Recently switched exhausts (Monster to Empire) & its spitting back through the carb, backfires through the exhaust, it does both simultaneously only on startup once the engine is warm. Jetting is 45 pilot & 2 1/4 turns out, I wouldn't think that would be lean, it doesn't backfire on de acceleration. I cleaned the carb thoroughly, blew it out with compressed air. New copper crush gasket when I put the new pipe on, no leaks in the intake, just adjusted all the valves recently when I installed the CRF cam, timing is dead on. I even tried another 450 carb and it still does it. This thing has been driving me nuts lately lol, any ideas on whats causing this?