Porting a china girl

johnsteve

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Jul 20, 2015
Messages
114
[FONT=Georgia, Times New Roman, Times, serif]So, I am thinking about porting my motor and I have never done any kind of porting work before. Hell I did not even know what porting was until I came here. First, I tried to call local machine shops that deal with motorcycles. They are like WHAT????????? What is a China Girl ? They had no idea. Another guy quoted me a price of about 750. That's a lot more than I want to spend so I am thinking about doing it myself. [/FONT]

[FONT=Georgia, Times New Roman, Times, serif]I also have been reading as much as I can from the forums and the dragonfly pages and I think this is what I should do. From what I have been reading I need to get a micrometer, and a hand dremmel with the bits. Then make the exit port about 21mm wide in the shape of an oval. Next the intake ports need to be expanded to less then 7mm which to me seems like a lot. The carbs come in at about 16mm[/FONT]

Other references point to taking off a small part of the skirt on the intake side and taking off part of the top of the piston on the exhaust side at an angle to about 2mm from the rings. Is taking off the lower part of the skirt on the piston and taking off a small part of the top of the piston the best way to achieve low end torque and a better midrange? Is there anything I am leaving out? Would this be the right way to port? Inquire minds want to know
 


Purple Haze

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Joined
Aug 21, 2011
Messages
374
Study up before attempting any port work; there are many good videos on youtube. It's easy to go overboard and kill your low-end torque. I'd do a port cleanup, for starters, there's lots of casting flash and roughness, that'll make a noticeable difference. Don't change the shape of the port, that's where beginners get in trouble. Port-matching is another thing that is easy to do and makes for a smoother-running engine. Most people port for high rpm, if you are riding around for transportation, that stuff is a waste of time.
 

sbest

Active Member
Joined
Nov 1, 2015
Messages
170
You can probably manage without the caliper.
Use HSS cutters rather than grindstones in the Dremel and run it on slow.

I was going to pull the cylinder off my bike to photo for you but thought it was quicker to just buzz up this extra cylinder.
The edges are not chamfered and I see lint from my rag but hope it gives an idea of what you can safely do to your exhaust port.

I've had good luck with an exhaust port shape like this:

The wide part at the top (closest to cyl head), no change to top or bottom of the port, just widen at the top.
If you go too wide you will stress the piston ring and could break into the cylinder bolt holes.

The whole point is to get more port window area.

This intake is stock except deburring the flash off it and is chamfered. No sharp edge.
The intake should be widened too, but I am debating the exact shape for this cylinder so I didn't touch it.
I'd suggest just widening it, but perhaps more at the bottom.
Trimming the piston skirt has an effect on port timing that could be bad. I'd leave that for now.

The transfers are "cleaned up". They come with a terrible ridge. Hard to get to.
Used long shank 1/4" ball and cone shaped cutters.

While this did not result in any RPM increase in my other cylinder, it did increase acceleration and hillclimbing ability.
Yeah, I know my photography is terrible and I didn't put a mirror polish on the exhaust.
Didn't on my running cylinder either. Fill yer boots if you want to.

Steve
 
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HeadSmess

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May 17, 2010
Messages
3,031
throw your dremel where it belongs, in the BIN!

get some dentists drills, easily available (unless you got FDA restrictions in case you feel like starting up a practices?) on fleabay, mine cost all of 15 each...45 degree and right angle heads, more than capable of porting even 30cc and less engines.



a dremel is a waste of time, you do more harm than good. they simply werent designed for getting into tight places, such as little kiddies mouths and two-stroke ports.

plenty of different cutters available, carbides and diamonds...

next time you get a bridge, ask him (her) if you can have the used bits, they throw them out anyways...health reasons.

you do need the hose connector thingy, an air compressor capable of at least 30 psi...and a lil foot valve helps.

plus you can spray air/coolant down some of them, and some come with LED headlights...really beneficial

the whole kit is still cheaper on ebay than a dremel and a set of cutters... you get what the dentist uses...a box on the floor, hoses, an air-compressor...

the neighbours will wonder what the hell you are up to... try screaming occasionally :D
 
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sbest

Active Member
Joined
Nov 1, 2015
Messages
170
Ha! You are Bang On Headsmess.
Very hard to get the Dremel into the transfers and the angle attachments are useless.

Any pictures of what works for you on the intake?
Especially for Piston port?

Steve
 

HeadSmess

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May 17, 2010
Messages
3,031
i thought i had posted a reply here...i mustnt have hit post :)

i had to give up china girls over a year ago when our transport minister BANNED the use of IC engines in bicycles.

its electric only now, here in NSW, australia.

of course, the new rules failed to mention external combustion engines, and thats where my main efforts have been directed,(long live steam power!) when i havent been in a depressed, "sleep my life away" state of mind due to having to really get out of the current job i am in...i cant get anything done at ALL when i have all this time on my hands and no money. a lot of plans have gone down the drain over the last year, im afraid. its hard enough to eat, let alone buy fancy stainless steels, high temperature graphalloy bushes, and new lathe tools etc... certain ideas i have need to be patented as well.

(in fact, someone even offered me a large sum of money -for something unrelated- the other day, i had to decline...until i get a working result with my OWN money first...i cant accept funding. i put myself in this position, i can dig my own way out! like when i bogged the tractor in my driveway...i just had to wait 6 months for the ground to dry up again, lol.)

and dont get me started on modern technology where you can lose photos in the blink of an eye, never to be seen again, despite having had several copies in various places at one point.

anyways... basically, after a lot of failures and experiments, (even made a dyno just before the laws changed!)i found the only way to keep a china girl RELIABLE was too leave it ALONE! a quick trim of the ports, smooth the edges, remove casting flash, match ports and manifolds, and whack on a good, properly designed tuned pipe.

nothing is more heartbreaking than spending two or three days carefully mapping out ports, modifying ports, reshaping and checking for flow/turbulence in the ports, only to have something like the ring locating pin pop out and destroy the cylinder on the first ride the next day. a completely unexpected and unthought of part will always undo your hard work.

note just how many threads on extreme porting etc there are, and note how many come back after 6 months and say that its still running strong ;) yet to see one daily rider do so.

they may be cheap but after destroying over a dozen cylinders and associated parts, you start wondering if the money hadnt been better spent on buying 1 denardis/morini that would have been virtually bulletproof from the word go. 6+hp from a 40cc engine, out of the box, with guaranteed parts backup? all you gotta do is fabricate proper mounts for them.

DO NOT RELY on a MODIFIED china girl. fun as a hobby, and if you dont mind the occasional walk home... you can get the things to really haul. could never quite break 9000 rpm though... but could easily run 33 and 36 T sprockets on 27"/700c wheels. pedalling? wha???
but when you have to travel a minimum of 15km just to get to the local shops... you start to leave them alone, i hate to say. ever walked 10km home with a backpack containing 30-40 pounds of groceries?

still have at least three engines worth of new parts sitting on a shelf collecting dust, where they will probably stay. i plan on keeping my license this time!

whereas, simply "blueprinting" them...you get pretty good mileage. i often embarked on 300km+ WOT rides. i never carried tools, just a bottle or two of petrol.

the best single modification is the tuned pipe, and re-jetting (the stock nt carb) to suit.
 

Bufu racing#1

Member
Joined
Feb 23, 2016
Messages
42
First off I'm going to say we got a stock completely stock cylinder, no porting!
52mph..

First off ,if you want to get into porting ,first thing you wanna do is buy a degree wheel.

Rather your running a reed valve or piston port motor... your always going to get more power by getting more port area
Widening the exhaust and intake.

Don't forget to use sand paper to clean up your ports make them smooth
 

Steve Best

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Joined
Sep 22, 2012
Messages
1,236
First off I'm going to say we got a stock completely stock cylinder, no porting! 52mph..
<SOME SNIPPED>
Don't forget to use sand paper to clean up your ports make them smooth
You have to tell me how you got 52mph?
Down hill with tall gears?

Steve
 

skyash

Active Member
Joined
Jun 26, 2015
Messages
755
You know 50 mph is like 80 kmh. You can get up in the 70 on stock motors 29inc wheels 36 tooth sprocket and a big hill. Maybe once but not on flat ground. Best we got all day flat ground was 65kmh
 
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