High RPM Stutter (Custom Bike)

Put an airbox back on it. Those filters are a pain in the ass to tune and never seem to work right. Or you can keep throwing parts at it.

 

It's a custom bike - there is no airbox to put back on it. I've already tried all kinds of different intake configurations (see post #17) to change the airflow characteristics with no effect.

 

Get it on a dyno 3 weeks ago.

 

Did you mess with the TPS yet? Try rotating the TPS to either extreme and see what happens. Also measure the resistance across the TPS per the manual just for fun. I know that the WR450 has a well known issue with the TPS for road riding as holding the throttle at the wrong position causes a stutter on the road. Ironically it's at the 35MPH and 55-65 MPH steady throttle position, which makes it so much fun to ride as a dual sport.

 

Thanks for the idea. I did mess with the TPS. I measured the resistance at all throttle positions as instructed in the manual and it is functioning as it should. I also ran it with the TPS unplugged and it still stuttered.

The next thing I think I'll do is weld an oxygen sensor bung in to the header and hook up an AFR gauge to see what's going on in there at those higher revs.

 

"Functioning as it should" is not exactly "functioning as it needs for this application." I would at least try it at either extreme just to see what happens. One screw and 10 minutes to test.

Edit: Not being snarky! I was pointing out that the manual has the best intentions, but you are outside the limits of their design intent. It's a part of the custom application thing. I know that when unplugged the WR450 TPS defaults the "neutral/idle" setting and doesn't allow the ignition curves to engage, so you get a bike with great road manners but no power above 3k RPM. So maybe try a couple of different positions with TPS and see what happens (or doesn't happen).

 

So you mean run the bike at 7K+ RPM (where it stutters), but rotate the TPS sensor within the carb to see if it makes any difference? It only bolts onto the carb one way and is not adjustable as far as I know.

 

Yes, that's a good start. You might move the stutter around. I wonder if the CRF is smart enough to know what gear it's in, and if that matters?

 

Does it eventually blow through the stutter and return to making power? Could be valve float if it doesn't.

 

Yea, it does. If I'm cruising along at a steady rate at high RPM where it is stuttering, I can whack it open and it will pull away no problem all the way to the rev limiter.