Street Legal wr400 chopping at low RPMs

WR400 street legal: shaking engine in the bottom/middle end below 3900 rpms

I have 1999 WR400 street legal that I ride to work. I live about two miles away and the bike only sees about 35 to 40 MPH with one stop sign all on paved roads. I religiously keep the oil and air filter clean along with all the other maint.

It has a Power Bomb Header, Baja Light Kit and Trail tech Vapor, 2001 YZ426 carb and DOT knobbies. (Tires are even balanced also.) Everything else is stock.

So here is my problem. When I’m in second, third of fourth, fifth gear running anything below 3900 RPMs my engine or the entire bike, starts to shake really bad. The vibration is transmitted to the entire bike, almost like I’m riding down a dirt road with all the ripples that the wind makes. I’ve tried and checked everything, balanced my tires, under inflate, over inflate, compression test, suspension loosen, stiffen…tight chain, loose chain.everything. The only way to get rid of the problem is to pull the clutch lever in just a bit with my two fingers, only enough to take up the slack in the cable. Then its smooth riding. The clutch doesn’t slip nor do the RPMs go up and my speed will stay the same. And there is no smell of burning clutch so I know it’s still making good contact. So my question is: does pulling the clutch lever in anyway change the mapping of the ignition.

P.S. It’s not the TPS…I’ve found it makes no difference with it on or off. So can anyone answer my question about the clutch.

From the sounds of it, you've got some kind of engine issue that needs to be dealt with. I'm thinking that hopefully your clutch basket is going bad. I say hopefully, because if it's not that, then I'd be concerned with your crank. This is where I would start looking. Perhaps some others can come up with some ideas too.


I had a similar issue with my plated 99' wr400 only not as dramatic just before the crank froze up on me riding home. After a complete rebuild top and bottom with a 426 crank upgrade I haven't had so much as a bog at low rpm's on the street.

hey mate its probaly your counter balancer it is common for the 400 to shear off the woodruff key on the counter balancer they fixed this with the 426 pull off your flywheel cover and see if u can rotate the balacer with your hand if so you need a new woodruff key should set u back $10

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