2008 YZ450F - Rough Midrange

Anyone care to help me out here? I just picked up a beautiful 08 YZ450F. Looks like it was ridden just a couple of times.

It has strong top-end, runs great on idle, but it's really rough on mid-throttle. If I slowly crank on the gas, it shudders and jumps throughout the midrange.

I've replaced the gas, changed the spark plug, and pulled the carb apart (looks like new).

Ideas? Any help would be greatly appreciated....



Start by looking at some possible causes within the carb. Note the needle position. Should be near the middle, in groove 4 from the top. Then, look at the vacuum release plate on the slide. It's possible to install this plate upside down so that the "round" end is down, and even looks as if it should go that way to many, but it needs to be assembled with the "square" end (with the hole near the edge) down. It also needs to be free of chips and cracks, but it's unlikely to have any. Also be sure the seal between the plate and slide is in good shape.

If all of that is good, unplug the TPS and try the bike without it connected. This will set the throttle load signal to the CDI at a default value, and timing will be adjusted only based on RPM while in this mode. If the problem goes away, you have either a a misadjusted or defective TPS. Check the manual for testing and adjustment procedures. One test that won't be listed is to read resistance across the black and yellow leads while you slowly open and close the throttle. The resistance should rise and fall smoothly as you do this, without any sudden dips or spikes.

Gray - thanks! The TPS is something I can try immediately.


update - TPS unplugged, vastly improved midrange. looking to price a new sensor out tomorrow.

gray - thanks again for the help.


That Grey, he's good people....

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