Also, I always re-ringed and replaced the piston in the engine every two years. Big difference when a fresh set of rings and piston went in.

You didn't say how old it is. The real early ones did not have power valves and were lightswitches (pre-1983 or about if my memory serves me). If newer with power valve, check the linkage and take the cover off and spray with carb cleaner. Try to turn in over by hand on the kick starter (with plug wire off). It should be almost impossible to get past TDC or really hard to do. If not rings at least. We would re-ring every 20 hours and pistons and rings every 60 hours on our race bikes. A 250 should be a rocket - able to wheely in any gear up and into 4th. I raced from the early 80's thru 2008. Broke more than my share of bones!

Eric, take notes:

YZ490 here.
Fookin crazy chit.

with the wacky rear swing arm shock setup?
Those were like light switches!
Friend had an old '77' or so YZ490

My first bike was a '78' RM80.
Bought it for 50 bucks in milk crates and boxes when my parents were away on a cruise and my aunt was watching us.
The old man was less than happy!
I put it together myself (old boy didn't think I could) and terrorized the neighborhood
for years on that thing.

YZ490, that was my last dirt bike. At 5'4" 165# that thing was CRAZY. Biggest problem was starting from atop a milk crate (and keeping front wheel on the ground). The seat was 4' off the ground, just like the front wheel when riding.

ok....so it was just rebuilt?? and a new plug doesnt help...
before you take anything apart, do a leakdown test, on a 2 stroke if you have the slightest airleak anything else you do is going to be chasing your tail
next make sure the powervalve is not gummed up and the airfilter is clean and properly oiled.
once you have done that, you can either mount an egt or become good at plug chops. once you get used to it you can get pretty close by ear.

Is it stock?? If some bubba with a dremel decided to port it, it may not run for chit no matter what you do. a well built and tuned power valved 2 stroke 250 should pull fairly strong all the way thru.

Got a CR 250 that I've made a pretty good woods bike out of. That thing spooled up to quickly for me so I put a Stealthy flywheel weight on it and the biggest rear sprocket i could find. Now it has smoother low end with good midrange punch. Helped tame the quirky throttle down a bit to.

Alright - I omitted some details.

Its a 2000 RM250. I rebuilt it from the ground up - pulled the motor down to bare cases and replaced everything -bearings, tranny bearings, seals, piston, rings, etc...... powervalve was completely dissassembled and put back together spotless. the only thing I didn't replace is the reeds, but you can't even see a flashlight through them so I don't see that as the issue.

Ill ride it it a couple times tomorrow and better understand it. It just honestly feels like it's missing, I guess I've just never ridden a two stroke so I don't know - my fourwheeler is 4-stroke.