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travismarshall 12-06-2012 01:28 PM

Newbie - Questions about engine for rebuild
Hi, I was wondering if you had time to answer a few questions for me. Basically someone gave me an old go kart chassis that has been sitting outside for a few years and although I am not mechanically inclined I thought it might be a fun project for me and my daughter to rebuild it and also save some money instead of buying a used one.
So hereís what I got. This is an old Roketa KTX-90. Originally it had a 90cc engine with automatic start and a hydraulic brake system. I know that I am going to have to get a new engine for it and Iím pretty sure that I need to get a new brake system as well. Here is pictures of what I got.

So a few questions.
1. Would something like this work for the motor?
2. How do I know what centrifugal clutch to buy? I see from Roketas site that the original chain is 420/48 and the sprocket on the axle is 420/38.
3. Will the cable that comes from the gas pedal fit into this motor or any motor that I buy?
4. Is this doable/worth messing with?
Thanks so much,


OzFab 12-06-2012 05:10 PM

Ok, before answering your questions, let's take a step back: What's wrong with the 90cc engine? Why do you think the brakes need replacing?

travismarshall 12-06-2012 06:32 PM

Well. Apparently the people that owned the house before my friend left the go kart in pieces outside, probably been sitting outside for years. I dont know anything about engines but the engine look bad to me and is missing stuff from it. Rather than try to put parts on an old engine and try to make it work just seems easier to me to pay $150 for an engine, convert to a simple centrifugal clutch, get a chain connect cluth to axle and be done with that part. Again, I am needing advice and wondering if its that easy or there is something i am missing. Brakes system is missing cable. also on the cylinder part, it seems after several years outside its bad. I tried to take the screws of the brake fluid container and couldnt get one out. Plus my kids are going to be riding this thing and dont trust them. I found the exact brake assembly on Roketas site for $80 so seems best thing to do would be just to replace the assembly and know they are good to go.



Amped 12-06-2012 08:14 PM

The 6.5 hp is around 190-200cc. The 90cc is around 3 hp. So double the power. And yes you can attach the throttle cable to it in several ways. But the 6.5 hp will probably have different mounting holes.This is not hard to fix though you may have to take it to a welding shop or maybe a lawn mower shop or small engines shop if you don't weld which I'm guessing you don't . I'm not sure how much it will cost.Ask all the local shops of the types above and tell them what you are doing and might need a new mounting plate made , aligned, and welded on. Get a price quote .The kart you have is $1000-$1200+ new so if the repairs are less its worth it. And you and your daughter will have share a great learning experience.The clutch will be $50-$100 w/chain ,brakes $80, engine $150 , Post us a lot of pics from every angle and close ups of all mechanical parts, cables ,wiring. Anything you question. There is a ton of kart knowledge and wisdom on this site.The more info you give us the more we can help.

travismarshall 12-07-2012 08:06 AM

Man, I didnt realize the 6.5 was so much bigger. Probably more power than I want the kids to have. I found this 2.5hp on ebay:

Would that work? I am guessing it still wouldnt mount up like the old motor since the old one was custom for this frame and used a torque converted instead of centrifugal clutch.

So as for the clutch, I understand that I just need to get the bore size to match the engine but from what i am reading since the axle sprocket is #420 I would have to also get a #420 clutch and chain? Would a 40/41 work as well?


jamyers 12-07-2012 12:53 PM

Does your kart look like this one?
If so, there's another guy here on the forum who is in the middle of rebuilding his...

In your Gokart 2 pic above, that's not a brake part - it's the throttle cable with the 'tophat' and slide from a slide-type carb still attached.

My bet is that the original 90cc engine was either a GY6 (looks like this - left view);

or E22 series (looks like this - top view),

Both are clones of Honda engines that have attached transmissions. Both come in a variety of displacements / hp levels as well.

Which means that if you go with a "normal" engine and centrifugal clutch, you're also going to have to do some re-gearing, probably a much larger sprocket on the axle (like 60 tooth) to match the 10 or 12 tooth sprocket on the cent clutch.

I'd seriously try and find out what engine/tranny was original, and either rebuild yours or replace it with a new one - in the long run it'll likely be cheaper than converting + re-gearing, PLUS the tranny's on these really make a difference.

Heck, you can pick up a 150cc GY6 (7-10hp, depending on the source) complete for under $300, and that's with electric start and sometimes a reverse!

TONS of info and parts are available for the "Chonda" (clone + honda) engines.

travismarshall 12-09-2012 08:25 AM

Yeah, that looks similar to the old engine. So if I were to go the regearing route would it basically just consist of replacing the axle sprocket with a bigger one? It looks like I would also have to modify the frame to fit a bigger axle but if I were to be able to get the old axle off and put a new 60 tooth sprocket on and then get a 10 tooth centrifugal clutch would that be all it takes? I probably would go ahead and go with a bigger engine since ive read that a torque converter gets a lot more out of an engine than a centrifugal clutch so that 6.5 would probably run similar to that 90cc.

I agree that the GY6 would be the slicker option but that just looks overwhelming to me and would require a mechanic or someone else doing most of the work. Instead of going that route I would probably just buy a brand new one for around $1000 delivered.



jamyers 12-10-2012 11:27 AM

Yup, to regear you'd have to replace the axle sprocket with something like what you described.

Yes, torque converters (actually a Continuously-Variable Transmission, or CVT) is a wonderful thing, way better than just a centrifugal clutch.

How much of the old GY6 do you have? I'm still thinking that rebuilding / replacing it and going back to stock is going to be simpler than modifying the frame, converting, etc. Plus you'd have electric start which really is a great thing for a kart... (I'm also becoming more of a fan of the GY6 the more I drive this chinese atv...dang thing started right up this morning (25 degrees F) and runs like a champ every time!)

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