Got ATV out of barn after somebody else had been trying to resurrect it. I replaced the battery, starting solenoid, drained and replaced oil, air filter, fuel ****, new gas, and bought and installed a new carburetor. Right out of the box it worked with no adjustment needed.

Ran great for first 2-3 weeks while taking on test drives around neighborhood.

Let it sit for 2-3 weeks while doing other things around the place and then struggled to get started. Needs to be choked to idle and when I touch the throttle it bogs and will die within a second unless I release it. Sometimes backfires when I shut it off indicating rich mixture.

I took the carb off and cleaned all the parts I could get at. Main jet, choke pin, other ports. There was a little bit of fuel dregs in there which I wasn’t expecting. But overall pretty clean still. I didn’t clean the fuel tank very well so whatever old grime had accumulated in there came down to the bowl I guess. This go around I also installed a fuel filter.

Got her her running but it was the same as before. Idles ok with a lot of choke. Wants to die when I touch the throttle.

I’m going to pickup an 18mm spark plug socket so I can check that out but I really just suspect some important pathway in the carb is gummed up or the fresh ethanol laden gas melted some cheap part in the non OEM carb.

Please advise.

 

A good idea to check/change the plug. As you surmise, it is probably running weak due to that dirt blocking a jet or passageway. A death white plug nose will confirm this, though running on choke may soot it up, masking this symptom. Did you check the float bowl for water? Common on bikes that have stood, water gets into the tank, and tiny drops get into the float bowl, you don't notice when running but after leaving the bike a while the drops collect together and block a jet. Yamahas are notorious for the inlet manifold cracking and letting air in, this would also give a weak mixture, which gets worse when you have pulled at it to remove the carb for cleaning.

 

I checked my plug. See photos below. But they look about normal to me. I replaced with a new one properly gapped to 0.6-0.7mm. No change in performance.
Backfiring means it’s fuel rich doesn’t it?

 

Yes, plug looks near right. However I would still check manifold for cracks and give the carb another clean. Backfires can be rich, weak, faulty ignition, or timing.

 

I was doing some reading the last couple days. I'm suspecting a problem with the idle/pilot circuit because my trouble starts as soon as I have applied any throttle and that subsystem is what is doing the work in the carb at the time. I know there's an idle air screw on the underside / engine side of the carb. It could be clogged. The crux of it is that SOB can't easily be adjusted while the carb is installed and not after the engine is running at temp. Maybe I'm a dummy and haven't found the right youtube video yet. I don't have a flat head screw driver with a right angle attachment.

Why the heck can't I find a dang diagram of this carb online?? Seems like Mikuni have their distributorships pretty tightly controlled. All I can get are an exploded view of a parts kit, and some generic tuning into for other models of similar carbs to the Yamaha which I think is a BSR33.

 

The workshop manuals tell you not to fiddle with that idle mixture setting. If you are working at altitude or have heavily modified the engine, exhaust, air filter, snorkel or cam etc, then you may have to otherwise, leave it alone. It worked fine with that setting for 2 weeks so why should it need altering?

Is this the diagram you wanted? https://www.yamahapartshouse.com/oem...9d3/carburetor

 

I wanted to try it because literature says that the issue could be that pilot/idle circuit. As soon as I touch the throttle but the plate is still mostly closed it will die. Perhaps in that last two weeks something disolved or crud became loose and lodged in some passage. I don't suspect any weak spark problem because nothing really should have changed electrically in the last few weeks either.

As it turns out I pulled that screw, cleaned it and reinstalled it with 2x turns off seat and nothing is different about how it behaves. I havne't tried to adjust it while it's running, it's not positioned to allow that very easily.

I bought a Mikuni carb rebuild kit. Should arrive tonight. I'm going to put the original unit back together with those parts and see if it improves. Right now I suspect something is blocked up or something is wrong with some materials in the cheap after market carb I installed.

 

Today I cleaned and rebuilt the original OEM Mikuni carb using a $20 rebuild kit from “Race Driven” https://www.race-driven.com/2000-200...-kit-carb-kit/
and she runs great again! Set the pilot air screw to 2-1/2 turns off seat. Didn’t even have to adjust the idle.


New, cheap, nonfunctional carb on left Original Mikuni and new rebuild parts on right.

New jets and seats installed.

Ready to install

 

Good news. I took the original OEM Mikuni carb and rebuilt it from a kit
https://www.race-driven.com/2000-200...-kit-carb-kit/
It started up no problem and the throttle works as expected. This clears up the question about where the trouble was coming from.