Well the topic says it all. I have an older Yamaha Kodiak 400cc 2WD ATV, and all of a sudden it has decided that it is time to run rich! At first it was just idling horribly, and couldn't pin-point the problem. I put it up and the next day I went to crank it so I could move it to the garage for work, and it ran flawlessly. This on-off normal idle/erratic sputtering idle continued for a month. Now it won't idle normal at all, and I have reached my limit of ideas. I have sprayed carb cleaner into the intake with the air filter off and run carb cleaner that mixes wit fuel through it, and nothing works. I know how to play with carbs, having restored a 1952 Ford 8N tractor and a 1957 PowerKing tractor, but this thing has NO adjustments on the outside! All I can adjust is the throttle-stop screw, and that doesn't help.

The reason I know it is rich, is because yesterday when I stopped while riding, I noticed faint black smoke from the exhaust. I revved it up a little and a cloud of black smoke came out, it stayed idled up high for about thirty seconds, then it idled down and started sputtering again, along with the faint black smoke returning. Normally you simply reseat the air-fuel mixture screw and turn it out about 1.5 times, but since this carb has no screws, I am confused as to what to do. I am in college right now and do NOT have the cash to go pay Yamaha to do something simple.

So is this something I can adjust? If so, how do I adjust it??

 

The mixture screw is on the underside of the carb, just in front of the float bowl It might have a brass plug over it, which will need to be pulled out, to get at the screw..and the mixture screw will only effect the idle to 1/8 throttle .
How the air filter element....cleaned and reoiled?

 

If the problem came and went at first then running rich is more likely a symptom than the cause. If you have weak spark at times it may not burn the fuel well or completely causing it to build up and run rich but adjusting the carb wont solve that. Most things that come and go erratically are electrical.

 

I cleaned and oiled the air filter, so I don't think that's it. I also doubt that it is the plug due to the fact that it runs so well at higher throttle settings. I have also read a few threads where a clip that prevents the pilot jet from rotating comes loose from time to time and can make it run rich or lean. I really do feel that this is a carb issue, but how could I check the strength of my spark without frying my multitester?

Oh, and the Yamaha service manual for the thing, it does not show an idle screw on the outside of the Mikuni carb.

 

Theres no clip on the pilot jet. Its just screwed down snug. They might have meant the mixture screw can back off, but theres no clip on that either.
Have you checked the enrichener, on the carb. They call it a choke, but its really an enerichener, and if the rubber piece inside the carb is leaking...itll run rich.

 

Haven't taken it off to tear it down yet, but I will keep that in mind. I have to figure out how to get the control arm that the throttle-cable goes into to come off so I can access the hex screw behind it to take that unit off before I can remove the carb. I did unscrew the choke cable from the carb and the spring was starting to rust, and there was dust in there, so maybe it is leaking.

 

Alright, after disassembling and reassembling the carburetor, I don't think it is my issue. Is it possible that a lock-nut on one of my valves came loose and is causing it to run rich? They're easy enough to get to, but I am not sure I want to unless it is a possible solution.

 

Remove and completly strip your carb. Acid dip it in one those 1 gallon carb. cleaning pail for 15 mins. Blow all orifices out with carb spray followed by compressed air. Check and reset your float level to OEM spec. Reassemble and install. Also check your fuel tank for debris and clean if needed. Check the carb mounting rubber for cracks,leak etc........ Air filter ?