Yerf Dog Engine Swap

[Grampy]

Hi, great forum. I just joined.

What challenges will I have to change out a 6.5 hp Tec with a Harbor Freight 11 hp unit? The Yerf is a 3203 model. Anyone tried this?

 

[anderkart]

Your 11hp will have a larger motor mount bolt pattern and a larger 1" output shaft (as oposed to the 6.5's 3/4" size) so you'll be needing a larger sized clutch or torque converter. Besides that it should be a fairly easy swap as long as you have the extra space.

We could probably give more/better info if you posted some good pics of your karts engine/axle area

 

[Blazkowiez]

The main problem aside from the clutch you will be facing is the fact that the rear end of those karts were amazingly weak and had a tendency to twist and break even with the stock engine, moving to the larger engine will tear it to pieces unless you take the time to weld bracing.

 

[Grampy]

Thanks to both who replied. Is there anything special I should know when ordering the correct sized clutch? I assume the existing one can not be properly machined to match the new shaft?

I do have welding equipment to beef up the rear of the cart. I just purchased this for my 10 year old grandson. He was looking for a project to do with his grandfather.

Any recommendations on a web-link for a clutch?

 

[anderkart]

Any recommendations on a web-link for a clutch?

Yer welcome. I've been running this 1" clutch model: http://out2win.com/catalog/noram.html#star on my 20hp. race karts for a couple years now, its still holding up great and is available in your choice of any #35 drive gears between 13 and 18 tooth.

This place: http://www.northerntool.com/shop/to...rt-mini-bike-accessories+brakes-clutches-hubs offers a couple 1" models at a lower cost but they don't offer as many drive gear options and I don't think there as high of quality as the Noram clutch I use.

But before you order any clutch I've got a few questions to help figure out a good gear ratio so you get one with the right # of teeth.

How many teeth are on your old engines clutch? (and does it seem to gear the kart the way you want it?)
How tall are your rear tires?
How many teeth does your Axle sprocket have?
Does your kart have a jack shaft? (if so, how many teeth do each of those gears have?)
Measure and post the clutch sprocket tooth-spacing of your old engines clutch so we'll know for sure what size gear your new clutch will need to fit your chain.
And finally where will you be riding the kart, (paved flat ground only, or off road/hill-climbing maybe?) and would you prefer better acceleration/climbing ability? Or a higher top speed? Or possibly the best compromise of those 2?

 

[Grampy]

Hi Anderkart:

Here is the information on the gearing:

Drive sprocket: 10T
Driven sprocket: 60T

I believe they use #40 chain

Rear tires are 19" diameter measured from the grount to the top.

The cart is used on mostly level ground in grass field conditions. Currently the engine lugs down some once it gets going. Not looking for more speed. Mostly looking for increased acceleration performance. I think it currently has a 30 series torque converter.

I am not sure what the jack shaft is that you refer to.

Thanks again for helping. This modification may not be worth all the effort as the cart appears to be in pretty nice shape. I have attached a couple of photos. I have not removed the guards yet, so they do not show much.

 

[slideways]

I would just regear the kart and put a heavy duty spring in the torque converter. That will help alot with low end if thats all you are after. Maybe a 9t 65t ratio. The key is finding a sprocket that will work on that axle if they are out there. The TC springs are cheap and on my Dingo they engage at a high rpm that actually spins the rear tires for a second it hits so hard.

 

[anderkart]

Yeah it certainly looks like you've got a torque converter under that plastic guard, so If you installed an 11hp you'd need to replace the driver portion with a 1" bore model at the minimum. (post a pic with that guard off so we can tell what series T/C you have there)

Your kart has a jack shaft, its what that rear-belt pulley (that's the driven half of your torque converter) and small chain gear mounts on. If you installed the 11hp with the same gear ratio you'd get better acceleration with the same top speed.

 

[Grampy]

I would just regear the kart and put a heavy duty spring in the torque converter.


That may be the easiest way out. However there seems to be alot of negative info in various forums on how weak the 6.5 Tecumseh is and the fact they wear out in 150 hours. Mine seems to run pretty good so far. As I indicated earlier, this is really a project to give my 10 year old grandson some practical experience with tools and ingenuity. Thanks for your input.

Al

 

[Grampy]

Yeah it certainly looks like you've got a torque converter under that plastic guard, so If you installed an 11hp you'd need to replace the driver portion with a 1" bore model at the minimum. (post a pic with that guard off so we can tell what series T/C you have there)

Anderkart:
I believe I currently have a Comet series 30 torque converter on the cart.

I found one with a 1" bore on this website:
http://www.mfgsupply.com/m/c/218355A.html

Al

 

[slideways]

I'd cross that bridge when you come to it. Looks like the whole kart and motor is in really nice shape. Nothing will work as good as a nicely maintained factory unit. If you keep the oil changed and maintain the Tec it will last a long time. The ones that don't are run low on oil or old oil and are turned too hard rpm wise.

If you go to the 11hp it would require you to do alot more fab work to fit it on. The motor plate needs to be redesigned and sometimes the muffler doesnt quite make it into the rear part of the caging. Plus the throttle linkage and getting the torque converter lined up right and don't forget you may need a new custom belt since the motor crank height will be different.

 

[Grampy]

I'd cross that bridge when you come to it. Looks like the whole kart and motor is in really nice shape. Nothing will work as good as a nicely maintained factory unit.


Slideways:

I agree with you. I think I will work to keep this one working well and what happens in the future.

Al