250-500cc 09 WR300 Cutting out in mid range?

Got an 09 300 that I love and it runs great. Last time I rode it I noticed it would hesitate right in the middle of the power range a little. I thought maybe bad gas but it did it again today more pronounced and normally it idles fine but died a couple times today too...which is unusual.

Its got the Keihin with the pre jetted kit in it. Sparkplug is always a nice dark brownish color.
Been perfect up to now. I bought it new in June 10 and it still has the original top end. A wild guess on milage is in the 500-600 mile range on it I guess.

Ideas?

Has there been a big change in temperature in your area? Maybe the needle position is too rich

Turn the air screw out a bit. If that doesn't clean it up, go to a smaller pilot.

what about droppping a clip on the slide?

Suggest you make sure your sparkplug cap is all the way onto the plug- had one arc-ing once and it made the top end ratty

More info firecrotch!!!

"it idles fine but died a couple times today too...which is unusual" - is this at cold start up or after you have been riding for a while? If it's dying after you've ridden for a while and haven't touch the jetting, it could indicate the float needle getting stuck (lean it over a bit and check if you see run off).

"hesitate right in the middle of the power range a little" - usually this is a combo of needle clip and airscrew/pilot jet.

You guys are heading or into summer correct? Whilst the Keihin isn't as sensitive to Mikuni's, you still need to compensate for changes in air density as the weather gets hotter - either turn the air screw out (max you should go is 2 3/4 before you go a smaller pilot jet) or the best way to tune is to warm the bike up, turn the air screw all the way in and the idle to drop that you need to blip, continue screwing out until the idle has risen to your liking (l have mine at 1800rpm) and then remember how many turns out.

If you are getting crappy response from idle it's likely that it will effect it up to middle position until it clears up - that's dependant on whether you ride fast open trails or ST's?

Either way, pull the carb off, give it a good clean up and then set your air screw/idle before changing clip heights.

As a point of reference, l have the air screw at 1.5 out during winter and as far as 2.5 out in the hot summers (38C @ 800-1000mt)...if it's on the sand, l change the main jet to 180 or 178 with the clip down to 2nd last as l'm mostly in 1/2 to WOT.

Thanks thats a good point about float needle. It happens after riding and warmed up not cold(which is a new developement) Temps havent changed that dramatically. At least it didnt happen this time last year. I also remember seeing gas overflowing once or two in recent rides out the bottom of carb a couple rides ago(probably still happening i just dont notice). Ill be pulling carb off this weekend and cleaning it thoroughly i guess.

I've had the same issue with my 09 250, it would cut out right before middle of mid range power surge and sometimes die like fouling a plug. I had this problem a couple of times a few weeks ago I tried changing plugs and same thing would go on, so every time it would start acting that way I would rev the guts out of it while riding and it finnaly cleared. Thought maybe cooler temp. outside was the cause but it did it during some warmer days too, I haven't had the problem in a few weeks now.

There is a possibility of dirt or clogging in the carb here but I suspect it is rich.

Anyway I have the Keihin on my 09 250 and I am using the Red needle on the 2nd clip from top with a 45 pilot and 175 main.

The carb came with the blue needle in it. As it is warming here now, I plan on going to a 42 on the pilot soon, but the bike is running super.

Unfortunately Firecrotch and fellow keihin users, as good as the Keihin's are they are very susceptible to float sticking which you'll see as overflow and it will effect the response of the bike as the carb is literally drowning at low rev's
Great tip when you find yourself with a stuck float or fuel overflow, find a stick and tap the crap out of the float bowl to free the floats - you'll get your response back and save fuel!!

Ok so I guess I misunderstood when I bought my pre- jetted kiehin carb. I didn't realize I still need to jet it. Just did it....holy Jesus it's like I gained 5hp. I can't keep the front end down and it's yet still rich a tad. It's like I out a new pipe on or soemthing... I am officially a believer in proper jetting now.

There is no such thing as pre-jetted and "bolt on" mods are rarely just bolt on and go. I guess it just depends upon how picky you are.

Pre-jetted basically means ball park jetting. Temperature, humidity, elevation, fuel type, exhauast type can all affect your jetting.

So, what did you end up changing from and to ?

Simply up one on the clip and play with idle. Not done though because now I am on a quest to get it perfect.lol I Had no idea what I was missing. It's amazing what you get used too.

Yep, you don't know what good is until you've had it.