Thing bogs on occasion coming off idle...I haven't experienced it yet, but my customer has.  Says the bike nearly dies.  I'm going to be playing around with it to see if I can get it to happen with me.  Just looking for some ideas of what to look out for.

Thing has <4000 miles.

I just upped the pilot a bit to help with cold starts.  Situation pre-existed and had no effect.

Valves are in tolerance.

Carbs are synced.

No vac leaks.

If the valves were out of tolerance or the carbs were out of sync in seems to me like it would be doing said problem all the time.  If the problem is intermittent I would be looking at suspect connections in the electronics like the CDI or for something that may be getting pinched when the suspension is bottomed out or when the bars are turned all the way left or right. 

Nah it's typically only going straight.  I think it was a air mix situation.  The bike also doesn't always maintain a consistent idle.  After closing the screws in 1/4 turn the stumble was reduced a bit.  I showed him how to adjust the screws and he's going to play with the mix and see how things go.

I know it sounds dumb but take a look at the plugs.  I've had plugs that gave hesitation but seemed fine on inspection.  Replacing them made it run better. 

oh yeah...she had new plugs in when I checked the valves.  old plugs looked fine.

I had a ex250 for 9,000 miles.  Those little 125cc pistons are picky about idle settings and that bog is really hard to get rid of!!!!!!

If you set the idle to the recommended 1300 you are almost guaranteed that bog that no amount of dialing in will ever fix.  I used to run mine at 1400.  I know it's a bandaid fix but most ex250 owners do this without issues.   

The right way with the idle rpm while turning the screws almost never works because the bike is really picky about temp.  When you're zipping around town it stays around 1/4 on the gauge.  When you idle long enough to set the screws it's at 1/2 on the gauge and it makes a difference. 

The easiest way is to just change the mixture screw 1/8 turn and ride it to see if it makes a difference.

The blipping the throttle test isn't very effective either for some reason, unless you're way off. 

It's hard to find the super narrow window that doesn't end up with a bike that jumps from 1400 up to 2200 for no reason, or dip down to 1000 then up to 1400 when you stop at a light, or bog on a start.  Lots of 1/8th turns on the mixture screws. 

I'm quite comfortable with dialing in carbs but it still took me a way more than a few tries to get a good consistant idle that didn't bog.  That's just my experiences.  Since you are a professional and I'm not please take it for what it's worth.