It's against my "religion" to have anything to do with "musical instruments", but.....why the big jump in main jets ?? Is this engine THAT radically ported ?? Don't forget that each 10 numbers in main jet size gives approximately 5-10% increase in fuel. Have you also upsized the carbs ?? How much bigger ?? What you MAY be getting here is serious overfuelling when you crack the throttle open 1/3 to 1/2 or so. This fuel "slug" will coat/puddle in the intake tract and/or crank chambers and allow the engine to keep running after the slides are closed. As the mixture leans out, revs increase....meanwhile the increased engine vacuum with the slides closed pulls in more fuel through the larger pilot jet to keep things going. If the engine has stock porting and you have only added chambers or low restriction intake filters, I'd start off with 2 sizes larger main jet and then start making test runs at no more than 1/2 to 3/4 throttle and TAKING NUMEROUS PLUG READINGS. When you have the idle to mid range sorted out, then try short blasts (1 or 2 seconds at full throttle under load) to check plugs AGAIN AND AGAIN. If you're not too confident of your abilities, you could try one colder heat range of plug to help prevent the dreaded seizure / piston hole. What the devil is a Banshee, anyway ?? Is that some kind of electronic piano ?? Just kidding...I can't resist the cheap shots at the Tuning Fork company. See...I can't even spell Yum...Yom...Yem...aww, to heck with it. LOLOL Let us know how you make out.