If I want my bike to run just a little better than stock will it work with a 26mm carb? Stock is 22mm. 120cc motor.
Nothing else is changed. Perhaps a new cam in the future.

 

need more info
120cc tells me nadda
what is it
what pipe carb air filter head combo is on it
is it a honda or a clone

 

Yes it is a 120cc Knock-off with everything else stock.

 

a bigger carb will allow more airflow total, giving you probably a slighty better top end pull. but having a bigger carb also slows airflow according to bernouli's theory of airflow charchteristics, wich will inturn drop the torque and response a bit, and give you less airflow in the lower rev range.

but if your racing it, it all depends on if your motor can breathe the extra air/fuel mix and make good use of it out of turns and on the holeshot, if that's the case a bigger carb is better in theory for all out top end performance,

i know this isn't a direct answer to your question, but hopefully it will give you an idea of how airflow works in an engine and allow you to make your own decisions and learn to use the knowledge you gain to help the next guy out, or just be the fastest guy on the track

 

That sure doesn't look like a stock 22mm carb to me.....looks way smaller.

 

Thank you guys
I have decided to go for a 24mm carb. A guy here in Sweden got 8 rear wheel hp in his knock-off bike (stock motor) and 24mm carb. Seems like the best way to go. Do not want to lose the midrange.

 

if you go to mikuni.com and look up the calculator it will tell you the right size for that motor.
depending on your elevation since you are in sweden different things will apply does the othe guy live at the same elevation.
also i can tell you another thing if you want top end run a short intake,if you want bottom end run a long intake, i run motard and shorttrack so i need my motor to pull very hard on the bottom side all the way through the curve,
secondly look at a stock xr 100 air box to carb set up.now look at your air box set up main thing that is different no air box and no flow tube to the carb.
the flow tube is the most critical. you have to have velocity but it has to be correctly mixed or does you no good.

 

http://www.mikuni.com/fs-performance_guide.html

 

it looks like a stock 70 carb that i run on my fiddy silver hehehehehe
you need an xr 100 carb for sure couldnt find the calculator on that page anymore hmmm