low.)  In standard form the current path is quite long and with aging connections after it has passed through  fuses, ignition switch, kill switch,ballast resistors and other assorted connections you are lucky to achieve 10volts at the coils.
The fix is to scrap the ballast resistors (if you haven't already) and the other thing you can do is fit an ignition relay switched by the existing circuit but giving the coils full voltage direct from the battery (via the relay).
Cheers
Norm.
 
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