New here and looking for some help!

I recently picked up a 1997 Polaris Magnum 425 6x6 and am having issues keeping it running. The ATV backfires upon start up and I have to throttle the bike to get it to start. I have cleaned the carb, installed new jets and fuel mix screws and set everything to stock settings, replaced the fuel pump, cleaned the fuel tank and mesh screens at the petcock. It won't idle and will die as soon as I let off the throttle. Seems like it's flooding out considering I have to go full throttle to get it to start and I am finding fuel in the downpipe. Exhaust leaks through the slip joints at the muffler but there is no rats nests or blockages. Air flow through muffler isn't great. Have not torn apart the engine yet. Bike also has trouble turning over from time to time. Seems to seize for a second, neutral light dims and then comes back. Installed a new battery as well.. Any suggestions or thoughts would be greatly appreciated.

Thanks!