Kimjon

Kimjon's not as cool or epic as Muncie's off-road goped Bigfoot scooter thingy

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Cool, good score.

Those motors go okay. That's what's on one of my drift trikes.

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You can get the pull starts off trademe for about $20. But on eBay they only $8 with free delivery if you don't mind waiting a few weeks. Try searching "G scooter" if looking for parts.

You can also start it with a drill too, for a short term fix.

I've put a bigger carb on mine and an expansion chamber and its doubled the power.

If you want crazy power then another motor would be the best option.

 

 

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Hey I've noticed something with mine over the weekend that might happen with yours with the new added power.

When you wind it out it'll be smooth as butter but when you get off the gas the ass end gets it's sway on when engine braking if I hit the back brakes it settles it right down the front brakes make it worse combo of back then front = no wobbles. Thinking back this is exactly what threw me off could you Chuck a caliper on the back wheel? Maybe onto the sprocket like the old tru-test mini bikes.

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Thanks for the heads up. That's a disconcerting scenario you've got there.

Normally the weight transfers to the front and the rear brakes in effect achieve nothing at all...often the rear wheel stops turning all together under emergency braking as it lifts off the ground. Here's a micro view of this in this clip of me riding the kids dirt bike...I stop hard 100% front braking only, feel the rear lifting off, reduce to about 50% front and lock the rear on.

Im not too sure why yours is doing that, best guess would be the geometry (or flex) possibly changing under hard braking?

I've looked at options for a rear brake on my goped, mainly for doing skids or wheelies with. But haven't come up with an ideal solution due to lack of room. I haven't given up on the idea...just need more absorption time to figure it out:-)

 

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No suspension most of the weight on the back wheel must upset things a bit I think the tall gearing might be partially locking the back wheel. pulling the back brakes slows the engine enough to disengage the clutch getting it back under control is my guess.

Regardless I think your gonna have quite the rocket extra brakes if possible can't hurt.

 

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Compression lockup 

That'll make your back end skip around a bit 

And the scooter

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Checking port timing after porting.

All measurements were taken from the top edge of the piston, when it reveled the opening to the port (not from the rings).

I then put a dial guage on piston and turned it over both ways until it reached its highest point running in both directions. There is about 6 to 9° window of movement where the dial neither raised nor lowered the piston at TDC...so I split the difference and set the zero in the middle of this flat spot. I repeated this until I was happy with its consistency.

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With that done, I got the following results for the blueprinted 3 port cylinder. I did this test 3 times over and got exactly the same numbers each time.

-Top of exhaust opened at 97° ATDC

-Bottom of exhaust at 163° ATDC

-Inlets x2, plus boost ports opened at 120° ATDC.

-Blow down time 23°

I didn't use a base gasket, I'm thinking I'll just use a sealant RTV type gasket maker there?
 

Next up is to calculate the port area. Basically done by shoving a bit of paper in the cylinder, rub the paper with a pencil or dirty finger...leaving an impression on the paper. 

IMG_20180215_124302.thumb.jpg.d4b4c7b81d45b593e0399656a08430c7.jpg

Do some basic math, and roughly I got the following areas:

-Exhaust port area 269mm2

-Total intake area 294mm2 (2x intakes, 1x boost port)

I ran those numbers via some experts and they feel only very minor improvements could be made on those results...so stoked!

Going to leave it there and look at assembling with new gaskets, bearings and seals.

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I'm looking forward to seeing you ride this wouldn't mind betting it's terrifying! And could put much bigger engines to shame.

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My crankshaft received some grinder love...(okay I knew that would sound dodgy, but I had no idea how bad until reading it back!)

IMG_20180215_194944.thumb.jpg.b781fc9d2cbff0f5218dfb831d920156.jpgBefore

 

IMG_20180215_201708.thumb.jpg.452eaecaa88ecec976501554ce078247.jpgAfter

 

The idea is to smooth the rough rotating edges and slightly point them, so it cuts through the air faster. It also lightens the rotating assembly a tiny bit, plus increases crank case volume. Normally you want to "stuff" a case to decrease volume...but that's for a piston port with the intake on the cylinder. This motor has reeds on the crank case, so volume is small and the flow path is restricted by the whopping big crank stuck right in the way, so an increase in volume (I'm picking?) probably isn't a bad thing here.

 

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I need to get some bearings and oil seals. I actually brought some today using numbers gleaned off the internet...whoops, bit of a mistake trusting the internet :-),  ended up with the wrong sizes.

I'll go back tomorrow and get the right sizes, then it's a simple matter of putting it all back together.

In the meantime, I dropped the frame off at a powder coaters. And did a bit of internet shopping on an American goped performance website. I figured it hadn't cost a lot of actual money to get to this stage...so splashing out at the end of the project is somewhat justifiable.

it may be a couple weeks before I have anything to show for it, but I think it should look pretty sweet once done.

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Still want to stuff a crankcase port engine crank, lowering the crankcase volume increases the primary compression and forces more air/fuel mix through the transfers and boost ports.

/Your shaping will likely more than overcome any negative effect from the slight decrease in primary comp.

 

 

//Giz a ride!

 

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42 minutes ago, Raizer said:

Still want to stuff a crankcase port engine crank, lowering the crankcase volume increases the primary compression and forces more air/fuel mix through the transfers and boost ports.

/Your shaping will likely more than overcome any negative effect from the slight decrease in primary comp.

 

 

//Giz a ride!

 

I thought the same, but the guys racing the same motors in their pocket bikes are getting much better results from stock cranks V's full circle cranks. I have to credit these guys for all the learning that I've done lately for these specific motors.

Apparently the Paloni (excuse spelling?) Motors benefit from the full circle crank which in effect is one way of stuffing a case, but these cag (Chinese) motors tend to go backwards in performance with a stuffed case.

The Paloni takes its air/fuel from off to the side and very little actually goes around the actual crank case (bypasses). Versus the cag, which draws it in from the bottom of the crank case and it mostly goes through the middle of the reciprocating crank assembly. Through experimentation, the pocket bike guys have found that these cag motors benefit from increased crank case volume to overcome the poor flow and (restrictive) characteristics of this design. Basically at high revs the piston wants to draw air in...but there's to much shit in the way crammed into such a small space blocking it.

On my goped motors, these are different and I've stuffed the cases with aluminum linings. Stuffing certainly increased performance on these.

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I improvised a crude crankshaft balancer.

IMG_20180216_074945.thumb.jpg.ef58d095ea4508b8c0d7cb1d27449d41.jpg

Keep adding weights (nuts hung from a wire hook on the little bearing end of conrod).

Once it stays put in any position, compare the added weight against the weight of the piston, pin and bearings. Then do some basic math and work a percentage.

The goal was 55% (as that's what the internet said). To achieve this you remove weight from opposing areas until happy.

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Onto setting the squish band. From all the reading I've recently geeked out on (mostly from pocket bike forums), the ideal squish is going to be somewhere between 0.030" - 0.035".

To measure this, there are various techniques. The one I liked the most was this:

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Tape a bit of solder inline with the pin bearings.

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Assemble motor and turn it over, squishing the solder as it gets crushed by the piston (hence term squish, as it does this to the air/fuel mix too). Pull out the solder and measure its squished thickness.

IMG_20180216_201725.thumb.jpg.0322710e7004cec0f62ef8a8289d7343.jpg

This was the extreme squish 0.2mm (pictured above).

 

Now I got the following data using layers of copper head gaskets to adjust the gap between cylinder and head:

I copper gasket = 0.2mm

2 copper gaskets = 0.5mm

3 copper gaskets = 0.85mm

 

So comparing that to the ideal range of between 0.030" (= 0.762mm) and 0.035" (= 0.889mm). Using 3 copper head gaskets gets me into that desirable range. I've emailed a couple mates who are absolute gurus on these motors as I want to know if it's worth machining the cylinder to get tighter tolerances(?). Hopefully what I've got is good enough... but if they say it's not, then I'll tear it down and machine some off.

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Until I hear back, I've decided to carry on.

With the huge head kit now fitted, I noticed the fan cover wasn't going to fit. 

IMG_20180216_205723.thumb.jpg.fc13376c8bf4eac02fe71c84f47d7036.jpg

So I marked it out, then set to work cutting the clearance required.

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Looking pretty tidy for now. I haven't decided the colour for the motor yet, but I'll figure something out soon.

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.75 to .80 is ideal hypo scooter swish too, this thing is going to move!!

Full circle crank is the go for 70+cc scooter builds with read valves, but either way yours is going to be nuts

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9 hours ago, Raizer said:

.75 to .80 is ideal hypo scooter swish too, this thing is going to move!!

Full circle crank is the go for 70+cc scooter builds with read valves, but either way yours is going to be nuts

Awesome! Thanks @Raizer great information you've been providing, much appreciated. I'm no expert, so every learning opportunity is genuinely appreciated. There so much information out there that sometimes you become paralysed with overload. I guess I just wanted to achieve a result...rather than getting bogged down in indecision. So I've been building this based on information mostly provided by a championship winning racer.

Yeah, the logic part of my brain says go full circle crank...and I'm lucky to have 3 of them to choose from. However the advice I've received from a master builder of this specific motor is differing to mine (and your) logic?

IMG_20180214_143520.thumb.jpg.2d437e82f8c1b3031ab26001ba173a6e.jpg

I have numerous windowed pistons, cranks, heads...all the performance parts.

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And after going through all these countless options...he says use:

-The stock piston, because it will rev higher with no window. Just polish it, and slightly taper the ring groove on the top side (using fine sandpaper) so it seals better under load.

-Use the stock crank, as they produce more power. The full circle is better when stroked, but mine aren't strokers.

-Use the stock cylinder, because there's less chance of leaking. But change the port timing, area and add a single large boost port.

So far I've followed his advice, however I did go with the two piece cylinder. I did this for practical reasons, as it was much easier to machine and set up in the mill without a head in the way.

I'm getting pretty exited, as in theory the motor is done. But I can't run it until I get the frame back etc...So I'll have to wait.

The good news is I still have enough parts to build 3 more motors. So I'll do them all differently next time, just to expand my own firsthand knowledge.

 

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Fuck yeah...best day ever!

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Seriously impressed...about 3 or 4 days ago I ordered some goped parts from the US. I now have those parts in my grubby (gigantic oversized) hands!

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Fucking unbelievable!

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