If you're at 3.5 turns out on the screws you definitely need to up your mains a size of two, or try the TBS needles or both. When I got my Scepters they came with new mains which I think were 135s and I had basically the same set up as you, but I'd cut the bottom out of my airbox. I still had a little popping on decel, but no backfiring which is what you don't want. I don't believe you will ever get these bikes to not pop some on decel. Afterwards, I got the FREAK which came with 160 mains and just recently I put in the TBS needles as well. The TBS needles work great, but you'll have to play around with possibly shimming them depending on the size main you use. At first I put in one shim and was way too rich because of my large mains, but once I took the shims out it ran like a champ. Try the TBS needles themselves first...if you still need to adjust the screw out more that 3.5 turns, it's time to up the mains. Also remember that the carbs probably won't take the same number of turns out on screws because of the mirrored setup of the carbs which evidently affects the fuel mixture flow inside. My left carb is always about 1 turn less than my right. One other suggestion. The screws in the top of the carbs are crap. They'll strip on you if you just think about it. Order some of the allen head screws to replace them and you'll save yourself a lot of headache. It's much easier to get an allen wrench in there than a screwdriver anyway. Best of luck with the testing.
