125cc atv engine on go kart

[gokart00]

Hi guys! I bought a 125cc 3 speed with reverse from China, installed it on my homemade go kart (used to have a 6.5hp). I test ride the first time yesterday and it was disappointing. I'm not sure whether the engine is defective or not, shifting from 1st to 2nd makes almost no difference at all, the engine barely accelerate and it just revs very high, forcing me to shift into 3rd gear. I thought the gearing is too low, 14T to 41T (the tires are 13”). Then I change it to 17T to 37T. It is better but still the go kart is noticeably slower than before. The engine just vibrates and revs too high. I thought the 3 speed would be an upgrade and gain more speed Does anyone has experience with these 125cc engine before? Please give me some opinions. Should I swap the old engine back. Is it a bad idea after all?

 

[Karts of Kaos]

well you could continue to gear it up 'til the engine either starts to bog, or you reach your desired speed. or you could swap it back to normal. I think on these chinese buggies the axle sprocket is very small because the engine has a 2:1 internal gear reduction. but i could be wrong

 

[Dingocat]

Wonder if its a clutch issue.

 

[Dingocat]

Wonder if its a clutch issue.

Or it's just made for something with really high gearing. Not sure as these arent my normal thing

 

[gokart00]

Wonder if its a clutch issue.

That is what I thought at first. I have adjusted the clutch but its still the same.

Anyway thanks for the help guys, I would continue to experiment with the gear ratio

 

[Karttekk]

What happens if you change the smaller sprocket to less teeth? What was the ratio of the 6.5 HP setup? Generally a 6 to 1 ratio works, looks like you're running close to 2 to 1 which isn't ideal.

 

[anderkart]

You need to buy the largest counter sprocket available for your engine (like maybe a 19-tooth)
And then also install a smaller rear sprocket ranging from like 20-25 tooth.

Racekarts with 125cc (2-cycle) motorcycle engines typically install counter sprockets as large as 19-teeth, and axle sprockets as small as 23-teeth. Those racekarts have 11" tall rear tires and can reach top speeds in excess of 100mph.
Here's a link to the sprockets they use: https://cometkartsales.com/-428-Shifter-Sprockets/