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Kimjon - seek medical advice!


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I made this one nicknamed the "slo-ped". I used a 20cc weedeater motor of very unimpressive port design from the factory.


As you can imagine it wasn't nicknamed "slo-ped" for no reason. However it worked well enough for a bit of fun. It did however look tough if nothing else! And that's the look I'm imagining for this new project...black and white...tough!

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Small set back, I found a broken stud in the little G23LH motor. Luckily for me I brought a frame for spare parts a while back...by chance it had half an engine lower case stuck on the frame! Seriously what's the odds?


It was in terrible shape. But using some trusty oven cleaner it scrubbed up okay. The bearings are fucked, as too are the oil/crank seals. I'll go to seal imports and get new ones.

So with that setback sorted. I thought while it's apart...may as well port the fuck out of it! So I did:


I always label my halves left and right. Plus draw the crankshaft rotation on as a reminder.

The black is bad. That's about as dumbed down as it gets.


So basically the blacks gotta go. It causes restrictions and ruins flow by creating eddies and turbulence. Note how badly the ports are mismatched...basically the air/fuel charge is hitting a wall! About 25% of the port is blocked by this sharp edge.



So, to speed things up I'm using a milling machine to rough out some larger parts. Also I milled the skirt down on one side by about 2mm all the way around.

Then it's time to rough out the rest with a burr in the dremel:



Roughed out. Now time to tidy up a little. Below is a great before and after shot.


Before. Note the sharp shelf and edges. Very bad for flow.


After. All edges now flow and transfer ports opened up and port matched...much better.

Finally. Here's a good side by side comparison of the cylinder. Look how much bigger the transfer ports are on the right hand (ported) cylinder...at least 50% if not more.


That should flow way more air/fuel.

The exhaust will get polished. The intake will be roughed up a little. But no real changes other than port matching the exhaust and intake will be needed as the specs for port timing are already very good. Only small wins could be made at the risk of ruining the motor. 

I'll replace the bearings and seals...slap it back together and it should be way better than before.


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Righto, frames at powder coaters getting a new look. I took parts of it back to bare metal to weld some stress fractures. Glad I looked in the usual places first.

Next was to carry on porting the motor. I've done this to death in the "not as cool or epic as muncies goped build". But here's the basics:


Intake: before (it's previously been mildly ported by me before...wider than normal)


Intake: after.

Intakes...only port downwards. You can go wider so long as your ring pins stay on a bearing surface on the cylinder wall.

Leave the intake surface finish rough. It helps atomise the fuel with the air for more efficiency.





Exhaust: before


Exhaust: after

Exhausts, only ever port upwards. Once again you can go wider, so long as your ring pins stay on a bearing surface on the cylinder wall.


Polish the exhaust port as smoothly as possible. This helps flow out/in the exhaust. And prevents carbon build up. Remember the exhaust plus air/fuel gets bounced back into the motor by the expansion chamber.






Next is to polish the piston. This helps prevent carbon build up.






Next I scribed the intake port onto the intake manifold. You can see a huge difference...now port match it.

I treated it to new bearings and oil seals. Only cost $24 from a bearing shop to get high end SKF bearings and seals. I reused the gaskets as they only seal at very low pressure on the lower cases. They looked fine...I'm sure it'll be sweet as. I used to get rid of the gaskets and just use RTV...however these cases had no end play and that would have loaded the bearings too much in this case.

So today's efforts, plus the massive transfer ports and flowed lower cases i did last week, should be quite an improvement.

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Oh...forgot to mention that I use a stone in the dremel to very lightly chamfer all freshly ported surfaces inside the cylinder bore. This prevents the rings catching.

And I also take the opportunity to true up all gasket surfaces on the linisher. You can use sandpaper on a sheet of glass and do it by hand if you don't have a linisher.


Here's a good photo of the surface trued up. Go easy...don't want to remove much...just get it flat and true.


So that's it. Wait 3 weeks for powder coating... and test it.

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Bit of a lazy day. Mocked this exhaust up.


I had something similar once, ugly as sin...but the midrange was unreal - pulled like a school boy!

I'm trying to recreate it from memory and using bits I have on hand. I remember the long 180° u bend having a parallel profile ie just a tube and thinking it was unusual, not been the standard tapering style...but fuck it worked well. 

Only one way to find out?

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  • 2 weeks later...

Okay, I'm full of shit and did a huge u turn on the colour.

When at powder coaters I saw this new colour with green, gold and black metal flake all mixed up together...yes...I like it. So as usual the guys did a fucking great job at the powder coaters and worked their magic with the application of this colour.




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Decided to use the first expansion chamber for another project as it was better suited for that one.

So I had to make up another one...like this:






Pretty happy with this. Should bring out the beast within the little motor!

I'm a huge fan of putting the exhaust around the motor. Doing it this way has saved so many of my little motors when you crash. The exhaust may get a ding or worse case senario destroyed...but that's cheap to fix. Unlike the motor hitting concrete and going from 19,000rpm to zero in an instant! No motor survives that.

It may be lacking in the carburetor department now, with the radical porting I did. I've pushed the port timing to the limits and it's going to draw a lot more air now. But that's just a bolt on part...so I'll try it like this first, then go from there.

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When you build these custom expansion setups do you know how sensitive they are in terms to sizing and the dynamics of the motor in question? ie is any chamber a good thing or do they need to be tuned proper? They look cool for sure just wondering if it's possible to design something that sucks in terms of exhaust timing (or other extreme get lucky and have something that rips)... 

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1 hour ago, KP_wag said:

When you build these custom expansion setups do you know how sensitive they are in terms to sizing and the dynamics of the motor in question? ie is any chamber a good thing or do they need to be tuned proper? They look cool for sure just wondering if it's possible to design something that sucks in terms of exhaust timing (or other extreme get lucky and have something that rips)... 

There's a bit of luck accompanied with some crude upper and lower sizes I work within.

I have a lot of off the shelf items that I own and have used. This gives me a guide. Some have much better traits than others...so I can try replicate the better traits. However, I'm somewhat hamstrung by what materials I have on hand...so that's where the luck comes into it, as I obviously can't do an exact copy of the store brought ones.


These expansion chambers are all factory made (some slightly modified). This is where I can test and get a fairly good baseline from.

I've filed them all with water to figure out the volumes of each. I even did it in stages to figure out volumes of each part of the pipe using an inspection camera. The funny thing is that the best factory pipe by far is the most fucked one, with a heavy restriction on a badly welded on 90° (header) after crashing and repairing it. It's on the red goped. It's the identical pipe to the one on the blue goped...exactly the same make and model, except the blue one has a nice factory mandrel bent free flowing header. 

I've made a heap of these and I'm sure there's always room for improvement...But for the most part I seam to be getting good results so far in terms of performance. Cosmetically...yep...lots of room for improvement.


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There's heaps of information online with some pretty good rules of thumb to follow. I basically use these to get attributes I'm looking for i.e. low end, or mid range, or top end. Hard hitting...or soft but wide power band etc...

I've tried a couple excel spreadsheets/calculators that I found on websites. They require very specific information, so only good if you've got all your port timings and areas recorded etc...which you'd have if tearing down and porting the motor anyway. But no good if you don't know the numbers. The design it popped out wasn't all that good, so I added more mid area into the pipe based purely on an assumption (It just didn't look right to me) and instantly gained a huge improvement! So like with everything online...take it with a grain of salt.

I guess I like to experiment, and build a basic knowledge by actually making and trying things out for myself. Trying to learn along the way from both successes and failures.


I'd call this an epic fail (red goped pictured above). I brought a tuned pipe for this motor...made to fit in an RC car. Had good reviews online, but wasn't very impressive. I ended up making my own and trashing the store brought tuned pipe...and the difference was night and day! My pipe turned it's performance from "sad"...to "wild"!!! So I learnt that even when you pay good money for a "tuned pipe" built for a specific application, it's still very hit and miss what you'll actually get.





These pipes all added huge improvements over a stock muffler. I'm sure there's a better design and/or way of doing this for all of them, but the results have been well worth the effort in terms of power increases obtained. Not just little improvements...massive power gains.

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