Bigger carb vs smaller carb

That sounds right to me.

The smaller carb has increased air/fuel velocity at low and midrange, giving better throttle response. A larger carb will flow more air/fuel at higher rpm and give more top end and higher peak hp.

Unlike many things when it comes to carbs, bigger is seldom better. The bigger the carb, the less pressure difference between the fuel bowl and the point at which the air flow passes the main needle jet orifice. The less pressure difference means that there is less of a 'signal' to allow fuel to flow from the fuel bowl into the air flow. Too large a carb will result in a flat spot when the carb transfers between pilot and main fuel delivery system. Also due to the lower air speed through the carb, atomisation of the fuel mixture will be poor, resulting in reduced horsepower. If you have to make a choice between larger or smaller carbs, go for smaller every time, and then fine tune the delivery of fuel using the jets, airscrew, needle etc...

Sully how do you account for the 39.5mm carb on the ktm 125? Bigger than most 250 carbs and yet it is without question the fastest 125. I am not saying you are wrong, but it would seem that bigger is better in some cases.

OK thanks, I was not aware of the KTM bigger carb(my RM has a 38mm) so it is slightly bigger you are right, and it certainly is the quickest 125 out there, without knowing the bore/stroke ratio and how it stacks up against the others it is not a straight comparison.
I was simply advising Homer88 that if he is going to shell out for a larger carb and assuming that a quicker bike will be the result it is certainly not the case, and he may be wasting his £££...sorry $$$

SULLY said:

I was simply advising Homer88 that if he is going to shell out for a larger carb and assuming that a quicker bike will be the result it is certainly not the case, and he may be wasting his £££...sorry $$$

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Thats good advise as in most cases a smaller carb will give you a better over all powerband.