Stock Chinagirl Riders- Got a minute?

66cc, 48 tooth, 29", nt carb, drilled stock pipe, level ground 29.8mph, slight down hill 32.6mph, slight up hill 27.9mph.

Now has shift kit and expansion chamber, but ran stock for upwards of 1,000 miles.

I'd imagine those tires would spin faster with a 44 sprocket, how was the torque?

I'm sticking to 44 teeth for now because of viscous head winds and hills, when I move to flatter ground I'll try a 40 and 36 tooth

I debated drilling a hole in my cap, but I can't undrill it if I don't like it

Once you get to ~30 MPH wind resistance becomes the biggest factor.
A short skinny guy tucked in and bent down to be aerodynamic will get an easy 2-5 MPH faster on the same the bike as a tall fat guy sitting up straight.

I have built dozens of stock 66cc (Yang Dong engine factory) distributed through China Gas as Skyhawk beach cruisers.
I rewire the stock ignition and drop an NGK 5944 (BPR7HIX) Iridium plug in.

Stock 44T sprocket.
26" 1.92" to 2.128" wide tires at 40#.
6' riders 150# to 200# sitting up.
32:1 mix after 25:1 one gallon break in.

They all do 30+ and don't vibrate but then again I replace the hard plastic grips with BMX foam grips (they will slide over the throttle barrel and feel great) and secure the engine metal to metal to the frame well, usually with a SBP front mount.

NOTE:
DO NOT TRY THE IRIDIUM PLUG IF YOU HAVE ALREADY INCREADED COMPRESSION!
The plug is a touch longer adding a bit more compression for stock, but will hit your piston is you have modified.
This goes for most every billet head.

Slogger said:

Thanks, Rudz
The exhaust system being gutted out seems to spin em up. I imagine it is a little louder.

Click to expand...

That is just wasted polluting power.
A bunch of fuel goes out the exhaust during the intake cycle.

A tuned pipe aka expansion chamber is designed to shove that mix back into the cylinder just before the port closes for a bigger bang.

Here is a little graphic that shows how it works.



It doesn't give you more power by RPM, it just makes a bigger boom on each stroke.

The trick is in the timing by pipe length between header and chamber.
Too long and nothing comes back, too short and you get exhaust back in.

You tune the length to give the best boost by RPM range via that pipe length.
Generally longer for top end power, shorter for low end torque.

Hope that helps.

Davezilla said:

Yup... I do a lot of race cars, custom hot rods and aftermarket suspensions that other shops wont touch and I get a lot of cars that have been rebuilt after being wrecked, a lot of times they don't get the frame as straight as it needs to be so aligning to specs will get you nowhere near driving straight. With these cars and trucks they need to be driven and adjusted sometimes several times before they drive right. Race cars also need to be aligned a lot different from regular cars so you set the toe out instead of in in order to bite into the turn much better, more camber also gives better cornering traction but not that rediculous looking fad that people are intentionally over cambering for looks... the caster is set for faster turning in favor over better stability, and every driver has their own settings they like. I get their business because other shops just turn them away.

Ive also done brake upgrades & diisk brake conversions on older cars as well as lift kits on trucks and 4x4s because other places turn these jobs away either because they never did the jobs before or just don't want to be liable if they cant get it to drive right etc etc....

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Damn Dave your shop and work sounds like a dream!!! I've done 4 wheel alignments before, let me work for you lol.

Wow thats nuts Dave, resilience is key in these those circumstances...
And i also feel you in regards to the weather as well, my knees tell me when its humid/ about to rain.

You ramped your piston too? By how much?

What do you think about the ramping i did for all 4 sides of my piston?


http://s296.photobucket.com/user/brandoncarpenter73/library/?sort=3&page=1

*This post was in reply to DJ but im not sure how to quote his post & notify him...
So anyone's input on my piston work is appreciated!

CarpsCustoms said:

You ramped your piston too? By how much?

What do you think about the ramping i did for all 4 sides of my piston?

So anyone's input on my piston work is appreciated!

Click to expand...

The intake port charges the crankcase under the piston so ramping the top of the piston on 4 sides shows that you don't understand how a piston ported engine works.

Piston ramping was a temporary tuning technique before you did porting on expensive cylinders.

None of the fastest race engine builders ramp the tops of pistons.

MotorBicycleRacing said:

The intake port charges the crankcase under the piston so ramping the top of the piston on 4 sides shows that you don't understand how a piston ported engine works.

Piston ramping was a temporary tuning technique before you did porting on expensive cylinders.

None of the fastest race engine builders ramp the tops of pistons.

Click to expand...

I thoroughly understand the concept & I've done research as well..The goal of slight ramping was just to provoke more air-flow to the center of the chamber & get a little less restriction, nothing drastic.
Could you enlighten me on what knowledge it is that you think im missing?

Here was one good thread from Roger for example:

http://motorbicycling.com/showthread.php?t=50905

For one, you ramped the intake side of the piston even though there is no flow over the top of the piston on the intake side.
Did you notch the bottom of the piston skirt too on the intake port?

MotorBicycleRacing said:

For one, you ramped the intake side of the piston even though there is no flow over the top of the piston on the intake side.
Did you notch the bottom of the piston skirt too on the intake port?

Click to expand...

I see..So why have others done it in the past???

& yes i made about a 4mm high "window" on both sides of the skirt for a little more duration

CarpsCustoms said:

You ramped your piston too? By how much?

What do you think about the ramping i did for all 4 sides of my piston?


http://s296.photobucket.com/user/brandoncarpenter73/library/?sort=3&page=1

*This post was in reply to DJ but im not sure how to quote his post & notify him...
So anyone's input on my piston work is appreciated!

Click to expand...

I can help you on this.... I do ramp my pistons but keep a stock uncut piston on hand in case the ramping is too much... what I do first is my "basic" port job on the cylinder by widening the exhaust port 2mm on each side then doing the same to the intake... this only allows more air fuel mix to get in and let's the exhaust out quicker without changing the timing. Then I raise the exhaust roof by 1mm and intake floor by 2mm, this increases the port duration and helps the mid range and top end power, if I was to install an uncut piston at this point the improvement in torque and rpm will be noticeable, but could be better.
This is where piston ramping comes into play... I now cut out 1mm from the piston crown at the exhaust port by placing the piston into the bore with the rings removed and the piston on the rod, then slide the cylinder onto the bottom end and make scribe marks at the exhaust and transfer openings.
Next I remove the piston and ramp it 1mm down at all 3 port openings tapering the ramps gradually to about 1/2" from the edge.. I want the ramp to be gradual and smooth so I start the cut with a Drexel and carbide rotary file but finish with a flat file to keep everything smooth and gradual as well as aligned with the port openings. After each ramp is finished I. Go back and round off just the edge of each ramp for a slightly better flow.
For the transfers I will make the ramps the same 1mm deep but direct the flow toward the center of the piston by using the dremel only to begin the cut and switch to the flat files to bring the edges of the ramps to meet the transfer openings keeping the ramp gradual to at least 1/2" from the edge of the piston, just this little cut can help guide the charge toward the center of the piston as the charge comes in.
Once I got the top of the piston done then I cut out 5mm from the intake side of the skirt for more intake duration. This cut is also matched to the intake opening and then smoothed out.
The end result was 42mph from a 26" mountain bike that weighed about 55lb with 225lb of me on it going thru the stock nt carb and the stock 44 tooth rear sprocket. I also used an expansion chamber pipe off a KTM 50 dirt bike that was modded to fit the engine and bike.

I could have gone back and raised the exhaust port, lowered the intake port, and put the uncut piston back in to get similar performance but still would have needed to ramp the piston at the transfers to keep the longer duration without giving up the added blow down if I was to raise the transfer roofs... more on this, but you get the idea...

Piston ramping is an easy way to gain performance but it can also be a way to gauge how far one can raise the exhaust or lower the intake before porting the jug since pistons are cheap... for daily street riding ramping the piston will give good gains, but for all out racing everything is usually done to the ports the self and the piston left alone in order to squeeze every ounce of power out of the engine since ramping does interrupt the squish band as well as drop compression slightly.

Thanks man. I did most of that. Im gonna see how she runs soon. Only difference was that i used sandpaper to finish not a file. But that was an excellent passage bro, thanks again