Originally Posted by merryman
The right bore for a carb depends on engine size and state of tune. No advantage will be gained if the original carb was right for the engine size you have. In fact performance will drop as the speed of airflow through it drops, causes less suction on the main jet and upsets the air/fuel mixture. With a performance exhaust, big bore kit, high lift cam, and/or big valves, an engine may need to get more air.
I'm with merryman on this...you could probably get a lot more out of your stock carb if other modifications are made. I significantly increased the HP on my scrambler 400, but it's still running a stock carb (modified).
My son's LT80 is having the motor built by Jack Moore and he only modifies the stock carb...
Same on my Yamaha Vmax 1200 - just stock carb with jetting changes and airbox modifications along with a high flow exhaust.
If you significantly increased the motor's requirement for more air/fuel, then it might be justified...but in many cases it is not. Stock carbs can be re-jetted and air boxes modified to get impressive gains.
But to get those gains the lungs (motor) needs to breath! Everything merryman suggested (I'll add port & polish the head) will work on the thumper and help it breath...