Hi everyone
I'm working on a big bear 400 and having trouble
figuring the problem. I've took carb part twice cleaned it checked float level, jet needle and so on. The trouble is at idle it sounds like skipping and some times will even go dead. When I throttle it up about halfway it starts popping and backing thru intake and exhaust. It seems like its rich with smoke and a black plug. Could this be a valve adjustment problem they have never been adjusted. On hot engine comp. is 145 psi. Any ideas would be appreciated.

 

Rather than pulling a carb off several times,just install a carb kit to eliminate it as a problem first.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
Intake = .06-.10 mm \ .002-.004 in
Exhaust = .16-.20 mm \ .006-.008 in
Plus plenty of basic videos on youtube that can help you check/set valves.

 

Thanks for your reply. I'm fixing this for a buddy and I don't want to pull the plastic off unless I'm sure that valves could cause this. I put the carb and coil off of mine on it and had same results. The reason I'm thinking valves is the popping and backfiring come from both intake and exhaust. I know with out compression release it will naturally be hard to pull start but this one is crazy hard. Also it seems as the engine gets hotter it runs even worse. Is it even possible for valves to cause this problem. What about the cdi can it cause it to backfire and pop. Thanks again.

 

Tight valve clearances can cause backfiring also and the hotter one gets the clearances can decrease even further. Just check/adust the valves when the machine is cold to eliminate this as a problem.If it still backfires move on to the next thing to trouble shoot.

 

Water in the gas tank was the problem. My buddy said he drained and put fresh in so I assumed that wasn't the problem. Draining from the petcock won't get it all out if fact I got about a pint of straight water out after draining from petcock. Thanks again for your help.

 

That is an easy fix. I once fixed a push mower that had the same issue.

 

I have a 2001 Big Bear 400 4x4, put a new Carb on it and it runs fine on the lower end. it cuts out and sputters at the higher throttle settings. I adjusted the valves and it idles better, but still cuts out when you get on it in third and fourth gear. I've also noticed it blowing fuel back toward the intake portion of the carb when I get on it. The spark plug is black (running rich). is the carb bad??? or is there a way to null the carb/ set to factory settings? any help would be appreciated. R/ mudduck

 

Your new carb will need setting to match the old one. Check the jet numbers are the same and that the main jet needle has the circlip in the same groove as the old one. If it runs OK at idle, the pilot jet is set right, your problem is the main jet.