At what point does the valve limit the A/F you speak of?
or the carb bore for that matter?
Not trying to sound like a smart *** because i really don't know, however you seam to have an understanding of it all.
Do I need a larger carb? If i do , do i need to install larger valves?
will a more aggressive cam help the flow to get to 1,500,000 cc/min?
i don't get your theory, please explain.

 

My theory for what its worth.

All the main jet does is limit the maximum amount of fuel.

With the 250cc, using the number above and lets say 14.7 to one for the fuel by doing some math, the 300cc cylinder is going to need more fuel and air.

Vacuum or venturi effect will be more, but the jet will only allow so much fuel to pass through it. So you need a bigger hole in it to allow more fuel to mix with more air to make more power.

I don't know if you need a bigger carb or not.

Post #1 described a too lean condition for the main jet perfectly. 3/4 to full throttle.

Sometimes a smaller carb will give different flow and possible more control on the low end, if that is what you are looking for.

David

 

Mine was running stupid like that and I put it on RES and it cleared up a little?
Not to sure what was going on.

 

Your main is on the small side for a 250, so it makes sense to go larger, but... it is not uncommon to have to jet DOWN when you go big bore.
More airflow across the same size carb will draw more fuel from the jet naturally.

 

Hard to beat experience when it gets to the physics of Carburation! I'm sure I would have gone up too much in size, even though I used to use Venturi's in respiratory Therapy equipment 30 years ago.

You've just saved me at least one jet exchange, assuming I do the 331 soon.

 

Went riding Sat. and the problem did diminish with altitude. I was sorting through the Dynojet box and I found the stock California main, it was a 118! And I also discovered the one DJ main missing from the box was not the 124, it was the 120. No wonder.

Unfortunately I'm hung up now with a stripped phillips head on the little (soft) screw that holds down the bracket that the push/pull cables are anchored to. It was either overtorqued or threadlocked in place. I'd be done now if not for this, might have to drill and extract the thing as it's just not giving.

edit: took the roundabout way and removed the cables from the bracket, a bit of a squeeze to get the carb out with the bracket still attatched. Got the first three screws holding the bowl on but now the fourth isn't cooperating. Same soft metal screws that want to strip.