Kart is to slow. gearing?

[chancer]

So I just finished this razor buggy conversion to Gas and it moves ok but has very little power and almost stops on the slightest incline.
Kart and rider weigh 158 pounds total. I stole the following (sticky)and input our numbers:
I have 6.54:1 gearing (11 tooth clutch & a 72 tooth axle sprocket), a 79cc predator 3.5 torque @2500 rpms engine & 13” tall rear tyres:
To find the available torque at the wheels, first you need to multiply the engine torque by the drive ratio:

3.5*6.54=22.90 ft/lbs

Now we need to find the ratio between the axle sprocket & the tyre; a 72 tooth #35 sprocket has an outside diameter of 8;814” so

13” tyre divided by 8;814” sprocket = 1.475

Now we divide the axle sprocket torque by the axle sprocket/tyre ratio:

22.90/1.475 which gives us 15.53 ft/lbs of torque at the rear wheels.
How many pounds of torque are needed to make this kart move.
158pound total kart and rider.
Should I put on a 60 tooth sprocket?
Try to find 11" tires? both?

 

[landuse]

I am not too sure about all the numbers, but I would gear even lower with that small engine to get you moving

 

[KartFab]

Gearing is fine. Engine is too small. 212cc would have worked out fine.

 

[chancer]

Gearing is fine. Engine is too small. 212cc would have worked out fine.

Bummer now what, I know the gearing should work. So if I am just under powered any thing I can do? I wanted a slow kart for my young daughter but This is too slow and the 212 is too fast.

 

[firemanjim]

Well CRAP! Can you put a TC on those? Seems like I saw one on ebay..... Imma go look around.....
You just don't have the power..... Work the engine over a little, then see how she acts.... You are probably gonna need the help of a jack shaft. I would have jack shaft-ed it to begin with. More room to play with ratio. No huge 72 tooth sprocket hanging down, looking for something to hit..... And I think I would have went with a 11-12" set of tires..... This is just me though.
Don't get me wrong.... ABSOLUTELY one nice looking kart. You did a great job! I'm actually looking for one of those, stupid cheap, to put this 3.5hp 2 smoke engine on... I think it would be CRAZY fun.....

I think I would consider setting up a jack shaft and ditching the 72 tooth. I should have said something before this point, but I've been mentally "preoccupied " lately. Not too much deep thought going on up there....
Does that engine have a smaller foot print than the 212? It does, doesn't it... Why did I ask a stupid ?? Like that.....
You probably have the needed supplies to build a j/s already. Just need one 5/8 shaft, couple bearings, some 1/4 plate (or 1.5" tube 1/8" [.120-.125] thick wall.... Standard kart bearing fits inside jusssst right. Only need inner spacer.....) I think you would be happier that way.... Then later, you can use that set up to power another project, after you put in a 212.....

 

[chancer]

I think I saw a TC on one of these somewhere. Also thought about a jackshaft and did not think it was necessary. (Plus never done one before.) Yeah I wanted 11 or 12" tires, These tires came with the wheels. It is hard to find 6" 3/4 drive wheels.
This engine does have a smaller foot print. It comes with a adaptor plate if you want to put it on a standard 5 or 6.5 hp mount.
What do you mean by "work the engine over a little"

 

[firemanjim]

Mod it.... Same as any other engine. Better intake/exhaust.... More fuel.... Adds up to more hp.....
Was gonna go look for tc, but got side tracked by jc Whitney..... ill go now.....

 

[chancer]

Using the numbers above from fabromans formula "drive ratios for dummies" If I go to an 11" tire I can increase my rear wheel torque from 15.5 to 19.08. that should help some.

 

[firemanjim]

Using the numbers above from fabromans formula "drive ratios for dummies" If I go to an 11" tire I can increase my rear wheel torque from 15.5 to 19.08. that should help some.

Definitely, but will the sprocket still have clearance?
I can't find a suitable tc.....

 

[chancer]

Barely. 72t #35 = 8.75" 13-8.75=4¼" /2=2.125 clearance. so 11" tires will give me 1.062. That's where like you said a jackshaft would be nicer.
I don't want to put exhaust on it but maybe an intake and jet.
Thanks for looking into the TC.

 

[chancer]

Affordable go karts sells some performance parts for this 79cc. Jets, intake etc.
What do you think of this how to, on drilling jets.
http://www.affordablegokarts.com/Drilling Main Jets.php

 

[firemanjim]

That's a pretty good article. I learned a lot from that, and their other tips.....

 

[chancer]

Do you think a rookie small engine mechanic would be better guessing (they suggest an 80)or trying to drill their own jet.

 

[firemanjim]

I use a welders torch tip cleaner to "adjust" jet sizes... It works out pretty good for me. I start with the rod that fits the hole, then move up from there. I take plug readings as I adjust jet size..... Not real difficult. If you screw up and go too big, solder it up, and start over. If you have to solder one, you WILL need jewelers drill to put new hole. The drill/bits are fairly cheap....

 

[planejob]

I have the same engine on a similar sized kart. I have 18:1 gearing (jackshaft) that allows the engine to wind out fully, keep the clutch engaged, and have a crap ton of torque at the rear wheel... problem is, it's slow, like 7.5mph while the engine is at full song. No problems climbing hills, or curbs, or small trees.

I looked for a torque converter for my engine but never foundd one that would fit the 5/8 output shaft of the 79cc harbor freight predator.

 

[chancer]

Cool thanks Plane job. I remember something about mud slinging torque on that kart! but you have to tell the story of Climbing small trees. But with those huge lug tractor tires she probably thought that is what it is for.

 

[firemanjim]

Here is the solution to the shaft size problem. Only question is, is the bolt pattern on the side of the engine is the same as a 212....

 

[chancer]

I will go check. But I think you are talking about going with a torque conv. I would like to try and do a jackshaft. It is cheaper at this point.
(EDIT) Ok yes the four mounting holes are the same. The manuals 212 and 79 show a circle connecting all four holes and they are both a 3.625" radius.
As far as a jack shaft how do you figure what you will want for a final drive ratio, Plane job went 18:1 but 10:1 sounds ok for me for top speed. I just don't want the thing to sit there and bog down like it does now at 6.5:1

 

[firemanjim]

http://www.ebay.com/bhp/reducer-bushing
There is the link I forgot to post earlier.... My bad...
Yes, a j/s would probably be cheaper.... The tc would just be more advantageous....

 

[chancer]

I was looking at those the other night. don't remember why? If anybody here (my house) gets a TC it will be me.