I have an 07 Big Bear 250 with a Mikuni carb but for the life of me I can't figure out which model it is. I've tried searching Mikuni's web site but have been unsucessful finding anything that looks like it. I'd like to get a rebuild kit for it but i want to make sure I get the correct one. Also maybe I'm crazy but is there no idle or air/fuel adjustments on this carb. I've attached some pics that will hopefully help somebody identify the model for me. Thanks

 

Have plenty of them on ebay. Probably a BST 34 Mikuni. http://www.ebay.com/itm/Yamaha-YFM-250B-Big-Bear-CARBURETOR-CARB-REPAIR-KIT-2007-2009/320814901056?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222008%26algo%3DSIC.MOTORS%26ao%3D1%26asc%3D20643%26meid%3D5440760676669969811%26pid%3D100005%26prg%3D9209%26rk%3D0%26rkt%3D6%26sd%3D360870912405&rt=nc
Supposed to have a cable operated idle adjustment. Ain't cheap,similar to the one for Polaris Mikuni carbs. Looks like yours just has part of the cable left. Item #39. http://www.cyclepartswarehouse.com/f...07&fveh=175024

 

Thanks for the info. The cable you see in the pic actually goes inside the throttle housing and controls how far you can depress the throttle

 

Cable I'm looking at sure looks like a broken idle adjuster?? ..This is the spot for it as the opposite end looks like the opening for the throttle cable that warps around the butterfly wheel channel and inserts into the cut out?? Have to turn my head sideways to look. Click on The Cycle Parts breakdown link I posted.

 

Okay now that I look at the first pic i posted which has the throttle cable removed and compare it to the diagram i see what you mean. All the small ATV's I've messed with in the past had a screw with a spring behind it that you used to adjust the idle. This one wraps around the carb and is held in place by the s shaped bracket (see pic #2). Nothing is broken that I'm aware of as it does increase throttle when i turn it just never saw it setup that way in my limited exposure to ATV carbs. Thanks again for the help and forgive my ignorance, kind of new to ATV maintenance

 

Thanks for your help. Got the carb cleaned up and everything installed back on the ATV. The question I have is how I know when my valves need to be adjusted. The ATV starts right up with no choke and the throttle response is good. The issue I have is with the idle. It idles fine for a minute or two and then for a lack of better words the engine stumbles and shuts off. Starts back up with no issues just can't get it to idle correctly. Thanks to the video you posted of the guy explaining the air mixture screw versus the fuel mixture screw I'm going to try and adjust my fuel mixture screw tonight but if i read your thread correctly you also mentioned the valves might need to be adjusted. What do you think?

 

Maybe both might be needed. Valves first as these are what a lot of people neglect. Bear Tracker video might be close,but check your clearance specs on your machine. Piston has to be at TDC. Check/adjust the valves if needed. if valve adjustment doesn't solve the problem and you're sure the idle is adjusted right,may need to adjust the fuel screw,BUT usually these are sealed under a small aluminum block on most Mikuni carbs,but can be removed if your Yamaha has the block. Just use a small drill bit(carefully!) then use a dry wall screw to remove the block. Count the turns you have now when turned in to a lightly seated position on the fuel screw just for reference. Start out at 2 turns out. This screw is best adjusted as the engine is running at an idle and warm. Like in the other videos on carbs you've seen,turn screw out to the highest rpm reached until rpms start to drop,then turn in until the same thing,counting turns both ways. Usually a setting between both points is where you'll end up at. Somewhere between 1 1/2 to 2 1/2 turns usually. Main thing is that you have no hesitation or bog when throttle is applied.

 

Sorry for the delay in getting back to you. I had to get to camp to bring home my utility trailer. I decided to take it to a shop that I've used in the past and they found two issues. Idle jet which I had cleaned previously had some blockage which they attributed to the gas. I didn't think it was that old but he said when he drained it the color was way off. Next problem was the exhaust valve was sticking. Runs great now. I was a little hesitant to attempt the valves myself which is why i took it to the shop. As far as the gas that was self induced, lesson learned. In any case thanks for your help

 

All it takes is a little trash/dirt in the jets. Keep stabilizer in the fuel and hopefully it'll do good for you from now on.