What RPM does your engine make it's peak power?
What RPM is your engine turning in 6th gear while you are going down the turn row at WOT?
I ask this question because I suspect that your gearing is way off for the amount of power your engine is making and having the need to run it at sustained top speed.
A big bore kit will go the same speed as a 250 if they are both turning 7500 RPM with the same gearing and tires, providing both engines have enough power to overcome the rolling resistance and aerodynamic resistance at that RPM.
The majority of the highly modified two strokes need a lot less fuel once they go pass the RPM where they make their peak power. Mikuni carburetors have an adjustable air correction jet that can help this problem but the Keihin dirt bike carburetors (PJ, PWK, PWM and Air Strikers) do not have this nice feature. Even when the air correction jet is properly tuned, it cannot make the abrupt fueling change needed to properly lean the mixture in the RPM range past the power peak. I suspect that you may be expecting the smart carb to do something magical and provide this abrupt fueling change that the Lectron, Mikuni, Keihin or any of the other non electronically controlled carburetors have not been able to do. I do not see anything in the Smart Carb design that would allow it to make this abrupt fueling change.
Like Dave RSS396 said in an earlier post, some of the 250 moto-cross and road racing two strokes had electronically controlled power jets. The power jet is turned on typically around 6000 RPM and operates in tandem with the main jet at WOT up to the RPM range where the power peak occurs. Once the engine reaches a RPM in the region where the power peak occurs, the power jet is shut off and the resulting mixture is much leaner. A fuel curve shape like the one just described, fits the fueling requirements of the majority of high performance two-stokes much better than what conventional carburetors can provide.
Most two stroke engines can be a little rich in the RPM range before the power peak without appreciably affecting the power in this range. A two stroke that is rich after the power peak will not rev and may have a rich miss-fire. The majority of the highly modified two-strokes I have worked with will be too lean in the RPM range before the power peak if we install a main jet that supplies the correct amount of fuel in the RPM region past the power peak. An engine that has the mixture correct in the RPM range past the power peak will have good over-rev and will run clean in this RPM range.
Knowing these facts forces us to make a choice of how we tune our carburetors, gear our bikes and then ride our bikes. Once we make these tuning and gearing choices we
must ride our bikes in a manner for which it was tuned.
Tuning state #1.
If we ride our bikes on a closed course track where we never reach a speed in top gear where the engine will need to rev past the power peak, we can install a main jet that will produce the best power and acceleration up to an a few hundred RPM past the power peak. If we take this engine package and gearing and try to run it WOT in top gear where it will try to rev a 1000 RPM or so past the power peak, it will probably have a rich miss-fire in that highly over-reved RPM range.
Tuning State #2
If we decide to take our closed course bike and not change the gearing and just run it WOT in 6 gear we can usually lean the main jet and get a few more MPH and not hurt the engine. With this leaner main jet the engine will probably be too lean in the RPM range before the power peak and we may hurt a piston if we ride it with these carburetor setting on a closed course or pull a hill where the engine is pulled down to the RPM range below the power peak.
Tuning State #3 If we want to do most of our running at top speed and get our highest top speed, we can use our carburetor setting and maybe richen it up a jet or two over what we used in
tuning state #1, providing we take enough teeth off of the axle sprocket or add teeth to the counter shaft sprocket so that when we run it WOT in 6th gear, the engine peaks or does not have enough power to pull no more than a hundred RPM or so past the power peak.
