Yamaha Big Bear 250 runs rough

[insanetreefrog]

Figured I'd ask here, seeing as how there's some really smart folks here...
My son's four wheeler, a Yamaha Big Bear 250, starts and runs, but runs pretty rough and "surges" and if left alone, eventually dies. It idles rough and when under throttle. It has a new OEM carburetor, and I always use ethanol-free gasoline.
Any ideas as to what could be the problem? I'm wondering if it has a fuel line problem, either an obstruction or clog, or clogged or bad fuel filter.

I've attached a quick video of it. What do you think is going on? Thanks all!

 

[Admin]

 

[DG Rider]

Did you do any adjusting on the idle air screw? Are there any air leaks in the rubber boot?

Pull the fuel line off and see how it flows.

 

[insanetreefrog]

Did you do any adjusting on the idle air screw? Are there any air leaks in the rubber boot?

Pull the fuel line off and see how it flows.

I didn't adjust the idle air screw, and the rubber boot looked fine when I replaced the carburetor a few months ago. Will check the boot more thoroughly and also see how the fuel line flows.

 

[russknight]

I had a 350 Wolverine and had that issue. Turned out the intake boot that connects the carb to the head was cracked causing a lean condition.

 

[sporttrac4x4]

Check aircleaner

 

[insanetreefrog]

Thanks for the suggestions!
Had a moment today to look at it. When you say "intake boot" or "rubber boot", are you talking about part #1 in the diagram? It looks OK from both sides, but I'm going to take it off and inspect it more closely.
The air cleaner looked OK to me, I also just took it off and ran it without it (after driving the four wheeler around a bit), and it still ran rough and surged.

 

[russknight]

Yes, part #1.

 

[insanetreefrog]

Got it, thanks! I don't see a crack in it, but there may be one I just can't see. I ordered a new one and it should be here by the end of the week.
I pulled the fuel line off the carb and noticed that no matter where I put the shutoff valve (off/on/reserve), the fuel still flowed constantly. Could that be related?

 

[russknight]

Got it, thanks! I don't see a crack in it, but there may be one I just can't see. I ordered a new one and it should be here by the end of the week.
I pulled the fuel line off the carb and noticed that no matter where I put the shutoff valve (off/on/reserve), the fuel still flowed constantly. Could that be related?

If your fuel inlet / needle valve isn't holding, yes.

 

[ByronM]

Why did you change the carb? Did it run the same before and changing the carb it runs the same? If so maybe it’s not the carb, how’s the valve clearance? Probably changed the spark plug but how about the spark plug resister cap?

When you say it has a new OEM carb, did you buy it from a Yamaha dealer, I seen some Chinese knock offs run like that.

If it’s not a vacuum leak it’s possible the new carb may have not been assembled or adjusted correctly, blow some carb cleaner though the passages and jets and double check the needle and seat and check the float level.

 

[insanetreefrog]

I changed the carb because the old one was a Chinese knock-off and needed a new bowl and jets. It wouldn't stay running, and was leaking gas out from the overflow tube (probably the float stuck I guess).

I got a new eom Mikuni carburetor and installed it, so that it'd run again, but now it's doing the surging, so I think it needs another part or two before it works right again.

We got the four wheeler in December, and the previous owner was pretty rough on it mechanically. He said it "never needed" an oil change, so he didn't do it. The oil filter was original to the machine. It's a 2009 model, so 11 years of possibly running the same oil, filters, and spark plug made it chug through some sludge.

 

[ByronM]

With that kind of maintenance record I am sure it needs a valve adjustment at least. Good luck with your project

 

[insanetreefrog]

Definitely a project, but it's fun. My 8 year old son is taking an interest to understanding what does what, etc. so that's cool.

 

[insanetreefrog]

Update: the intake boot did indeed have a tear in it. See pic.
I also changed the fuel shutoff valve, as the old one didn't work. The old one was halfway dissolved, was covered in gunk, and no longer had the screens. I'm going to rinse out the fuel tank too.
Thanks for all the suggestions! Really appreciate the assistance!

 

[russknight]

Update: the intake boot did indeed have a tear in it. See pic.
I also changed the fuel shutoff valve, as the old one didn't work. The old one was halfway dissolved, was covered in gunk, and no longer had the screens. I'm going to rinse out the fuel tank too.
Thanks for all the suggestions! Really appreciate the assistance!

View attachment 289145 View attachment 289146 View attachment 289147

You got it running yet?

 

[Jerryg]

Update: the intake boot did indeed have a tear in it. See pic.
I also changed the fuel shutoff valve, as the old one didn't work. The old one was halfway dissolved, was covered in gunk, and no longer had the screens. I'm going to rinse out the fuel tank too.
Thanks for all the suggestions! Really appreciate the assistance!

View attachment 289145 View attachment 289146 View attachment 289147

Judging from pics, you may have gotten debris in the jet passages of the new carb.

 

[insanetreefrog]

It's not running yet. I still have the thing apart. I'm going to flush the tank before I put it all back together.

 

[insanetreefrog]

Got it running last night. It's really smooth now. I cleaned the tank and installed a little fuel filter between the selector and the carb. Thanks for everyone's help and suggestions.