300ex low end bog

[riley1374]

I have a low end to midrange bog i can not cure. Bike is 2002 300ex with FMF exhaust, port & polish, titanium valve springs, big cam. I am using the stock carb, and the thing starts up and idles great. As soon as i start to let out the clutch the thing starts bogging and spitting and sputtering until it climbs to the upper RPM's and then it runs great. Is this a jetting issue or does it sound like something else. I even switched carbs and it ran better but still had a pretty decent stumble revving up. Also, it seemed to get worse as the motor got hot. Any help is greatly appreciated.

[riley1374]

anyone?

[Honda TRX250ex]

Either its your timing, jetting, or your sparkplug/airfilter
check all of them

[riley1374]

Well its definetely not the plug since its brand new, but the air filter is old and dirty. The thing that gets me though is it runs fine at idle and at high RPM. Would a dirty air filter do that? Timing is the other thing i am considering because i bought this motor used and it has had a fair amount of work done to it. Keep the suggestions coming, i want to hit the trails this weekend.

[bwamos]

You need to raise your needle in the slide (drop the clip) to allow more fuel in at the 1/3-2/3 throttle range.

[riley1374]

Thanks Bwamos, i will try that and report back!

[riley1374]

Bwamos i tried raising my needle and now i have a hesitation while revving it up on low end, definetely a bog. Realised also that the pilot that was in there was a 45 so i swapped that out for a 42. Im going crazy with this thing and i want to hit the trails this weekend. Any help is greatly appreciated. Is this jetting or something else?

[bwamos]

With you're setup you should be really close to a 42 pilot 2.5 turns out. Needle clip in 4th slot, washer above the clip (Slot 1 is the slot on the blunt end of the needle), and about a k135 main.

Try raising the needle 1 more spot. You should be close. It will be obvious when you get the needle in the right spot. That 1/3 - 2/3 bog will go away. The needle isn't a ver subtle adjustment.. either it works great, or it's having a fit. ;) Makes it pretty easy to adjust. Since you're full throttle is running well.. leave that and the idle circuit alone, until you get the needle set.

Bog = lean. You just need a little more fuel in the needle circuit.

Keihin Carb Jetting (http://www.duncanracing.com/techfaq/Tech_keihin-carburetion-jetting.phtml)
Should be a nice read for you, to help you understand what's going on.