Hey guys, first post here.

I've been reading around multiple websites and forums for information to fix the issue i'm having with my Big Bear. It had been sitting for 10 years, when we put it away it ran fine. Naturally I completely gutted the entire carb, boiled it in cleaner, the whole works. Put everything back into adjustment, turned mixture screw out to 2-1/2 turns. Everything looked good. choke wasn't working so replaced that. Threw a new spark plug on it for good measure, etc.

In firing up the quad, it seems to idle fine, responding to carb adjustments with the idle screw and such. However, when I rev anywhere past 1/4 throttle, the engine just starts sputtering, backfiring and in some cases it just completely bogs out and will die unless I let off the throttle. The only way I can drive it is in the first 1/4 range. Now I have completely cleaned everything, again, and there was absolutely no dirt in anything. I put a new petcock valve in because the old one was goofed. Before all this I forgot to mention I also drained the tank. So new fuel as well.

The problem seems to get a little better when I pull the choke out, but even then it persists. The problem worsens with the mixture screw turned in further. I have the needle on the slide in the max fuel setting, its as high as possible. There are no vaccuum leaks in the system, I checked that already. After driving it for a while, I pulled the plug to read that, and see that it's a nice brown color on the top, and a little black around the sides and on the electrode.

What I seem to be seeing is a lean condition. However even with all the adjustments being set to max fuel, i'm still running in to this. Thinking about setting valves but I think if they were that screwy they would affect the entire rev range. Not spark because fuel wouldn't make a difference to that anyways. So i'm left with a lean condition, or possibly not enough engine vaccuum to pull the fuel in and lift the slider. I have around 90 psi compression atm which, I know is a little low, but I didn't think it would cause that much issue! Anyways. Any help or ideas would be appreciated.

 

Double check the diaphragm for holes, and that the piston slides up and down in the carb without sticking. Re-fit the old plug, a new one doesn't always equal a good one. Check the fuel flow from that new petcock. My guess would still be dirt in the main jet or emulsion tube. I never clean carbs with anything except compressed air, and it seems to work for me.

 

 

So no holes in the diaphragm that I can see. I am taking off the valve covers tomorrow and setting the valve lash, I bet it's never been done by any previous owners and I just want to make sure it's not that. Valve lash problems can cause low compression and as a result low vacuum which could cause a lack of fuel getting into the cylinder. Anyone else have any ideas I could try while i'm at it? Running out of things to try here...

 

Your sure the air box is tight and no leaks anywhere in the intake?.

With the wheeler idling id spray starting fluid by the intake and carb boot etc area and see if idle changes. May have a small crack in the intake boot causing it to run lean.

As for the airbox, i know some machines are tuned to have the air box sealed tight for it to draw in exact amount of fuel/air mixture.

 

It sounds like a fuel restriction problem either in the carb main jet, fuel delivery, or in the tank itself. I had similar problem with a generator engine carb and spent hours trying to find/fix it myself. The Yamaha shop mechanic cleared the fuel passageway obstruction quickly because he knew where to look.