Advice Needed: Engine Bog @ ~6k 1/4 Throttle

[MaxxVE]

So I bought my bike last year. Its a 2006 and had under 1,500 miles on it at the time.

The carbs were gummed up (bike didn't run) so I pulled them, cleaned them and reinstalled.

I rode it all summer and put another 1,000 miles on it. One thing that always bugged me was a bog or stutter that occurred around 50mph while just cruising at ~1/4 throttle in 6th gear. It almost feels like you're hitting a heavy head-wind.

The bike runs very well otherwise. WOT is perfect 0-12,000 rpms when taking off.

I pulled the bike out of storage and as I wait for warmer weather (thank you Michigan) I figured maybe I could try to fix this problem. Would shimming the carb needles help this 1/4 throttle stutter in 6th gear? I'm seeking some advice before I tear into it.

Thanks!

[quarterliter]

Is the bike stock?

[DaBlue1]

Why cruise @ 50 mph in 6th gear? Why not fifth? At 1/4 throttle in 6th gear you are not getting enough fuel for necessary power.

Here's a 'lil something you can do to help determine throttle position when tuning your carbs.

-From Motorcycle Carburetor Theory 101

Place a piece of tape on the throttle housing. Place another piece of tape on the throttle grip and draw a line (while the throttle is at idle) straight across from one piece of tape to the other. When these two lines are lined up, the engine will be idling. Now open the throttle to full throttle and draw another line directly across from it on the throttle housing. At this point, there should be two lines on the throttle housing, and one on the throttle grip. Now find the half-way point between both of the lines on the throttle housing. Make a mark and this will show when the throttle is at half throttle. Divide the spaces up even again until idle, 1/4, 1/2, 3/4, and full throttle positions are known. These lines will be used to quickly find the exact throttle opening while jetting.

[MaxxVE]

Originally Posted by quarterliter

Is the bike stock?

Yes it is.

[MaxxVE]

Originally Posted by DaBlue1

Why cruise @ 50 mph in 6th gear? Why not fifth? At 1/4 throttle in 6th gear you are not getting enough fuel for necessary power.

I guess maybe I wrongly assumed 6,000 rpm wasn't too low. I use the little 250 as a commuter bike so gas mileage is important. I want to be in the highest gear possible.

What do you and others use as a normal rpm target for extended driving?

[quarterliter]

Are you running ethanol free fuel? Also the advance curve at 7 grand changes and there is a flat spot if I remember correctly. That may be part of the problem. Check out a dyno chart of the 250.

[DaBlue1]

Originally Posted by MaxxVE

I guess maybe I wrongly assumed 6,000 rpm wasn't too low. I use the little 250 as a commuter bike so gas mileage is important. I want to be in the highest gear possible.

What do you and others use as a normal rpm target for extended driving?

If your rpm speed is 6k you should be good for cruising @ 50 mph, just don't expect a sudden burst in power when you whack on the throttle. If you are getting a bog or stutter, you may want to look at possible intake restriction. An old air filter or a heavily oiled one can cause the symptom you are having.
Also shimming the needles doesn't hurt either. For some bikes it may offer a small performance boost.

[MaxxVE]

Originally Posted by quarterliter

Are you running ethanol free fuel? Also the advance curve at 7 grand changes and there is a flat spot if I remember correctly. That may be part of the problem. Check out a dyno chart of the 250.

The gas I use does have ethanol unfortunately. I'm reading this may make the issue even more pronounced.

Maybe this is just normal and I happen to hit it all the time because of a long 50mph stretch on my way to work. I was hoping shimming the needles would allow more fuel at the 1/4 throttle point. But first I think I need to use the grip & tape method to find out exactly where the throttle position is at this speed. The 1/4 throttle is just an estimate.