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Redyaris, glad you were not hurt and Gray wasn't damaged too badly. Sometimes the crash repairs provide an opportunity to infilct modifications, sounds like you are going to take full advantage of that.
On the carb size. The size of the carb is more relevant to engine power output rather than engine displacement.
The ninja 250 puts out 37.4 hp and has 2X30mm carbs for a total carb area of 1,413 sq mm, divide this by 37.4hp gives 37.7 sq mm /hp.
The gs500 puts out 47 hp and has 2x34mm carbs for a total carb area of 1,815 sq mm, divide this by 47hp gives 38.6 sq mm /hp. Pretty close to the same.
By going to smaller 30mm carbs on the gs500 gives 1,413 sq mm, divide this by 47hp giving 30.06 sq mm/hp.
As a cross reference, a Yamaha tw200 1X28mm total carb area of 615 sq mm, divided by 15 hp gives 41.05 sq mm/hp
So it looks like a carb cross section area somewhere between 37 sq mm and 41 sq mm per horsepower seems about right. Motorcycles are usually carbed to flow the maximum amount of mixture the engine can flow which directly translates into the ability to horsepower.
Motorcycle carbs are what was once known as variable venturi. The size of the venturi changes as the engine can accept more and more mixture. It's like having a small carb at low engine horsepower outputs and a large carb at higher engine outputs, so a smaller carb probably won't increase efficiency much at lower speeds but would definitely hinder top-end output. If the engine was optimized for lower speed operation by substituting lower lift and/duration cams (hindering top-end output), the original carb size would be too large to utilize it's potential, therefore a smaller carb would be better matched. It would also increase throttle response at lower loads somewhat.
It would be interesting though to fit a ninja 250 programmable fuel injection system and see what the result of a dialed-in injection system would provide. I saw a link a few weeks ago and it looked promising, just can't find it now though. It may be just the ticket as the top-end output of the GS500 is more than you need anyway. The injection system can be more finely tuned to the engine than a carb can, though we're really splitting hairs here.
I'd look at cam alterations to improve low speed operation first. Compare cams to other bikes optimized for lower speed operation. Look to decrease valve overlap, increase dynamic compression and decrease reversion into the intake so the carb doesn't try to meter fuel to air twice. Test. Then re-jet as needed. Could be that just advancing cams a few degrees would have a beneficial effect for not much invested.
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Last edited by beatr911; 06-27-2011 at 12:20 PM..
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So I have to choose then what to waste: brake pads or gas... (because I have to start the engine at the transitions between engine braking and coasting)
