Rob,
I think you said you have a 1600cc SP. Does that come with a 34 PICT-3 ?
I had the same problems with my engine (type 4) and it worked out that the idle circuit was lean. I don't think you'd have the backfiring with a rich circuit. I'm just learning how the carbs work as well.
Do you have any vacuum leaks ? This could cause the idleing to be lean as well as air is getting into the mix.
Hope this helps in the meantime.
Sandeep
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