OK....need some help. Purchased a 96 Timberwolf 4x4 a couple of months ago for ice fishing. It ran a little rough but had low miles on it and was super clean. Once I hit mid to upper throttle in 3rd gear on the engine it hesitates and really struggles for power. I just rebuilt the carb completely and it definitely needed it. Now it starts, idles, and runs AWESOME in low gears but I still have the issue with power loss on top end and higher gears. New air filter. New plug....I did go to a hotter plug thinking that may be the issue to.
Any ideas??
Thanks in advance!

 

Forgot to note that I do get a some backfiring and sputtering when I hit the mid to upper throttle position as well. Tried different carb adjustments still no help so I'm back to original 1.5 turns out on the fuel/air and 4th notch on throttle needle.
Checked for air leaks on intake as well. Is it possible the timing is off???

 

The timing is fixed by the CDI box and not adjustable. It is not likely the problem.

Backfiring is usually a sign of a lean condition. When you set the main jet in the 4th
notch, do you mean to the rich side of center, or the lean side of center?

If it is an ignition problem, then look for a damaged spark plug wire that shorts to ground at higher RPMs.

 

sr.ruedas
Thanks for the feedback.
4th slot was on the rich side. Tried the 5th without much difference as well as the 3rd which made things worse. Today there wasn't much in the way of backfiring just major hesitation and sputtering once I hit 3rd gear and mid throttle. I didn't check the fuel filter but there's plenty of fuel coming out of the line when I open to RESERVE or ON.

 

When you rebuilt the carb did you also remove and clean the petcock and dump
the old gas in the tank? Really bad gas (I have seen plenty) can cause poor running.
If your float bowl is not filling properly, that would explain poor high speed running, too.
Check the needle and seat again for debris.

 

No I didn't remove petcock and drain fuel but I'm sure it needs it anyway after nearly 20 years of use. I'll do that next.....

 

The YFB250 Timberwolf was Yamaha's entry-level 250cc adult utility quad in the 90s, and came only in 2WD. It was replaced by the YFM250 Beartracker, which was almost the same. I have had both, and they are decent basic utility ATVs. the photo shows a 97 Timberwolf.

As for backfiring, it is important to differentiate between backfiring into the carb, and backfiring out the exhaust. The former is what I was talking about above, and often sounds like a tapping sound with a temporary drop in RPM.

 

Love the pic....mine looks EXACTLY like that except is has "4x4" decals on it. Same colors too.
Thanks!

 

Hmmmm....wonder why you have "4x4" decals on it. Did the previous owner add them? Did they also add 4WD to the machine? They woulda had to transplant a whole engine/drivetrain.

In 96, Yamaha made 3 4x4s: YFM350 Big Bear and Wolverine, and YFM400 Kodiak.

 

Trying to insert pic for you but everything is original from Yamaha. It has blue 4"x4" decals on both the rear fenders and also in the Timberwolf logos on the gas tank.


Checked the petcock and fuel filters. Actually cleaner than expected. Only a little debris on the reserve filter. Drained tank and blew everything out and even went back through the carb one more time. As before, it starts, idles, and revs at low rpm like a champ but I haven't been able to run it anywhere yet to test out mid throttle and higher gears. I will post once I do.