Need help with jetting 2006 yz250f with Athena big bore kit ?

[D-Brown]

Hello everyone, I have a 06 yz 250f with Athena big bore and Dr-D exaust, Looking for some jetting tip's. Im in California at a couple hundred feet abouve sea level, Just need a base line. Thanx D-Brown

[mn519]

Your stock jets should be a good baseline. some ppl say they had to go leaner, some richer, might just depend on the bike.

on my 290kit I had to go richer on the pilot 2 steps, and leaner on the main 1-2 steps. But that was with pipe and cams also. your stock jets should be a safe ballpark to start with.

[MotoSenior]

I am running the 290 kit with head porting for increased flow. I had to drop the main and idle jets. I think the general rule is that the more flow (suction), the leaner you'll need to adjust your carb.

As mn519 says, start with your stock jetting. Ride the bike at a good pace with the throttle open, then stop and check the plug. It should be burning a tan color. If it is sooty, reduce the main jet size until you get a good tan color. You should always re-tune the fuel screw with the engine fully warm each time you change any jets.

The pilot jet is running too rich if the motor stumbles and stutters as you crack the throttle open (first 1/8th turn), an then cleans up as you continue to open the throttle. Reduce the pilot jet size until you get a crisp clean response in the first 1/8th throttle.

It might take a few iterations, but you can make theses changes through the port in the bottom of your float bowl without removing or tilting the carb. Once you have the jetting dialed-in, your big bore will rip! Well worth the effort.

Good luck.