Axle Sprocket/Tire Size questions...

[jonbenderr]

So I've been looking into ways to increase speed on the yerf-dog go-kart.

The obvious first option is removing the governor. According to the gear ratio calculators this would net a theoretical top speed of 50 mph! Yikes! It's a bit much for my single acre of open property. Everything else are little trails through the woods, so I feel like that's kind of overkill and also not worth some of the potential risks of the modification.

So I've come to the idea of reducing the axle sprocket and increasing tire size.

The axle sprocket would go to a 47t (current sprocket is 60t).

This would net a theoretical top speed of 36mph! I feel like this would almost be perfect.

Eventually, not right away, I would increase the tires from 16" to 18". This would bump that top speed up to 41mph.

My question is, what other things do I need to take into consideration with these changes? How will it effect performance in negative ways? Will it decrease climbing power for bigger hills? Will acceleration be reduced making it so I'm never actually able to see those speeds on my short open/straightaway areas?

Another reason I want to make these changes is for when I go down to the "farm" with family. They all ride four wheelers and keep some speeds up on trails around the 30-40 mph mark. It would be nice to maybe have a chance at keeping up with them. This might be a pipe dream, but a fun goal to shoot for.

Any help is greatly appreciated!

***EDIT***
After some more searching I found the incredibly informative post on gear ratios located here.

Seeing that a 4:1 ratio can seemingly insta-destroy a clutch I'm now reconsidering my plan as the ratio I had in mind would be 4.7/1 which I'm assuming is still pretty dangerous?

Is it possible to get a better clutch that can handle the lower gear ratio?

So maybe I will remove that governor after all.

 

[anickode]

With 16" tires, a 6:1 ratio is already pushing it for a centrifugal clutch, esspecially trail riding, where you don't run flat out all the time, allowing the clutch to slip.

A 30 series torque converter with an 11 tooth jackshaft sprocket would give you better performance. The 5.45 ratio, combined with the 0.9:1 overdrive of the 30 series would get you to 35 mph with the governor set at stock 3600. As long as the kart isn't real heavy, it should run quite nicely.

That said, you have to factor in the power of the engine. 6.5 hp (just guessing) might not be enough to push the kart that fast. Opening up the intake and exhaust, removing the governor, and richening the jetting a bit will let you some extra power and speed. Even still, a torque converter would be a great improvement overall. It would allow you to utilize your full throttle range, or more importantly, the bottom half of your throttle range, without cooking your clutch.

 

[jonbenderr]

My model yerf dog (3202) actually comes with a torque convertor stock. Not 100% sure, but it may be 30 series. When referencing manual part numbers, replacements come up as 30 series.

 

[gr8hairy1]

Removing the governor will not force you to go overly fast. The same as a car, your foot controls the speed. But you will need to pay more attention to the sound of your motor, listening to rpms, listening for valve floats. Higher tension valve springs would reduce valve float. If you want to be able to hold the gas pedal down continuously and not worry about blowing the engine, then leave the governor in. But higher rpms are usually the first step towards getting better top speed.

Playing with the ratios affect both top speed and acceleration when you change them. Are you happy with the acceleration now? Are you ok with accelerating slower to gain top speed? Also, think about the trails. How often are there hills or loose terrain that slows you down? If it's most often flat and you have great acceleration, choosing a smaller rear sprocket might be what you want.

If you want to keep your acceleration where it's at, then you'll need to (1)change the engine's max rpm and/or (2)increase the power of your engine (high flow air filter, high flow exhaust, and likely change the jet on the carburator). Increasing the power will give better torque (ability to accelerate) which will give you greater flexibility on do-able ratios.

 

[jonbenderr]

Thanks for the tips and advice.

Really starting to lean more towards governor removal.

Is adjusting the throttle so it never hits those rpms that cause valve float common practice or something worth doing?

Also what exactly do I need for carb rejetting? Any recommendation of a specific part? Still a little confused by this despite some research.

This is all very new to me to be honest so I have a hard time even knowing hot to search for some things.