Does that centrifugal clutch give you a low enough gear ratio? I built one for the kids years ago and just used a slipping belt for a clutch and it was geared too high. Back tires were old garden tractor rears so they covered more ground than yours though. Have to say my brake (external contracting band) was less than spectacular as well. They eventually broke the back axel and it got parked. Put the carb off a 18 HP Briggs on an old 10 HP Wiscosin and bumped up the timing at the magneto. That thing screamed! A little too good actually...
Gearing has been an issue, largely due to a lack of selection of tire rim packages, they are wet weather gokart tires. The problem is of course the fixed driven axel, you need a rim you can fix solid to the axel. I had a sprocket on the back that was virtually running in the gravel to keep it slow enough. I need to find a clutch with less teeth;mine is a 17, I need an 11. I have since put a smaller sprocket on the rear to make it well clear the gravel but its harder on the clutch and has poor acceleration. I think we have stuffed the clutch now because of this. The solution is to put a counter shaft with a gear down by 50% in the system. I want it to accelerate hard so they can practice power slides on the gravel and get a top speed of say 50km/h I think it's theoretical top at the moment is 70. You can buy clutches with 11 teeth. Problem is getting small sprockets in 35 link . I wish they were made out of 40 link , things would be much easier to find.
Yea... I came to the conclusion I either needed another drive or a small differential on the back. Ours wanted to go 60mph but didn't have enough power to get there. We clocked it at 45 mph on a field trail until I got airborne going across the yard and almost ended up in my mother's flower garden. Even bad brakes are horribly ineffective when the tires aren't touching the ground. Hmmm....shoulda had air brakes!
That is quite the cool little rig you built and I always wished I had something like that ... never did though.
Its hard to make one gear ratio work well for the whole speed range and often thought a snowmobile engine ( liquid cooled with rad and fan ) with its belt variable speed could work or even more cool would be a street bike engine with its 5 or so gears ( liquid cooled as well as air cooled wouldn't like being out of the direct wind ). Can you imagine a 750 twin or larger engine's power, the smoke or dirt would fly from those tires . Wouldn't exactly be a child's toy then LOL.
If you use a bike or snowmobile power train I strongly advise transplanting the brakes as well. I've seen them with snowmobile parts before, but again it's all about the gearing. They weren't as fast as I would have thought. By the time you get over 700cc you may as well be building one of those old VW powered dune bugs.
Some small engines have a threaded hole on the PTO side or a bolt pattern that a bracket for a countershaft can be mounted to. This is an older competition cart that I had for my kids. I quickly decided we were not going to mess with one gear ratio competition machines unless they had 8 or 900 horsepower and our budget mysteriously got much bigger. But I forgot to buy lottery tickets, so MX had to do.
This one has been refitted with a 3 HP and does about 15 MPH all day long in a power slide on the lawn using pavement slicks. Those are just welded together sprocket hubs with brass bushings inside mounted on a 5/8's bolt.
I think my oldest son is going to run my 20 year old KX 500 motocross bike in the Pro A class this Saturday, because no one has been able to beat it in practice this summer, and the fricken 450 has cut its engine in half twice with its connecting rod. I'm going to have to take a day off to go and watch the old 2 stroke.
Cool.... get there while you can. I was messing with the cart stuff when my boys were like 12 to 14. They were both getting taller than me so the cart conveniently ended up being more "adult" sized. Had fun with it while it lasted. Oldest grandson is now 9 so we might have to drag it out of the long grass and try a 2nd version rebuild with much much lower gears.