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Old 07-27-2013, 05:06 PM
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Default Honda GC160 won't move mini chopper

New to go karts and mini bikes but I came across a free mini chopper with a 49? cc weed wacker engine, rebuilt the carb, rode it and thought a bigger engine would be more appropriate for me, at 6' 200 lbs. I got a stock GC160 and a couple boxes of go karting parts and got to work. To get the bigger motor to fit I chopped and extended the frame, made engine mounts and put it all together. With a small engine tach I'm getting 1250rpm at idle and max is about 1500rpm, which isn't enough to move the bike with me sitting on it but the bike will take off if I pick up the rear end.


Honda gc 160cc from a pressure washer with Governor removed.
Cam timing was verified
Stock exhaust and a modified air filter to get it to fit in the small bike. The filter is 2" in diameter and 1 and 1/2" in height with 8" of 1/2" tubing from carb to filter.


Wil attach pictures of bike if needed.

Chain is tight and square from clutch to rear sprocket
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Old 07-27-2013, 06:27 PM
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Before with 49cc engine


Removing governor from the 160






Before I realized the engine runs counterclockwise


All together


Chain tensioner
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Old 07-27-2013, 08:18 PM
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Welcome!

max @ 1500 rpm? that must be a typo right?
if not that's the point
The clutches I know about are mostly set up to fully engage @ 2000-2200 rpm so it wont move you an inch @1500 rpm.

And: We looove pictures, so maybe you can add some
[EDIT]
Ah there they are...

'sid
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Old 07-27-2013, 08:36 PM
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Your issue is gearing. Your reduction ratio is too high! Please post the tooth count of the clutch, tooth count of the rear sprocket, and the overall rear tire diameter.

You can't exceed 1500RPM because your clutch is TRYING to engage, but your engine hasn't enough torque at that ratio.
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Old 07-27-2013, 08:50 PM
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Default No go scooter

Nope not a typo. I posted some pictures but it seems a moderator has to approve, possibly because of my low post count, 2 lol
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Old 07-27-2013, 08:56 PM
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Whoops- Missed that, sorry. Should be good now.
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Old 07-27-2013, 09:05 PM
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Will check the teeth count and report back in the morning
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Old 07-27-2013, 11:08 PM
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Your 49 cc engine must have had a gear reduction built in. I can see enough of your rear sprocket to know it aint going to work like it is now. Your going to have to build a jack shaft system or use a much bigger rear sprocket.
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Old 07-27-2013, 11:22 PM
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Clutch: 17 teeth
Rear Sproket: 40 teeth

So 40/17 = 2.35 which is too low of a gear ratio to really do anything. Cost wise it seems that getting a new sprocket for the rear wheel should be much cheaper and easier than getting a torque converter or jack shaft so I'll look into that. Would a bike sprocket be fine, somewhere along the lines of at least 90 teeth to get a 5.29 gear ratio or is that still too low. I hate to get a new clutch but they maybe a better option overall

All the smaller engine had was a chain tensioner, granted it only went about 15-17mph

Last edited by Mike:; 07-27-2013 at 11:37 PM. Reason: Added
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Old 07-28-2013, 01:11 AM
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get a smaller clutch sprocket say 10 or 12 tooth count and rear sprocket of say 70 and you would be good to go. also i would get a TC when you get the money.it will blow a cent. clutch out the water in any condition
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Old 07-28-2013, 01:54 AM
philip metzner philip metzner is offline
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Unfortunately I cant just figure it in my head. The thing is that rear wheel size also changes what gear is need. How much distance does that wheel cover in one turn compared to a standard mini bike wheel. Your going to have to do some research to figure it out. I bet you will need a rear sprocket almost as big as your rear rim!
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Old 07-28-2013, 07:14 AM
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I very much doubt you're going to achieve a low enough ratio simply by swapping in a larger rear sprocket. That 17T clutch really ruins the numbers for you, and (although I still don't know the diameter) that rear tire looks pretty big.
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Old 07-28-2013, 08:25 AM
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Hmm, toystory is more likely right, that this gear ratio is not really a good idea.
But I think it should rev up and move you at least a bit (possibly burning your clutch)
Assuming you have a 120/70-10 rear tire (dia 42cm or almost 17")

Why? well the Joker Felt Slater motorized bicycle

Has a very small ratio too as you can see (looks like 12-40) and allthough it's still better than yours it surely does have a larger rear tire dia
And from what I've been told the TC doesn't really compensate for that.

Oh that's a GC160 too btw

And if the Felt moves (and it does) yours should move too -lousy maybe- but it should move
So, could you rev up the engine without load to at least the 3600rpm it should have?
Maybe an issue with compression?

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Old 07-28-2013, 08:37 AM
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Sid- The CVT most certainly does help compensate for a poor ratio (incl tire diameter). That's what makes a CVT the beautiful thing that it is. At the point of engagement, the CVT adds a reduction ratio of about 2.5:1- So- If you have (example) 3:1 between the sprockets, start-off with the CVT will be around 7.5:1! You can sure bet the vehicle will have an easier time staring off at 7.5:1 than at 3:1.

I don't think there's anything wrong with the engine. These centrifugal clutches begin to engage at around 1500RPM. If the engine hasn't the torque to get you moving at a ratio that is too steep, then the vehicle (and the engine) simply will not accelerate beyond that point.

Also- not that it is the critical element here, but do ensure that there's a bit of slack in that chain. Having it too tight will do more harm than good.
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Old 07-28-2013, 08:47 AM
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You definitely know more about all that stuff than I do;
But from the back of my mind I think I was told to not take the TC into account for calculating my gear ratios.

Well then Mike, nevermind me

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Old 07-28-2013, 08:54 AM
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You're right- I generally do tell people to leave the CVT invisible in the equation, as it (generally) won't affect top speed gearing- which is what the calcs are for. Done that way, you end up with excellent acceleration to go with your maximum speed. Here's the fine print:

When using a CVT, you can often gear a little steeper and get away with it, because of the engagement ratio. But there's a lot of room for trial-and error, so I don't generally advise it from the start. Gear TOO steep, and you can accelerate well enough, but not have the torque to attain or maintain the projected top speed. I had a kart like this- I actually GAINED top speed by gearing DOWN because the original ratio was a touch too high.

In addition, some asymmetric CVT's actually feature an overdrive factor of about 10% at the top end. Between the engagement ratio and this overdrive ratio, it is a LOT simpler for the newbs to just forget about it and gear normally. Doing this, the overdrives becomes a surprise bonus of a couple KM/h in the top end.
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Old 07-28-2013, 09:48 AM
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Assuming 17" diameter tire, this will correct ratio problem without adding a cvt or jack shafts.
Once mounted, all that will be needed is longer chain.
Martin #35 112 tooth sprocket
Attached Thumbnails
112 sprocket.jpg  
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Old 07-28-2013, 12:12 PM
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The 49cc motor appears to have the 5:1 reduction box they tend to come with.
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Old 07-28-2013, 02:24 PM
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The rear rim is 11" and the rear tire is 16"

So the consensus is that I should look into getting a 10T or 12T clutch as well as a larger rear sprocket upwards of 90T. This is the epitome of a budget build and I really need to keep it that way so with a clutch ($50) and sprocket ($40) am I still cheaper than a jackshaft/TC setup?
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Old 07-28-2013, 02:27 PM
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A TC is upwards of 150$. A jackshaft is about as cheap as you can make it. just need bearings, an axle for it. just a keyed shaft or even just a round bar stock can work. and a couple of sprockets. i think you might be cheaper than a jackshaft

EDIT:what speeds are you looking to obtain?

Last edited by gvfc2; 07-28-2013 at 05:40 PM. Reason: Changing beards to bearings
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