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Kimjon - builds a 49cc goped


Kimjon

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12 hours ago, cletus said:

What is the advantages of a full crank vs a counterweighted one?

In most 2 stroke applications the FCC fills the case more, and this increases crankcase pressure in return. The increase in case pressure speeds up the transfer of the air/fuel into the cylinder when the ports open and more of it gets into the top of the cylinder before the ports close off...this makes more power.

Now the catch, this motor is poorly designed and is more for tools like weedwackers or chainsaws where its designed to run at near peak rpm all day long. The flow of air/fuel has to get past the crankshaft which is right in the way. This is the most restricted path and fundamentally will inhibit this motor from ever being awesome.

For a chainsaw having restricted flow doesn't matter as long as enough power is made to do the job, but compared to a European motor of equivalent cc capacity these motors are way down on power, ie. This 49cc motor in stock form is likely to be 1.5hp-2hp. A 39cc (yes 39cc) air cooled  polini could be as high 6.5hp straight out of the box!

I'd like to get 3.5hp or higher, which isn't as impossible as it may sound, as simply adding an expansion chamber alone with no other mods could get 1hp additional power just like that. Bigger carburetor adds a bit more, raising the compression, setting tighter squish band, changing the port timing, flowing (blueprinting) the cases and matching the case/cylinder ports...etc etc...all adds up. 

Main advantage with these motors is they're cheap, and produce acceptable power...and they're cheap, oh and cheap too. When all done, this motor will owe me fuck all V's a starting price of $1500 for a polini or other European motor. For someone like me, who enjoys mucking around, cheap is good, as if you take a gamble and it doesn't work...who cares, you wasted $40 for a replacement part and a bit of your time. Not a bad price to pay for an education...

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There's a school of thought that FCC's aren't the best idea for this style of motor due to the FCC further blocking the flow path of the air/fuel.

But I personally think that's not the real reason why the FCC's get a bad wrap in these motors. They are so far out of balance that they rattle the shit out of the motor and that causes no end of issues like bearings failure and poor rpms. Most users are teenagers with a pocket bike and without the ability to fix these problems.

My logic is to balance the motor the best you can and those issues should go away (ish).

I guess someone will be right. If it's not me...then so be it, I've wasted time and money on dumber things before and it won't be the last time...

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Out of interest I measured the stock cranks balance factor and was surprised to find it was 38%. So 40% isn't looking too bad all things considered.

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Remembering that this isn't pure science - more just getting it to feel good on average, as its impossible to balance a single cylinder motor like this. 

So like everyone one does in this day and age...I googled enough times to support my own opinions and ignored the sites that contradict my viewpoint. So 40% it is;)

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Then matched the exhaust port on the cylinder to also match/blend in smoother than it was.

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Much better now. 

Gotta remember the exhaust flows out into the expansion chamber, then bounces back into the cylinder again...so the transition has to flow reasonably well in both directions at this point. There's limited material thickness here to work with, but it didn't take much to much a huge improvement.

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  • 1 month later...

Been crazy busy of late with real work, so haven't spent any time on my fun projects. But for a bit of timeout today I took a break from work and did this:

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Dome fitted to cylinder, this allows me to set correct squish band tolerance.

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Pull starter cover now doesn't fit, so a bit of nip and tuck...

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Tadah...sorted.

So nothing spectacular, but it gave me 30 minutes of tuning out time and a few steps closer to finishing it.

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  • 4 weeks later...

Finished the expansion chamber, welded and painted in high temp paint.

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Usual story when welding thin metal...alls going well, then you blow a hole in the thinner side...then make it worse, wose again and much more worse as the hole grows bigger, not smaller the more you try to fill it...then it all comes good again just as you're thinking about throwing it out the fucking door!!! 

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  • 2 weeks later...

Started assembling the motor. Missing some keys for the crankshaft (both clutch and flywheel key onto a tapered shaft). But other than that its pretty much all there.

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All new SKF bearings and oil seals going in.

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Cutting gaskets to match the bigger ports.

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Thermal conductive grease, helps heat transfer from cylinder into top heatsinks.

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Long block fully assembled.

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I skipped heaps of boring steps...but you get the idea. That's as far as I can go without the 2 missing keys, but fuck yeah...its looking the part anyway...

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Added a chainsaw style "pumper carburetor".

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I had to make an adapter manifold for it all to work. Luckily the spacing on these carbs is fairly standard on most chainsaws, weed eaters and gopeds. So I was able to mill up an aluminum plate and use a goped thermo plastic manifold to adapt it to the pocket bike engine.

This is far better system, as these carburetors are fully tunable with a needle screw. Way better than fucking around trying out different jets and settling on something that's never quite right but runs.

Another advantage is you can mount the tank anywhere, as it pumps its own fuel, unlike the gravity fed carburetor that's now filled in the rubbish bin!

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  • 3 weeks later...

Got home and found an aliexpress package...best day ever!

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My woodruff keys arrived, so no fucking around...lets get some shit finished!

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Okay, not so awesome. Problems...

The flywheel I'm using is supposed to convert it to easy pull start by shifting the leverage outward, thus increasing torque on the pull start. This certainly works...but there's not enough clearance to simply bolt on the pull start.

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This is how much clearance I need, so I'll space it with washers "for now, cough cough". And make a proper aluminum spacer plate for it later. #shitthatwillneverhappen #washersforlifemofos

Makes me wonder how normal customers get on with this type of purchase. Not everyone buying cheap parts of ebay or Amazon etc have access to a lathe or milling machine to then make said cheap parts actually fit.

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Then a bit of clutch bell restoration.

I've put it all back together and it's now ready to start.

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Squish is a shade loose at 0.82mm

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Tested with solder. I only turned the motor over for one stroke, so possibly a bit of spring back in this measurement? If I pulled it over a couple times I may get closer to the 0.7mm I was hoping for? But it's so close that I can't be fucked changing it right now. 

At 0.82mm it's still going to be awesome, and it wont brake shit like pull start cords etc like my last one does (with insane compression). I can also use pump gas without detonating. So maybe a mildly conservative (conservative is a relative term, like saying a 2JZ "is only running 32psi boost or something like that") squish band setting isn't a bad thing?

I'm pretty confident that if I tested it again and pulled the motor over a few times it would further squish the solder closer to the 0.7mm, so I actually think that's more likely the number than my half-arsed test. The main thing I've confirmed is that the piston has clearance to actually rotate and not strike the head, plus the squish is a lot tighter than factory 1.5mm, so I'm happy.

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  • 3 weeks later...

Something that I've been putting off for a longtime. Not sure why, but I hate making solid round bar into tube...knowing that I could buy it with a hole already in it (but never do, becauseI don't need 6m of random tube lying around afterwards).

Anyway:

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Working out a baseline to start with.

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Doing the part I didn't want to do. Took less that an hour start to finish...so I'm not sure why my whiney bitch side of my brain took over here? But 4x "tubes" precision engineered to within near perfect digital vernier accuracy of each other.

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Setting out the sprocket I'm going to use. Blue Mark's new holes on corrected pcd.

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Test spacer. If you going to make a change...make a big one. Best advice I've ever received in life. By doing big change, you get instant feedback ie. you have a baseline, then make a big change and you now have a range. Now just rationalize it and you're bang on in 3 easy steps.

And here it is. Perfect! 

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Not bad for something that was never made with this feature. Sounds like a simple task, but getting it to sit as tight as possible to the frame, but not interfere with it was a challenge. I even machined 1/3 off each bolt head to get it about 2.5mm plus another 2.5mm closer each way. Chain only has minimal clearance on tyre...but it works!

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My chain tension (lack of) was going to cause issues, due to the combination of parts landing on a half link.

I started making a chain tensioner device, but I was struggling for room. I kind of mocked up a working solution...but I wasn't in love with it.

Then I had an epiphany...i could add a spacer block under the motor to lift everything up to create the desired effect.

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So that's what I did.

I can fine tune with the threaded rod and pivoting engine mount. That was always the intention, however it would be asking too much to move it as far as I needed to, just to set the tension pre chain stretching.

I'll tidy it all up and get it painted now that I know it's going to work.

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  • 2 weeks later...

Okay, I'm starting to realise I may be pushing the limits here.

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There's a lots of motor on this scooter now.

It's all but done, but I can't start it as the current pull start can't handle the compression. I tried to start it with a drill and fucked my drill!!! So I've ordered a different "easy pull" that I'll adapt to fit this motor. Basically it shifts the diameter of the pull start outward to increase torque in your favour.

So...more waiting on Aliexpress to deliver the goods...

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  • 1 month later...

Finished it:

I've tuned it since this ride, now running much stronger. It's got huge torque, but the tyres on it make the handling feel like your riding a loaf of bread...

I'll swap the tyres out and possibly upgrade the handlebars, as its sketchy as fuck to ride. It lacks any feeling in the controls and just loosely goes "about where" you want it to, unlike my other gopeds that handle much better.

Anyways - my motor works, so that was my main motivation for this project, so I'm stoked with that!

 

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