Getting near the end of a rebuild on a QuadRunner that had been "played" with by several people before I got it. Many settings were disturbed.
It has a Mikuni BST-31 carb.
Does anyone have a manual for readjusting this carb.
I'm starting easily and idle is good. Exhaust looks and smells good (not burning rich) But I can't get it to throttle up. As soon as I hit the throttle it wants to die. I can force the unit to rev up...but the engine won't run stable. If I hold the throttle in one spot, it will rev up and down on its own. Just can't get it to run smooth except for idle.
I've been adjusting the "Pilot screw" for idle mix, and that seems good.
Then the "jet needle" for a throttle response. but this hasn't been very successful.
I'm suspecting the float adjustment may have been tampered with, but I can find any info on the net to tell me what the float height should be.
I'm desperate for some guidence.

 

Check for vacumn leaks too.Is the intake tight and leak free? Could there still be a blocked passage in the carb? Were things put back together with all the parts, and in the right place? what year is this quad?
Is the air box stock and the lid is on?

 

The unit is a 1997.
Total milage is less then 800.
I've got a parts blow up, and all parts are present, clean, and assembled correctly. Of the six different adjustments I'm aware of, 4 are fixed hole/port adjusments that can't be altered except to exchange them for larger or smaller sizes. The ports are all factory original sizes.
The carb to intake manifold connection seems tight. I'm not sure how I can check it for a leak. But I agree with your assumption, it does react as if there were a vacum leak. The carb has a vacum line going to the fuel pump and another going to the fuel tank supply valve. They are both good, I checked for leaks. The only other line is a snorkle line that goes from the carb to an area under the top shifters. It doesn't hook to anything but a "T" shaped plastic fitting. It seems odd, but I found nothing that it appears to hook up to.
I've had it running excellent on start-up, and running pretty good when warmed up. But I can't get both.
When I adjust for a good warmed-up throttle response, it sucks on a cold start-up.
When it runs great at a cold start, I can't get it to throttle-up when it warms up.
VERY FRUSTRATING.