Bought off-road kart/engine question

[briggsandstratton123]

G'day, I've gone and picked up this unidentifiable buggy thing for $200.
It needs major brake repair, seat adjustment (cos even though it is long, there is mesh against the roll bar that is preventing me from moving it back, (i will get a nicer seat though).
I have a 3 speed semi-auto 125 cc which should have enough power
(I think). my main concern was with the gear ratio; It has 22'' tires with a 31 tooth axle sprocket (which is basically permanent), and the engine has a 13 tooth sprocket. Now i'm not sure wheather the 3 speed ratios would allow that, and i have almost no room to upsize the back sprocket without serious modification. in terms of engine mounting.
Here are some average pictures, and I'll keep update with my progress.
Any help would be appreciated thanks

 

[ezcome-ezgo]

These images have a quality best described as random. I still don't know what this buggy looks like.

 

[RustyRedneck]

With the information given, we have enough to figure out the potential MPH it will reach at a guessed engine top speed in an assumed direct gear (probably 3rd gear). If my math is correct, that puts you at 82 MPH at 3000 RPM in direct gear. That's pretty $%$#@$#@ fast over here in America. Interstate speeds even. I might be looking for a way to slow it down too if I had it but I might just put shorter rear tires on it. 18 inch tall lawnmower tires would dramatically drop your top speed. But the formula I followed is based on theory that your engine will rap that fast (very likely) and your transmission has a direct gear (hopefully). I might get it running and try it first. Maybe I'm way off.

Engine RPM / total gear reduction = wheel RPM
Wheel RPM x tire circumference = IPM
IPM / 12 = FPM
FPM / 5280 = MPM
MPM x 60 = MPH

3000/2.384=1,258.4
1,258.4x69.08=86,930.2
86,930.2/12=7244.18
7244.18/5280=1.37
1.37x60=82.2

Hope this is helpful.

 

[briggsandstratton123]

gosh, That's way off
I could change back tire, and downsize engine sprocket to 10 tooth
I don't know if that would be enough though.
Jackshaft wouldn't be an option would it?

 

[itsid]

You all read that we talk about a geared engine with an internal clutch, not an industrial engine, don't you?

Those engines ALWAYS have an internal reduction ratio otherwise that gearbox won't work

But.. it's unlikely we can find out by googling it

chinese engines are the worst documented stuff you can get for some reason.
Very few infos on the net, and what's worse .. you are mostly unable to find the mfg of the engine you hold in your hands and all of them are different

Anyways primary reduction is likely in the realms of 3.5 :1 (+/- 0.3)
then follows internal gear reduction with the highest gear being around 1:1 (+/- 0.2)
and in some cases a secondary reduction (if not omitted 2:1 or so)

Let's just say there is no secondary reduction, the chain ratio pushes the
overall gear ratio still to 8.35:1 in highest gear.

with the engine having 6 reliable kW of power (8hp)
and 8 bragged kw (10.5hp)

I think that chain ratio isn't too far off to be honest.

bad news is, we don't know ANY of the internal ratios for sure withoutmessing with the enigne,
and since it's a semi auto we'd need to pop it open to find out.
(a manually started shifter we can just remove the plug and start counting to get the overall ratio per gear [which is enough to know])

You will have to guess and hope for the best.

I'd say the chain ratio is alright'ish
yes a 12T engine sprocket would be better pushing the overall ratio to around 9:1
in highest gear

BUT it's easy to find out...
install it, try ONCE if the kart takes off happily.. you're golden,
if not.. stop trying, and get a new sprocket to save the clutch from burning.
(judge by the butt... 12, 11 or even 10 teeth depends on how badly it tried)

For now.. not too concerned at all..
(Oh in case you don't find smaller sprockets for that engine.. buy smaller wheels for the kart 18" should work fine with this setup!)

'sid

 

[briggsandstratton123]

yeah, I've got 18'' lawnmower tires that fit on the rims.
I shhould have mentioned, the Kart apparently used to have a 200cc honda 5 speed semi auto,
and it would reach speeds of 85 kmph (I'm not looking to reach those speeds),
but would a 3 speed likely have simmlar ratios?

 

[bob58o]

So when I looked up 125cc 3 speed Semi Auto Chinese Dirtbikes,
this is what I found....

7.75 HP at 7500 RPM
6 Lb Ft at 5500 RPM
12" Rear Tire
130 lb Net Weight
132 lb Max Weight Capacity
262 Max Loaded Weight
41 Tooth Rear Sprocket
14 Tooth Front Sprocket
2.93 Chain Ratio

Your tires are 83% bigger in diameter.
Your loaded weight is probably 100% more.
Your 31T:13T Chain Ratio is 23% taller than the bike's 41:14

I'd say either the Chinese 125cc bikes flip over as soon as you hit 5500 RPM, or you may be a ways off with the gearing on your buggy.

I'm just playing the proportion game here. It's fun. If you have no life like me.
Playing that game with a 31 on the Rear, you'd want a 3T sprocket on the Front. LOL

 

[itsid]

So when I looked up 125cc 3 speed Semi Auto Chinese Dirtbikes,
this is what I found....

7.75 HP at 7500 RPM
6 Lb Ft at 5500 RPM
12" Rear Tire
...
2.93 Chain Ratio

Your tires are 83% bigger in diameter.
Your loaded weight is probably 100% more.
Your 31T:13T Chain Ratio is 23% taller than the bike's 41:14

Noooo.. nonono
https://www.powersportsdistro.com/S...-Bike-Free-Shipping-p/ssr-sr125-semi-auto.htm

3-00x12 rear tyre is what I read..
that means a 3.00 rubber round a 12" rim
that's 18" in total

so that wheel is only 22% bigger not 80something...
and yes, the chain ratio is 23% bigger

weight still is.... well an issue really.
But I can't tell how heavy this thing is, nor do I have any indication about the driver/passenger...

I still stick with "remains to be seen"

and yes.. if It were my thing I'd find a 11T sprocket just to dial in towards safety...
or..well.. use the 18" tyres which I guess is the better/quicker option for a testrun here

But even the 13T wouldn't concern me much until it fails to provide "sufficient" take off on the initial test.

mainly because it IS a pitbike engine, which usually means wet clutch and a good chance of a living clutch after first gentle (maybe failing) test

'sid

 

[briggsandstratton123]

okay, so the most I can do at this stage with minimal cost is buy a 10 t sprocket and use the 16" tires I have that seem to fit

---------- Post added at 10:58 PM ---------- Previous post was at 10:22 PM ----------

I weigh around 62 kg (skinny fella), and kart without me probablly 80-90 kilo (only guesstimate), It is more of an atv engine as it has reverse (same basically i guess)

 

[bob58o]

Noooo.. nonono
https://www.powersportsdistro.com/S...-Bike-Free-Shipping-p/ssr-sr125-semi-auto.htm

3-00x12 rear tyre is what I read..
that means a 3.00 rubber round a 12" rim
that's 18" in total

'sid

Nice catch, I guess that is a different story then.
The site I was reading also said "Rear Tires: 3.0 - 12, Dirt"
"Front Tires: 2.75 - 14, Dirt"

Other places is says 14"/12" Wheels.

Another place says 14" Front and 12" Knobby Rear Wheels



Makes sense in the pics since the sprocket is should be 6.5".


http://www.powersportsmax.com/produ...2IK7CVxfL9QyXa2Kk9YxN-Yl5P-J7QpxoCui4QAvD_BwE

---------- Post added 12-09-2017 at 12:05 AM ---------- Previous post was 12-08-2017 at 11:28 PM ----------

For the ATV's I get 18" Rear Tires
14T Front
37T Rear
222 lbs
7.7HP at 7,500 RPM
8 N-m at 5,500 RPM

This thread talks about how it won't go uphill without getting off and pushing it. Also where I got the sprocket counts.
https://atvconnection.com/forums/1-engine-problems/340968-coolster-125-no-low-end-power.html
________________________________________________________
I would gladly trade some top end for grunt
any idea where to buy the rear sprocket?

I looked at my sprockets, by my count the front one has 14 teeth and it says 420, the rear looks to have 38 (manuafacturer says 37, I could have miscounted)

front sprockets seem to be available in 12,13,14,16,17 but I havent seen anything but 37 for the rear.. would dropping 2 teeth to a 12 in the front make much differance?

thanks

edit: I actually found someone who will make a steel 42 tooth sprocket, but it would be much cheaper to just change the front if that would do it
________________________________________________

I have the same issue with a 125 coolster. It will get up to 30mph on the flats with my big butt on it, but if my little girl who ways 50lbs is on it and there is any hill or incline at the start the thing just sits there and will not move unless I give it a push.

__________________________________________________________--
mines the same way.. goes plenty fast in the neighborhood, but hit a trail with some incline and you have to hop off and push if you slow down too much

---------- Post added at 12:17 AM ---------- Previous post was at 12:05 AM ----------

okay, so the most I can do at this stage with minimal cost is buy a 10 t sprocket and use the 16" tires I have that seem to fit

 

[itsid]

<200kg...
is great.. that thing looked heavier on the pics really
and if your weight estimation is correct, you're still in the realms of the total weight of the dirtbike
and not hit the weight limits of the aforementioned coolster 125 atv, the people are complaining about.


Anyways..
I think those smaller wheels alone should do really,
first you said they're 18" now they shrank to 16"
if they're indeed 16".. they will do the trick,
18" might do the trick.
And I'm only saying that because you have them laying around, not because I'm against swapping the sprocket for a smaller one

And bob..
yeah, that's the beauty of chinese engines.. you'll never know what you'll get.
improperly jetted engines, faulty recitfiers/cdis ... glazed clutches... a bag of marvels.

semi auto transmission might not work as one would suspect (on these engines)
as far as I can tell it's indeed a shifted gearbox, that (unfortunately)
decides itself which gear it wants to be in..
(that's what explains people needing a "run at" inclines and "ease into it" and such at the forum you linked)

And here's the thing.. if you want to go up a hill you rev up hard (well I do)...
and the engine thinks (oh look.. no load and 5k rpm.. I shift into 3rd)
and there goes all the torque, since naturally you cannot climb in highest gear on a shifted vehicle.

But I agree.. that does not sound promising!
I'd still assume these are not properly working engines (lack of compression.. insufficient fuel, faulty gearbox...)
rather than a wrong setup.
The chinese very well sell stuff that might not work (lack of proper quality control)
but I never saw them selling stuff that CANNOT work (lack of skill)

So my guess is: if the engine is holding up to it's advertising (torque, hp, compression etc..)
the setup should propell the atv just nicely around.

I still think quickly mounting the smaller wheels and giving it a try is the best option atm,
just because it's no purely theoretical discussion anymore,
but we get a true test of the engine's ability to propell the vehicle in sufficient manner (or not),
and we can work from that

'sid

 

[RustyRedneck]

I absolutely agree Sid. Trial and error is so much fun. Who knows, it might be so much fun the way it is you won't want to change anything about it. Worth a shot.

 

[itsid]

I absolutely agree Sid. Trial and error is so much fun.

well it is NO fun if you burn your clutch on an engine like that,
so I totally understand the concerns...
I'm not too concerned myself,
but that doesn't mean anyone should just risk their engines/clutches.

And in fact it's great to have bob in this thread to still raise the warning flag.
It's a discussion forum and
we generally want to see safely and reliably working karts/mini bikes.
And occasionally we're making wrong assumptions.
I have not seen a small dirtbike engine alike that's not at least 3.5:1 internally reduced, that doesn't mean there is none.
I am assuming a 3.5 internal reduction should be able to move the kart easily with the smaller wheels and current chain setup
(I could still be wrong on that)

And all the risk of burning a clutch is up to briggsandstratton123, not me...
and it's his decision to make..
I say small wheels and a single careful test could give us the information we need to be sure about if it works or not
and bob (not unfoundetly) says he'd rather go hunting for a smaller engine sprocket,
which is for sure the safer thing to do.

'sid

 

[bob58o]

Well 10T -19T 17mm sprockets are available in #420 and #428 for cheap, but some of the smaller sprockets (10T and 11T) seem to have a snap ring instead of two bolts holding the sprocket.
These are cheap to experiment with... Not sure if you have a snap ring slot?

17mm 10 Tooth #420 Sprocket.
$3
https://www.ebay.com/i/391185915384?chn=ps


17mm 10T-19T #420
$3.50
https://www.banggood.com/420-101112...cYa91xoCD7YQAvD_BwE&ID=47766&cur_warehouse=CN

 

[briggsandstratton123]

thanks,
I won't actually know wheather the thing will work until the engine is fully mounted