Honda 125 4-stroke engine (sprocket?)

[rednecktech]

I have started fab work on my barstool kart. I have an old (atv motor) Honda 125 4-stroke engine with a 5 speed shifter (R-N-SL-1-2-3-4) electric start and reverse.
My question to all is, How many teeth should I use on the rear axle sprocket?

I am using the stock engine sprocket (13 tooth) 428 chain, I have my tires which are 7.10X11.5X6, and I have a desired top speed of about 40 mph.

[Canadiankid]

Did a little research and I'm pretty sure 125 atv had 48 tooth rear sprocket and 22" rear tires and I'm guessing a top speed of about 40 mph. If that is true with your 11.5" rear tires I would put a 15 tooth front sprocket & a 30 tooth rear should get you a top speed of 40 mph. But hang onto that sucker cuzz first gear should peal you off the stool if you peg it.

[RUBICON]

I have a honda 90 clone with 14/44 and I see 45mph....i would expect your engine to turn within 500 rpm or so of mine....

[Canadiankid]

An atv tranny has lower gear ratio than a bike engine. The ratio I suggested might be tall but I would guess to acheive 40 mph with a atv donor engine 14/44 maybe a little slow. But then again I may have know idea what I'm talking about. Atv's do have a lower gear ratio do to weight and available traction.

[RUBICON]

I can't say where I am on final output ratio...but most stuff is near 1:1 in 4th gear...and I would guess the engine I got was for a small motorcycle or such...it had a strange lever to start it without the electric starter that led me to beleive it was for one of those bikes you see in National Geographics that has 3 people, 4 bails of hay, a pig in a crate and 2 chickens in a cage...tied on the top....if so the wheels would be larger than the ATV and therefore lower ratio....Best advice...build something and see where you are...then begin the fine tuning....mine has been redesigned 4 times....the last was nitrous...good luck

[Canadiankid]

How many bails fit on your stool & do they stay on when you hit the NOS. lol. Trial and error is the best way and the most fun. Garage time is good.

[rednecktech]

thanks for all of the imput I think I will start with a 13 engine and a 35 axle and see how it goes.

[Canadiankid]

Make sure you let us know how that gear ratio works out. And post pics of build up and finished product...good luck.