Hey all, I got a 2002 big bear that I'm having problems with. I did a top end rebuild on it last year due to oil consumption and ever since its been back together it has been backfiring on decal. I installed a bigger pilot jet and adjusted accordingly. That didn't fix the problem. I then installed a bigger main jet and that didn't fix it either. I removed all of the exhaust thinking there might be a pin hole in one of the pipes with no luck on that either. I reinstalled using a little rtv on the header pipe to make sure it wasn't leaking at the exhaust gasket. That did nothing also. After that I installed a new timing chain cause the other one was stretched. I completed that and adjusted the valves and I thought that I had it cause it barely did it after that. The next day I took it again to go riding and it was backfiring out the exhaust just as bad as ever. The spark plug is a grayish color which seems fine to me. Can anyone tell me what I'm missing? Any ideas would be greatly appreciated. Thanks

 

Something as simple as as a leak in the intake manifold or worn parts in the carb could still be causing the back firing. I'd check the manifold out and install a cheap carb kit since it's 12 years old and see if that solves the problem. If it doesn't then you've eliminated the carb as a problem along with valve clearance and other things you've looked at. Could end up being an electrical problem if all else checks out and that's where a service manual comes in handy. http://www.ebay.com/itm/Yamaha-Big-Bear-400-2000-2001-2002-Carb-Carburetor-Kit-/380908087044?pt=Motors_ATV_Parts_Accessories&hash=item58afe42704&vxp=mtr Here's a free link for a service manual.yamaha big bear 400 4wd 2002 2004 service manual.pdf