1992 Yamaha big bear 350cc 4x4 1992

[Johnnie]

Hi I have a Yamaha big bear 1992 and I have just put a new carb on it and when I get to abit half throttle it starts to splutter on like it’s not sparking but when it’s under load it won’t really do it did this with original carb aswell thank you 

[Mech]

I'd try a new spark plug first, and then completely strip and clean the original carby, check for air leaks at the manifold, or any small vacuum hoses, and for blockages in breather pipes. If the original carb used to go ok, then that's the one I'd use. The other one might have a fault or not be setup for that bike. You are introducing uncertainty by fitting another carb.. Strip and clean the original really thoroughly.

The symptom you have sounds like the slide needle needs moving up, but I would check everything else before I did that because they don't usually adjust themselves..  It's more likely you have a low float level, a blocked jet, or the emulsion tube is blocked. The emulsion tube is the brass tube that goes up from the main jet to the slide needle. You need to take that out and clean the small holes in the side of it, and clean the drilling it fits into. Blow out the air jet near the aircleaner end that feeds air to that emulsion tube. Check the diaphragm on the slide for small splits too, they might only show up when the diaphragm is stretched. Check plenty of fuel is getting into the carb by undoing the drain screw and watching the fuel flows readily.