1992 350 Big Bear Cuts out under load

[speely31]

Currently working on a 1992 Yamaha Big Bear 350 that idles perfect but cuts out with any kind of throttle. We replaced carburetor and head to carburetor boot. 

Put in a new spark plug, the old one was black as hell, unsure if it was caused by old carburetor or current misfire. This problem similar to the rev limiter you see on newer models when

differential is locked or in reverse. I don't think this machine has that feature but I could be wrong? I suspect an ignition component is breaking down but at the moment I don't have access to a wiring diagram or workshop manual but I'm working on that. Are the e-manuals any good?

Any ideas?

[quadmaniac]

Did you figure it out? Almost sounds like a possible timing issue.

[Curtis Underwood]

I have had this on several machines I have worked on and every time it has turned out to be one of the following. The air filter not on machine, cracks in the intake adapter, float not adjusted right, carb air fuel mixture not set correctly, or intake/exhaust valves not set correctly.  I would start by first making sure the air filter is on the machine these ATVs will not run good without the air filter in place. The second thing I check is to make sure there is no cracks in the intake adapter and to make sure it has a good seal(this will give it to much air and cause this type problem). Next check the float level in the carb to ensure it has plenty of fuel in the bowl. I do this with carb on machine by taking a clear hose and putting on the drain tube of the bowl opening the drain with the tube running up the side the carb. If fuel level shows where the top of bowl lip meets the carb then float is set correctly if not carb will starve for fuel when you push the throttle.  Another problem that can cause this is the valve setting. If the valves are to tight or to lose they will cause this type problem.
On one occasion I had a coil bad that when engine was running at high rpm it would start cutting out and miss firing. But it would rev up just would not go down the road at higher then 1/3 throttle.
From your description I do not think this is timing or ignition problem.

[spock58]

I'm in agreement with what was listed above. All important stuff for sure. The airbox & filter should remain stock on utility quads. The CV style carbs they use depend on this design. You cannot just run an air filter that just bolts on the carb with stock jetting - it's a hassle to get them running properly. I'd like to add that the diaphragm on top of the slide on these carbs is important as well. They must seal well or you won't get proper acceleration.

[speely31]

Problem solved! I did resistance checks on all ignition components and found high resistance on the secondary coil windings. Replaced ignition coil and it runs awesome!

Thanks everyone for your help!

[cookmb11]

Great idea on the float level test!