china bike 122cc dohc 4 valve project

Advertise with us
Shop deals on ebay
Find deals on amazon
Mini Dirt Bikes & Pit Bikes Forum

Help Support Mini Dirt Bikes & Pit Bikes Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
tormentandglory said:
If you have one in pieces, and if its not to much trouble, could you count the teeth on the primary gears and let me know what they are please? That'll save me the trouble of pulling mine apart :)
Click to expand...

i think my SDG 3V is pulled apart, i'll see if i can get time to check it out today (have Pitboss and Vert 3V too don't konw how different they are?)
 
that gsxr tacho, where does it connect to? gsxr and twin cam?

it does sound to be revving alright, but not to 10.5k, could be wrong but if it connects to the coil (assuming the gsxr runs a single coil) then it will be giving a reading 4x higher than true, if the gsxr runs 2x coils then you receiving a reading 2x higher then true.
i don't think it sounds like its idling at 4-5k.
 
Gsxr runs 2 coils, one fires cylinders 1 and 4, the other fires 2 and 3. Both paired cylinders fire simultaneously. One spark per 360deg rotation for each cylinder. Just like a china bike.
 
just rethought my theory and that would put it at reading half what is happening, forgot about the china bikes firing every rotation. that puts the guage reading true.

well done on getting this thing running so well, can't wait to see it completely tuned and dynoed.
 
Hi!

A really cool projekt! I was thinking about do somthing by my self, but with a moded cbr600r head, as basic. But there are some many things how can make big problems........... (btw i work in a custombikeshop, www.erbacher.biz)

Can you make som pix from the inlet and exhaust ports????

REgards

PS:Sorry about my english, im from switzerland
 
He's right mountain ....... although 4 strokes only have a power stroke every 2 crank turns (720 degrees) , the ignition fires every 360 degrees because the flywheel is turning one to one with the crank ........

In other words every second rev there is a wasted spark in a single cylinder 4 stroke engine that uses a crank triggered pickup ......

The only way around it would be to time the spark off the end of the cam which runs at HALF crank revs like early Honda SL/XL and Honda posty bike engines do .... or in the case of a 4 cylinder bike .... run a camshaft driven distributor like a car does .......

Incidentally , since 4 strokes only fire every 4th stroke ie 2nd crank rev ..... they SOUND like they aren't revving as high as they really are (they'll sound like they're doing half their actual revs) ..... So you CAN'T accurately judge how high an engine is revving by the sound of it's exhaust note ..... Only the crank which makes no "Bang, bang" noise revs ...... the piston which is driven by the "Bang , bang" noise reciprocates (goes up and down like a yo-yo ... LOL) ..... so you'd need either a mechanical crank driven tacho or an electronic tacho that measures pulses every 360 degrees ....
 
Last edited:
Breth said:
Hi!

A really cool projekt! I was thinking about do somthing by my self, but with a moded cbr600r head, as basic. But there are some many things how can make big problems........... (btw i work in a custombikeshop, www.erbacher.biz)

Can you make som pix from the inlet and exhaust ports????

REgards

PS:Sorry about my english, im from switzerland
Click to expand...

I don't have any pics of the ports, and its all assembled and running now. I fitted a 30 tooth rear sprocket tonight. Gonna take it for a blast around the block later :)

I wanted to strip an fzr400 engine to source parts for my twincam but I'm to cheap to buy one so it didn't happen.

I'm currently looking for cheap/broken/free 125 or 110cc donor engines. I'm gonna start work on the 244cc vee-twin china engine next. Billet alloy engine cases. Mostly stock components. I don't think it'll need to be super tuned to make big power with 244cc.
 
Wow thats fantastic. It sounds like it'll be mad in a 50! You must be pretty good with this kinda stuff coz i couldnt make heads or tails of those pics. I only knew wat they were coz u labelled em lol. I hope it all pays off 4 ya! And great work making ya own parts!
 
tach,sprocket

f.y.i. early gsxr tachs indicate @1000rpm higher than actual rpm , and you can get a 17 and 18 tooth sprocket for that motor
 
Last edited:
Hey been following this thread from the start, still haven't seen any video's how's the bike going have u got it tuned and dyno'd yet, any updates would be great
 
I haven't been playing with the xr much lately. Its geared way to low and no fun to ride. If I use different sprockets to gear it up, the chain fouls on the swingarm badly.

I just got my z50j registered yesterday. It looks so weird with a number plate on it. Still feels like I'm breaking the law when I ride it :). I wanna put the twincam in that. I'll buy a yx150 for the xr. Trouble is that the twincam engine is longer than the z50 motor and it hits on the front wheel. I'll have to change the forks and clamps. Gearing isn't a problem on the z, coz the swing arm angle isn't so steep. I can use a tiny rear sprocket.

I'm changing jobs in a couple of weeks to, and won't have access to any cool machines, so doing any complex engineering is going to be much more difficult.

Does anyone know if there is a taller primary ratio available for the china engines? I beleive the engine I have now is about 4:1. I saw specs for the yx150 somewhere and apparently that has a primary ratio of 3.7:1. If anyone has a 3.7:1 gearset I'd be very interested in buying it off them.
 
I got the bike dyno'ed at buzz bikes today. 10.7hp@11,800rpm. Apparently its the most powerful bike that they've had on the dyno. Next best was a 140 with 9.8hp. The twincam is a giant killer! :)
 
The sheer fact that your engine is still running is testament to your engineering ability ... :cool:

A certain "engineer" on another forum couldn't get his engine to survive for a few minutes ... OR get past 5,000 rpm where it would be making far less power than a stocker at peak revs ... :p
 
You must log in or register to reply here.

Latest posts

Back
Top