'02 WR426 popping

First time Yamaha forum poster. I am looking into getting a second bike an '02 WR426. I have a line on one at an excellent price. The reason for the great price is that the bike has had a problem since new.

The bike has been ridden for a total of 4 months in 2 years and only has 2 races on it. It has all the extras for desert racing and looks new. Here is the problem: It pops and surges during hard acceleration and deceleration. The owner changed the black box, plug(many times), numerous jetting combos including taking the carb off another 426 that ran great. The only thing he did not addresss is timing. I heard from a friend of his that he did change the bike to YZ timing, but am unsure of that.

I heard a rumour that there were some cam issues with in '02. I KNOW this is THE forum to get the BEST advice possible on this bike, so I am asking for your expertise before I buy it. OR should I steer clear. :)

Thanks in advance for all responses!


Have not heard of any cam problems with the WR. The change to YZ timing may be the problem. Some have tried this and rotated the exhaust cam the wrong way. Motomans site here on TT has good instructions on this mod. Its easy to pop the cam cover off and check. WR timing has 13 pins between the punch marks on the cams and YZ has 12. If turned the wrong way there will be 14. It may also be the throttle position sensor if he has taked this off. I suspect the fix is simple, these bikes are pretty bullet proof.

My dad have shared an '02 WR426 for nearly a year now - I ride it 9 out of ten weekends and he rides midweek twice a month. The bike has probably done 3000+ miles in that time with at least 50% off-road, including ten or so races including MX and Enduros. We regularly change the oil and just switched over to YZ timing at the weekend - when we did this we checked the valve clearances and they have barely moved from spec and did not need adjusting. The bike has proved extremely reliable and only used standard consumables such as sprockets, wheel bearings, etc.

Incidentally, in follow up to tctrailrider's suggestion, when we changed the cam timing I rotated the crank a 1/4 turn while the cam-tensioner was disengaged and the inlet cam jumped a tooth, so it should be relatively easy to mess up the change.

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