Brap Brap Brap

This past winter I had my cylinder bored, ported and polished. The engine makes great power. I have ridden the bike about 10 hours since the break-in, all on my home track. Riding on a track environment I never really used the midrange of the engine, i.e. the needle.

This past weekend was my first woods experience with the new cylinder. The first thing I noticed was that my needle is just in the wrong position. Whenever I was at mid throttle the engine would breakup badly (hence the Brap Brap Brap subject line). It was bad enough that I had to ride at the extremes of the throttle. I assumed that this was indicative of a lean condition. So at the lunch break I lowered the clip from the third position to second from the bottom.

Here is the problem; I think the problem got worse. I’m hesitant to go the other way on the clip and lean it out further because I don’t want to do any damage to my fresh piston. I’m starting to think that I’m chasing the wrong problem. Is it possible that my spark plug is fouled (lots of slow speed single track riding) but is only showing itself at mid throttle?

I’m going to install a new plug tonight (after I clean about 10 pounds of mud off the bike). But if that doesn’t do the trick then I’m back to my jetting. Any thoughts before I potentially fry my piston?

I did do a plug chop on the main. Never thought to do one on the needle. It just seems strange the way the bike behaved. It certainly acted as if it was running lean, but dropping the clip made it worse. That’s why I think I’m chasing the wrong problem. I’ll try another plug read on the needle if a new plug doesn’t sort it out.

Installed a new plug (BR8EIX...I know, a waste of money) and raised the clip to the second position from the top. All feels good and the plug chop shows correct jetting.

I just misread my initial problem. After speaking to Mike at RRP (the guy who did my cylinder work) he explained that if I had a lean condition the bike would have fell on its face at mid throttle settings. I had a hesitation/surge condition.

I just assumed that it went lean figuring that the greater flow through the cylinder and head would require MORE fuel. I would highly recommend some cylinder/head work for a KDX. This bike just rips. It has power everywhere and is much easier to ride.