I have a friend who has a stroker 440 (460 kit) and he runs an TRX450 FCR carb, but he also has a ported head with larger valves, stage 3 cam, no air box and a custom large diameter header with a Supertrapp muffler and the whole thing is dyno'd for maximum HP - he races and this weak link now is the drive line - he breaks transmissions 3 to 4 times a year, goes through axle bearings like his dad works at Lonestar Racing and buys drive chain on 1000 foot spools. But he has more money than brains and spends more time working on his quad than racing it.
Best carb for a simple big bore kit is the stock OEM carb with minor jetting changes to compliment the modifications to the exhaust and intake.
I'm not there to listen to it or ride it, but what you describe sounds like it might be jetted too rich on the main jet.
I have a 450 carb on my 400 and it was a easy mod just you have to tune it and run something like near 170 jet size. You can use the stock intake boot just you need to drop into boiling water and let it sit for about 15 seconds and right away force onto the carb. I don’t remember if I put the 450 throttle cable or left the 400 one. You will need to plug in the hot start with silicone or job weld. Cut the TPS wires, reroute the breathing lines on top. The only drawback is you can’t use choke but you will still be running lower than lambda 1 so you should start above freezing temperatures no problem.
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