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2 Stroke Bicycle Engines & Kits 2 stroke engine kits need careful installation and setup, find out how from our professionals here!

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Old 10-09-2012, 09:24 AM
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Tyler6357 Tyler6357 is offline
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Default China Girl Motorized Bicycle Problems

I put a standard stock center mount 66/80cc 2 stroke china girl kit on my mountain bike. It ran great for many months until the needle bearing broke and parts of it went through the piston/cylinder and out the exhaust pipe. I bought a new china girl 66/80cc slant head replacement engine. I replaced the head acorn nuts with lock nuts from my local hardware store, mixed up a gallon of gas at 20x1 dino oil and fired it up. It was running fine as I broke it in. I went through the first tank no problems. I never gave it full throttle. I poured the rest of the gas mix into the tank and kept breaking it in driving it a little bit every day for short rides, gradually giving it more throttle and going full throttle for short bursts. When it got to about 1/4 tank gas left the engine stopped and wouldn't start. It was clear it was losing compression because I could feel the air against my leg as it shot out from around the head every time I popped the clutch. I figured out that the lock nuts I got for the head were not screwing down far enough on the studs. They would screw on a little bit but then would get locked on the studs until tightening them just turned the entire stud. I removed them before I stripped out the stud's threads or the threads on the bottom. I tried regular non-lock nuts and was able to get some that would screw down on the stud threads but no matter how tight I screwed them down the head kept leaking air. I replaced the head gasket with the one from my old engine and tried it again. Result: It started up but would bog down losing all its power every time I popped the clutch also smoking like crazy, oil dripping out of the exhaust pipe as it cooled. I ordered two new stock NT carbs (BGF $12 each) and two new head gaskets (dax). I also got a new spark plug (dax). I emptied the gas tank and cleaned it out with straight gas and drained it again. I mixed a gallon of gas at 32x1 with Castor 927 racing oil. I put one of the new carbs on and put one of the head gaskets on the head and made sure the head was tight with a decent amount of torque. I screwed in the new spark plug, refilled the gas tank with my new mix and tried to start it again. Result: It will only start and run when I have it choked all the way. It bogs down, losses all it's power, and dies when I pop the clutch. It's not smoking like crazy anymore though. When I twist the throttle and then release it revs up high but doesn't rev back down very quickly, it comes down but very slowly. It just seems unresponsive coming down, Turning the idle screw counter-clockwise to lower the RPMs makes it die.

Questions: Why am I losing all its power when I put a load on it? Is this still a compression problem with the head or maybe somewhere else? A carb leak or some kind of other air leak? How can I test? Could the throttle controls need adjustment? It seems like the fuel/air mixture is wrong. Should I adjust the carb needle setting or jet? Is the engine broken in now? Did I break it in wrong?

No wonder so many people give this up and get moped or a motorcycle instead. All advice welcome.
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Old 10-09-2012, 11:05 AM
crassius crassius is offline
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Default Re: China Girl Problems

It's possible that you still have a pool of oil/fuel in the bottom of your engine from when the head was leaking - I'd go back to the carb that you had when it ran right, then try to keep it running as long as you can to see if it clears up.

Taking the muffler off & making sure it's clear and not full of fuel wouldn't hurt.

If it doesn't clear up, maybe it will get bad enough for the problem to become easier to spot.
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Old 10-09-2012, 11:19 AM
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2door 2door is offline
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Default Re: China Girl Problems

You have an air (vacuum) leak somewhere. The most common areas are the carburetor to intake manifold and/or the intake gasket at the cylinder. Other areas would be one of the two crankshaft seals. Your symptoms are consistant with excessive air entering the engine someplace other than through the carburetor.

Also if the locknuts were turning the studs that tells me the studs aren't tighten sufficiently into the case. To fix that remove the cylinder head, double nut the studs and torque them into the case (120 to 140 inch pounds) then reinstall the head and torque the nuts to the same value.

Good luck.

Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"

Last edited by 2door; 10-09-2012 at 07:16 PM.
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Old 10-09-2012, 01:44 PM
vachon644 vachon644 is offline
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Default Re: China Girl Problems

Great advice 2door.

To test air leaks use brake cleaner or carburetor cleaner to spray on the areas which could be culprit;
All engine gaskets,
Crankshaft seals (49-66cc 2-strokes are famous for seals pushed halfway or crooked..)

Get the engine running even if it requires quite a high idle then proceed to spray and check for changes in idle speed.
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Old 10-13-2012, 10:50 PM
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Tyler6357 Tyler6357 is offline
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Default Re: China Girl Problems

Okay, first of all, I want to say thanks you guys for all your help! 2 Door is right, I had a vacuum leak but I have good news to report. First, I tried to spray some carb cleaner to see if I could locate the leaks and as it turned out, that was a very good idea (thanks Vachon)!! It was still slightly leaking around the head and also around the carb/manifold seal.

I decided to put my original old carb back on. I was able to tighten the head nuts a bit more (unfortunately I don't have a torque wrench to measure). I had to stick with the non-lock nuts for the head because the lock nuts just won't screw down on the stud threads enough before getting stuck. Also, I found out that I was also leaking air around the spark plug too, it just wasn't tight enough. After I fixed all these things I got most of my power back. I no longer have to keep it choked to get it to start. It still bogs down a little bit at lower RPMs but it is now going about 25MPH up a steep hill. I'm still losing a little power but I think it is because I have a very small leak at the cylinder/case seal, its kinda hard to tell. It's not perfect but it's 1000% better than it was. It's not smoking anymore either, it is running much better with the Castor 927 oil. Can I risk going 40x1 when my current 32x1 mix runs out?? Hmmmmm, anyway, it's nice to be back on the road again! Thanks again to all 3 of you, your advice really did help me.

Thank you, thank you, thank you!!
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