Not doubting your tuning ability & love posts like yours to following how you get it sorted. I'll go back & read MarkT's long post tonight as value his advice on just about everything--ok--everything-- but would reducing the air jet richen the needle due to less airflow rather than lean it out--just curious?. I started mx in about "62 but now mostly race my rocking chair haha. Had all the big bores up to yz490k. Just realized yours is a 250 which was a perfect model in "78 with alloy arm. Just looked right!!. Cheers from NZ.

Sorry, one more-- is it deffinitely rich or just go through a flat area then come on strong. I bought an it400 motor for a project that someone had ported to give close to 200 degrees exhaust duration & the pipe I adapted to fit was  bit short in the mid section. The power came on like a 125 on steroids & was unrideable in the dirt. So added 4 inches to the header as easiest to do in my case. This made it manageable & spread the power out overall.

With the "primary choke" needle jet that most of these two strokes have, the air that goes through the passage provides a "vacuum effect" that pulls fuel from the float bowl.

Take carb off bike, slide out, liquid in float bowl (or bottom of needle jet in liquid).  Use an air nozzle to blow into the air jet passage.  Liquid in float bowl will be sucked up through needle jet and the bowl will empty pretty quick. 

Very dangerous to try with gas as you'll spray gas mist out of the top of needle jet. 

Somewhere I have a Mikuni book that has a chart showing how air jet affects full throttle...  bigger air jet richens mixture at full throttle slightly as RPM (and air flow) increases.  Effect is greatest at full throttle and high RPM.  Really doesn't do anything below 3/4 throttle.  It's a way to "fine tune" the main circuit... and I mean "fine tune".  Makes very little difference so usually not touched except perhaps by the engineers and tuners at the factory. 

It certainly won't fix idle circuit richness if that's what's going on here.

First i can not even post a photo for some reason.lwt alone a video...haha..
Deffinately to rich off bottom at lower rpms and that is why I was leaning toward something done wrong just like the two pilot air screws in carb...one in pilot air screw location and one to adjust idle, haha....
The stator ignition wires wore through insulation and had contacted wires, stangely bike still ran....had 2 socket head bolts heads for main cases popped off so when pressure checking crankcase i had air leak from breather, luckily new bolts cured it..
Will add more info soon as I did get new jet needle but after measuring it i am not sure it will help much..
I did set timing....i can tell you if anyone has owned one of these it kicks back really hard at times, very surprised it did it so retarded timing to 1.8 and it is better...just wish I didn't sell all my jets years ago ....SOB!

Hi Giys..
The 6f15 jet needle ( on 1 position, clip top) and pilot air scrrw at 1 3/4T out did it, Bike rips now..
I have read the 250E is like a 125 and can attest to that...
Thanks for help guys...If i make a video i will post...