My stock gearing is 13 up front and 38 in the rear and my chain size is 520/86, if i drop down to a 12 tooth spocket up front will i have to go with a different chain size?

 

No, unless your chain has no more adjustment left to compensate for the smaller c/s sprocket

 

Why do you want to put a 12t on the front? The best setup is to put a 14t on the front. With a 12t, you could climb any hill but you top speed might be only about 30 to 35 mph.

 

that same chain will be ok, if it isnt just grind a link off. You will defently loose speed though

 

if i put a 14t up front wont i loose low end though?

 

the 300EX has lowend to spare!

I'd say go with the 14T up front, and you'll be plenty happy with the results

 

30-35 mph?

by that logic, if I put a 14t counter sprocket on I could go what, 70 mph?

don't think so, changing counter shaft sprocket only one tooth will change top end by maybe 4-5 mph.

dropping down to 12T is a good idea if you never see 5th gear (tight woods, deep sand, lots of mud, ect.) may have a hard time keeping the front end down though, he-he
---------------------------------------------

 

totally agree with wikmaan, theres no way your top speed would be 30-35.

listen to wikmann

 

I dropped one tooth on the cs sprocket on my 400EX this year and a 200cc Yamaha motorcycle in 1998. Both received great new low end starting power. However my motorcycle has a speedometer. Under ideal conditions with the original sprocket the bike would do between 60 to 70 mph depending on the road, wind, etc. Now with the sprocket change it always does 65 mph, and gets up there much quicker.

 

Azrob is right, you'll never know till you try. I am running a 13/39 combination in my 400EX with Holeshot XCT'S and its got the power I want for trail with less strain on the motor. It can be compared to racing electric RC with the wrong gearing you never finish the race cause battery will drop.