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Old 10-30-2018, 12:20 AM
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Default 79cc Mini/Dirt Bike with Clutch Accelerating Poorly

Hello, I recently converted one of those little Razor electric dirt bikes to a dirt bike / mini bike using a 79cc Predator Engine. Most of the parts are new.

Currently, I have an 11 tooth centrifugal clutch, a 60 tooth sprocket and a 15 inch drive wheel. It probably weighs less than 100 lbs. It's top speed is about 30 mph.

Problem: Even when my little brother, who only weighs ~75 lbs., rides the bike, it takes a considerable amount of time to reach top speed on level ground, approximately 15+ seconds, and has virtually no low-end torque. The clutch is brand new, the throttle is connected in such a way that it does reach full throttle quite easily, and the clutch is secure on the drive shaft.

What can I do to improve the take off time and the torque? Is it an issue with the engine or the clutch or neither?

Thank you. Help is greatly appreciated.

By the way, this is the clutch I am using:
https://www.amazon.com/Jeremywell-Ce...h+35+chain+12t
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Old 10-30-2018, 12:34 AM
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Welcome to the forum!!

The problem you have is your gear ratio. You are geared too high at 60/11 = 5.45:1. You are going to have to gear lower than what you have at the moment. Especially with only a 79cc. You are going to have to gear at least 8 or 9:1.

Seeing as your clutch is already a small tooth count, you are going to have to go up on the sprocket size. Considerably. As high as you can practically go. AT LEAST a 72T, but preferably higher.

You will lose a bit of top speed, but you will gain some low end
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Old 10-30-2018, 12:37 AM
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Thank you very much!
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Old 10-30-2018, 12:44 AM
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Thank you very much!
No problem
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Old 10-30-2018, 12:56 AM
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One more question: Can I preserve the top speed and gain low end while using approximately the same gear ratio if I switch to a torque converter? If not, how much of a difference is there between a 212cc vs. the 79cc in terms of power overall? My go kart, which probably weighs 320+ lbs. uses a 212cc with a torque converter and a similar gear ratio, and that reaches 36 mph quite quickly.
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Old 10-30-2018, 09:29 AM
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They don't make 5/8" bore Torque Converters.
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Old 10-30-2018, 09:53 AM
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Alright. Thank you for the input. That is good to know in the future so that I don't spend an hour scouring the internet for a 5/8" bore torque converter. Haha!
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Old 10-30-2018, 11:53 AM
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Let me influence you.
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5 8 X 3 4 X 1 14 Shaft Adapter Sheave.jpeg  
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Old 10-30-2018, 11:58 AM
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Let me influence you.
I've tried finding one. It isn't easy, and the key will be a big issue because it won't be long enough unless you went for something like a 30 series 1" bore and then made an adapter from 1" to 5/8".

Something you might want to ask you local machinist very nicely for or give him a few $$ for it.
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Old 10-30-2018, 12:27 PM
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Well...this might work. https://www.gopowersports.com/colema...converter-kit/

It has a 3/4" to 5/8" sleeve but I'm not sure if the pto on the 79cc is long enough to work properly.
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Old 10-30-2018, 03:19 PM
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That GPS kit probably would work.

But for $180 bucks, you could buy a predator 212 AND an Chinese 30 series and get WAY more bang for your buck.

The fact is, you've got a wee little engine on there. There's only so much power available no matter how you gear it. You might be able to shoehorn a 212 in there using a remote fuel tank... Another consideration is that the torque converter will stick out much farther than the clutch does, and due to the location, would need to be run with the guard on, which makes it even wider, so it might make for an awkward riding position.
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Old 10-30-2018, 09:11 PM
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I know!!! 5/8" BORE V-PLEX!!! A--HAAHAHAHAAA
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