China Girl Motorized Bicyle Build

Re: China Girl Motor Build

Tohri said:

Biknut, if you can get those guys to make up a couple cranks, I'll buy one, with the crankpin moved 3mm further towards the edge. I'll balance it myself.

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I'm still waiting and haven't heard a thing yet. I will though, I'm sure of that. I seem to have a kind of curse about getting work done for me at machine shops. It always takes about 3 months.

Re: China Girl Motor Build

maurtis said:

Any update on this? I would love to put one of these in my upcoming build, and have zero confidence in my ability to do it myself in my garage...

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So far so good. Unfortunately I haven't been able to ride much in the last couple of weeks, because of weather and other obligations. I'm still in breakin mode with only 135 miles so far.

My riding impressions have not changed, about this being the smoothest, and quietest China Girl motor I've experienced to date.

Performance is still a little bit of a question mark, but it's loosened up enough that 35 mph with a 36T is a given. The tuning is not accurate yet because I'm getting a lot of 4 strokeing at top speed. Even so, power in the mid range is coming up enough that I'm starting to detect a little clutch slip under full throttle. I'm not going to worry about that till I get it completely broken in, and change the fuel mix ratio.

Re: China Girl Motor Build

maurtis said:

Excellent, good to hear. Having done the majority of the tuning and custom work on my motorcycles, I know that I should never be allowed inside of a case... ever. I am good with getting to second base on my motors.

How difficult do you think replacing the crank is on one of these? I have read Norman's thread on disassembling these HTs, and it looks a little daunting in print.

I am very comfortable working up top, but wondering if I should find someone professional to open up the bottom end?

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I feel that if you take your time, and ask questions, there are many members on this site that can get you through a complete build. These are the easiest motors I've ever worked on. Other than the gear puller tool that comes with most kits, any other special tools you'll need are easy to make yourself.

This is a very handy tool I made to help split the cases. Now there's no need to beat on the crankshaft, or pry the crankcase apart. Anyone can make one of these. A propane torch is about the only other special tool I can think of you might need. Other than this, a hand full of proper hand tools are all you really need. And like I said, ask a lot of questions, and don't get in a hurry.

Re: China Girl Motor Build

I moved my exhaust from here



to here



To tell you the truth, I thinking about moving it back to the rear. In the forward position I'm having trouble keeping out of the peddles. I can adjust it so nothing touches, but after a couple of miles it shifts and starts rubbing the peddle. In the rear position it's very dependable. Also I'm not sure I don't like the power delivery better in the rear position. It definitly has better mid range like that, and the top end isn't much different. As it is now in the forward position the top end is ok, but I miss the mid range.

I think I'll try one more time to fix it in the forward position, but if it still gives me trouble then screw it, I'm going back to the rear with it.

Re: China Girl Motor Build

There's been a minor set back. At about the 165 mile point the stock needle bearing on the small end gave up. One of the rollers broke a piece off, and it fell into the crankcase where it got sucked up onto the piston top. As I was riding along about 30 mph the motor all of a sudden sounded like the piston was hitting the head. I killed it and when on backup power (peddles) a mile home. At least I wasn't too far from home.

The damage was fatal to the piston, but the MM head can be salvaged. There was no damage to the bottom end.

I had just installed a MM adjustable rotor before the failure, and initially I thought I had advanced the timing too far and siezed the piston, but at least that didn't happen.

I can say that advancing to timing did improve the performance, because right before it failed it was running better then ever. How many times have you heard that lol.

I did originally try to source an aftermarket small end bearing but motion industries told me it was an odd size , and I probably wouldn't be able to find one. I should have tried harder to find one. Today went on a mission, and after going to five different bearing houses finely hit paydirt. i was able to order a Japanese made IKO part # KT-101415 10x14x15. These are the same brand as the crank bearings I bought.

I now have about a week to clean up the gasket surfaces, and repair the dings in the combustion chamber.

I was kind of wanting to take the cylinder back off anyway. After getting the motor running I became aware that people were cutting the bottom of the piston skirt to match the intake port, and wished I had done that before sealing it all up. I'll take care of that this time, unless someone knows of a reason whay I shouldn't do it.

Re: China Girl Motor Build

One more thing I'd like to comment on, is how the break-in process seemed to be going. I ran one gallon through it at 24:1, and the second gallon was about 28:1. i didn't particularly take it easy on it. Maybe the first 50 miles, but after that I was getting it up to 35 mph often, and holding it there for as long as 2 blocks sometimes.

I was interested it see how the piston skirt looked as far as blow-by was concerned. Both the piston skirt and cylinder looked like new with no sign of any blow-by, or any problems at all, and the rings looked great. I conclude that not babying it wasn't causing any excess wear, and actually looked like it was breaking in well, and the rings were sealing well.

Re: China Girl Motor Build

My new small end bearing came in today. I started putting the top end back on. I cut the piston skirt this time.

I was able to sand most of the dings out of the head.

I think between cutting the piston, and the adjustable rotor it should run pretty good now.

After I get it broken in and tuned, and after I'm used to it, I going to install my Dax carb and see what the diff is.

Re: China Girl Motor Build

I got it back together again. I installed a new cylinder, and piston, and new small end bearing. This time I cut the intake side of the piston.

Right before it failed last week, I was right in the middle of playing with my new adjustable magnito rotor. It was rippin and then kaboom. Now it's all better than ever, again.




I heat cycled it 5 times, while out riding tonight. It's running good. It feels even a little better. Seems like advancing the magnito rotor added a little more uoompth over stock. Now I gotta play catch up breaking it in again.

Re: China Girl Motor Build

Mobbin'Deep said:

where you order that small end bearing from? just finished building my ht and that was the only one i didnt replace...lol

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Goat Herder said:

The little bearing on the wrist pin is truly one the weakest spots on these motors!! One of my buddies just blew his second motor to this cheasy bearing and I told him to replace it long ago! I know another guy here locally that blew a bearing too.. Although I have not gotten one of these...this bearing looks like perhaps one of the best designs to me? It looks a lot like a Morini wrist pin bearing I think? http://www.spmotostore.com/SearchResults.asp?Search=10x14x15

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I have to agree with you. For some reason they seem to be hard to find, but not impossible. I assume the one in your link is a good one because it's probably not Chinese. The stock chineses ones are crappy crap for sure.

These Japanese one are higher quality.

The one I bought is a Japanese IKO part # KT101415
which are 10x14x15 mm, same size as stock. I paid $6.42 ea for 10

I bought them from a bearing house

Bearco, Inc.
3220 irving Blvd.
Dallas Tx 75247
214 631 8701

I think you can also get them from motion industries if you give them this part number.

Re: China Girl Motor Build

Oh no, I might have caused another problem. After the first 2 or 3 rides I noticed the head bolts seemed pretty loose. Each time I torqued them back to 145 in lb. Finely after 6 times they started staying tight. It was mystifying. Then I happened to see why it's happening.

The cylinder base gasket is squishing out from between the block and the cylinder. I think it's the fault of the gasket. It was a little different from the other ones I've bought. Thicker, and silver color. I thought it looked like a better one, but that's apparently not the case.

I put it together using super tack gasket sealer, but I don't think that caused this. So far there's no oil leakage coming from the joint, but I have a bad feeling about it.

Other than that it's running great. Maybe squeezing the gasket out raised the compression. If it starts leaking I may be tempted to lift the cylinder up and apply some type of gasket sealer directly to the block and bolt it down without a gasket.

Who's tried this?