Can someone please help. I have cleaned and reset this carp to specs but it is still spitting back through the carb. Could this be a valve problem? Also the owner has gutted the muffler, does this have any effect on this machine.

 

Could be the valves, get a manual and adjust them, Ite pretty easy on the Big Bear.

check your spark plug. See if it is a nice tan color. Or replace it.
What do mean gutted the muffler?
If they "gutted" the muffler then u may be running lean.
What throttle position does it sputter at? 0-1/4, 1/4 to 3/4, or 3/4 to full?
clean and re oil your air filter.
I have a 99 Big Bear also.
I had to completely disassemble the carb, including removing the jets and blowing out each jets chamber with carb cleaner and compressed air and adjusted the valves...
It runs like a champ now.

 

Thanks LAcop. I think I have eleminated the carb as the problem I took one off of another bike that was running good and it did not make a difference. I may have and exhaust valve not seating properly or sticking. It is firing back though the carb from 0 to half throtle but if I am gentle with it sometimes it will throtle up but start to cut up again when I come down and try to go back up with it.