First, let me say thanks for all of the contributors here. I wouldn't have had the throught of diving into a new ATV project without a lot of help from these forums. Thanks.

I recently inherited a YFM250 BearTracker in reasonably good shape. Only thing I have found issue with is running a little lumpy at idle, which I resolved by cleaning and tuning the carb. It idles just fine now at about 1650 RPM according to the digital tach. I'm OK with that. The idle kicks up a bit when I pull the choke to first stop; goes wild if I have it all the way out.

I have an issue now with increasing the throttle; when I gas it it bogs down. It won't completely die, it will bog down to almost stall and then recover while it audibly isn't happy about it. I can feather it and keep it near 4000-4500 rpm, but can't get it any higher. It happens on and off of choke.

I have cleaned the carb twice and replaced the usual rebuild kit parts with the stock equivalents, checked the diaphragms, checked the vacuum operation of the slide. I eliminated the fuel filter and replaced the fuel line. I checked the bowl fuel level and it's between 1mm and 2mm above the mating surface.

I'm completely stumped by thing nearly dying at WOT throttle. There is a tiny 3-4 MM hole in the exhaust can, but no leaks near the the head, no vacuum or air leaks that I have found with smoke testing. The situation doesn't change with or without the air filter and/or foam.

Only thing I'm not good on is a new, fresh spark plug. I'm picking that up this afternoon.

I don't have the appropriate adapter for this heads threading to be able to measure compression, so I don't know if that is OK or not.

Does anyone have an idea that I might have overlooked or something to help get me through this?

-radio

 

New plug seems to make it weaker.

I took a video of the thing making me sad, it's uploading now...

 

they do make a compression gauge that has a tapered rubber end that can fit in the threaded hole. i have one and it works great on different quads. i'm still scratching my head on your quad's problem . hmmm....

 

link to the video I mentioned.


The clip on the main needle is at the bottom of three stops right now, I moved it there from middle stop a couple hours of troubleshooting ago. I'm going to move it up and check each time for progress/change.

what a pain. this bike should run better than this.

 

just double checking....did you pull the jets out and clean them or did you just replace them? especially the main jet. sounds like it is starving for fuel due to not meeting fuel demand. i'm leaning towards a fuel delivery issue here.

 

Cleaned the whole thing once, and wire stuck every orifice. Dipped it in Napa carb cleaner, and then cleaned it all out again. Blew it all out with air. Then replaced the screw, jets and needle.

I'm about to move the needle clip now, and will report back shortly .

 

Put the air box cover on, all problems solved.

This dummy learned something today. Small engines need recirculation air.

Thanks all those that read and Jaster for some input.

-radio

 

well i learned something too. i wasn't totally for sure on the recirc air, but now i know. thanks. glad you got it going!

 

First I wanted to say thanks to radio_spadio for taking the time post the solution to this problem. I have been wrestling with this same problem for a week and was completely stumped. I thought that I had read EVERYTHING about the Bear Tracker on the internet but somehow missed this.

The tags to this thread are a little lacking so I wanted to take the time to add a few that might help others.

1999, 2000, 2001, 2002, 2003 air fuel mixture screw needle pilot idle jet main floats intake manifold filter rich lean

Again, thanks radio_spadio

 

this guy is a genius!!!!
never would have thought it all needed to be buttoned up for the bog to go away!!