My latest issue with my DT250B is an erratic idle and whatever it is actually prevents you from riding it due to the engine simply cutting out. Here are two videos if you are willing to listen and make suggestions for my troubleshooting efforts. I assumed carb issues so I disassembled it and put it thru the ultrasonic cleaner and made sure nothing is blocked or plugged. The o-ring seems fine because its fit is noticeable when you attach the float bowl. I would change it if had some on hand, but again, it seems to have a snug fit.

I am wondering if it is an ignition problem causing it to stumble like it does. If you try to pull away it simply dies within a couple feet. As always, I would appreciate any suggestions you have for my troubleshooting efforts.

youtube.com/shorts/9iAAtSR7wCM?feature=shared
youtube.com/shorts/rujc5r6Zzpk?feature=shared

It's electrical, but which part. Look in the flywheel in the dark & see if points go into arc welder mode at the misfire indicating condenser or if the very small spark points give in the dark see if it intertmitantly stops sparking for that fraction of a second. Maybe check points gap isn't way too wide. Check coil mount to frame. Check engine mount bolt tightness. First check points gap. 24 thou gap max. Check wire terminal to points is tight, that sort of thing. Does anything alter when lights are turned on. I'll get into that later as possibly primary coil earth's into lighting coil so are any of the coil windings loose on their posts. BUT I forgot the simple things--faulty plug cap or plug set way too wide in thd gap!!. The more I think about it--points are too wide. Out past 15. Does it kick back more than it used too. I know it's a 250 but they can kick back if over advanced.

It did kick back for the first time ever……

All great suggestions Morley and I will start with your points related suggestions.

When I went to install a new spark plug the cap came off the coil wire in my hand….. So I trimmed it back a little and reinserted the wire into the cap. It didn’t resolve the problem. I have spare coils to try as well.

I probably won’t get back at it for a few days. My 5 year old grandson is here to stay with us for a full week! I will give an update as soon as possible.

Thanks again!

One more random thought re-kick back, if the points aren't very wide maybe check the flywheel crank key but seeing its running timing must be in the ball park & within reason timing won't cause erratic spark that you have & would just have it firing in the wrong place. Yours is running but faltering when spark momentarily stops--in my opinion that is!!. Have a good week.

My first thought is change plug and make sure it's gapped 0.020" to 0.024"

It's not clear to me what happens at higher revs because you're focusing on idle in the vid.  If it breaks up worse and/or dies as you try to rev, condenser would be my next suspect.  

Mark I cannot get it to rev up without it faltering to the point of stalling. I will add the condenser to my list for trouble shooting. I simply HATE trying to solder the wires to the condenser. I don't have the knack of doing a clean job. It ends up looking like a spitball on the chalkboard. 

Edit:
It has a brand new 4054 plug in it but I gapped it at more like 26-28 so I will close it up.

when you rev it, it gets worse a few secs. .

Reminds me of when I turn off the fuel valve on mine and let engine run.

fuel flow, petcock not clogged, no vapor lock from cap.
float level not too low
slow jet is snug

other thoughts:

cracked plug
plug cap not well attached.
points contact

I do have a brand new Yamaha petcock on the shelf which I plan to install as one of my first tasks.

I did notice that one of the floats did not simply fall off its guide pin when I inverted the bowl during disassembly. Had to shake it a bit. Put it back on and it did the same thing again. I can’t feel a snag or why it hangs up. I do have a spare carb and will swap out that float.

But the fact that the miss is intermittent like it is leads me to believe I have an electrical issue. But yesterday I noticed a puddle of fuel under the bike. The float bowl overflowed. Petcock was in the off position.

Thanks for your suggestions.

I’m thinking my problem is the flywheel key. Had a hard time pulling the flywheel and this is probably why. It appears the key is partially sheared.

What causes this to occur? I don’t want to have a repeat issue when I install a new key. Or is this one of those things that just happens now and then?

imgur.com/a/tSznzfp

Crank key is on the way out for sure but still might have points gap too wide as well.
Only happens because its not tightened enough.
I keep using my pic as an example & i know its over the top but never had a fail & all my cranks a pre stuffed by previous owners not tightening enough.
I don't mind being dragged over the coals for my methods haha but just putting it out there.--if i can find pic.
photos.app.goo.gl/qod67Q3QXyZ61mNcA