I am new here but I was hoping to get a little help.
I have a 2006 yamaha grizzly 660 and it is not running well.
It starts and runs but tops out at about 28 mph then it starts sputtering and back fireing. I drained the tank and put fresh preim. fuel and changed the spark plug, changed oil, cleaned air filter nothing helped. 2 days ago it was running great. I was driving it and it just losts the top end.

What do you think??? any help would be great

Crap stuck in the carb jets or the muffler interior is caked with mud and not allowing exhaust flow.

not the muffler. I am working on getting the carb screws off. Will let you know what I find

Sometimes just draining the float bowl will fix the problem, other times it will not flush out and the problem will return.
There should be a flat screw on the bottom of the bowl that you can loosen off (NOTE: not the air/fuel mixture screw)

I took apart the carb cleaned the jets and anything else that I could poss. clean. It is running Great now. Thanks for all of the help

I would put a fuel filter between the tank and carb to stop any more dirt from getting in there. It would hurt to change the plug and check the valves.

Fuel filters are a good thing

My grizzly was doing exactly the same thing. I get upto exactly 28 miles an hour and it starts cutting out. The problem has come and gone. I took out the bottom of the carb and it was spottless. The jets were clean and the the tank filters were clean. I changed the spark plug and no luck. I changed the gas and no luck. It is very perplexing.

The problem exists on the needle jet as the throttle reaches a certain position. Any help would be welcome.

Hydroguy

Another thought, could it be the rev limiter kicking in?
Sometimes water gets into the front diff electrical connection and involks the rev limiter. Remove the connectors, clean and apply di-electric grease.

Yup, rev limiter. That's the speed it kicks in at when in diff lock. Hold in the overide button and see if it runs better.

Dmetzler wrote:

I took apart the carb cleaned the jets and anything else that I could poss. clean. It is running Great now. Thanks for all of the help

Every once in awhile, specially if riding in the rain, add a couple of ounces of Isoheat (I believe that's the name). This product is sold at NAPA and other automobile parts store, and is used to remove moisture from the gasoline in the fuel tank. I use the red-color bottle, not the "Heet" one which is yellow.

Mine did that couple times buy it would quit....the vent line on the gas cap was almost plugged off not letting fuel into the carb