Any other mods (pipe, cdi, air filter, anything that will affect how it breaths?). If it's just snorkels you should be fine with the stock jetting, I had snorkels, K&N filter and muzzy pipe with headers and jetted from stock 158/155 to 160/158. I'm in Grande Prairie, about 2300 - 3000 for most riding, ran good. If it's backfiring through the pipe jet down, if it's popping back through the carbs jet up. Shouldn't have to touch mains or shim needles yet.
Got it running fairly good now. I'm at 2900ft and ride to 4500ft. I'm running a 140front/144rear (dynajets) with the needle on clip 3 & airfuel screws out
1-3/4" turns.