400 Big Bear timing
Have you ever adjusted your valves?
You could have blockage in your carb.
You can check to see if your timing chain is streched by looking at your adjuster. The adjuster should be on the rr side of the cyll. Pull the center bolt outalong with the spring. Then remove the upper and lower bolts on the adjuster. Check to see how many clicks you have left before its all the way out.
I have never worked on a bigbear. All other quads that I wrked on had chain adjusters like this.