66cc China Girl Motor runs too rich at low throttle, can't adjust anymore!

Hi eh!

Just got my China Girl ticking, it uses the stock exhaust and NTTC (NT?) carburetor with stock air cleaner, the square black plastic one. I'm using the stock 10-tooth engine output sprocket and 44 tooth rear sprocket.

Initially, it ran extremely rich. It wouldn't even fire at all with the choke on, when I took it off it fired right up. It four-stroked badly at all throttle settings and bucked like crazy at low speeds.

I started by taking the 0.028" stock jet and soldering it up, then drilled it with a 0.022" drill, which ended up giving about a 0.024" hole. The full-throttle running is now excellent and top speed is thoroughly nuts for a 44T rear sprocket, probably around 55 km/h (35 MPH) or so

Problem is, at low speeds and small throttle openings, it still four-strokes like crazy. I adjusted the jet needle to the leanest setting (C-clip at the top end of the needle) but it doesn't seem to have helped. At low speeds the engine runs rough, bucking like a bull and trying to run the bike at normal pedaling speeds results in four-stroking as the bike accelerates to a steady-state speed at around 1/4 throttle or less. The four-stroking only stops if I apply more throttle, but of course I end up taking off eh! Fuel consumption is, of course, higher than it should be at low power settings. At about half-throttle or more it runs great and the spark plug comes out at the ideal light-brown colour.

I want to be able to run the bike across a wide speed range, but can't use the small throttle opening range. I'm all out of adjustments! I'm currently using Castrol Super 2-Stroke oil at 20:1 ratio. Running less oil will only enrich the mixture, making the problem worse.

Any ideas? I'm surprised that adjusting the C-clip on the jet needle across all of the grooves seems to have almost no effect (I'm currently running it on the leanest setting, to no avail). Has anyone ever modified an NT carb to incorporate an air bleed?

Thanx!

Ian

Re: 66cc China Girl runs too rich at low throttle, can't adjust anymore!

dan428, the needle that fits into the throttle slide controls the mixture at up to about 2/3 throttle, above that the main jet controls the mixture. Tune the carb for best full throttle running first by replacing the main jet, since you're 4-stroking use a smaller jet.

Get that right, then adjust the clip on the needle to give best running below 2/3 throttle.

Finally, for best running at idle and small throttle openings/low speeds, you would adjust the idle air bleed, which unfortunately these little carbs don't have!! Maybe I'll try retrofitting one.

Ian

Re: 66cc China Girl runs too rich at low throttle, can't adjust anymore!

shwnrttr said:

mine had a similar issue, i ended up having to bend the tabs on the float so it would sit lower in the bowl. try that, u might have to play with needle clip setting after that. good luck

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I did that, but it had no effect. The needle is already at the leanest setting and adjusting the float height has no effect (strangely enough). Engine runs well at half- to full throttle, but tends to 4-stroke below that, it's especially bad at low speeds.

That's how these engines thank you for fixing all the intake leaks

Re: 66cc China Girl runs too rich at low throttle, can't adjust anymore!

fonzie said:

Just for a test, remove the air filter and see if the unit acts like it is lean at WOT. You will know immediately. Do not run it lean for any length of time. Might help your low end .....then rework your C-clip, and main jet to compensate. Find out what works. If the unit runs better then put about a 1" hole in the air cleaner housing and put it back on. These engines are all different and what works on one might not work on another. You need to try it and see if that works.

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Hi Fonzie (EEEEEHHHHHH! *thumb up*)

Thanks for the tip, this did cross my mind, but I'm not too keen on running without an air filter for any amount of time!

I figure since the engine runs well at larger throttle openings and the bike goes so damn fast ('till my butt cheeks are liquefying from the vibration!), the air filter is fine, as a restrictive air filter would cause the most problems at high speeds. It's very low speeds/small throttle openings that cause the bike to 'buck' like crazy at idle with the clutch out and it also four-strokes a lot at small throttle openings when the bike reaches steady-state speed.

Perhaps I'm just pickier than many other folks and they don't notice it, or don't care much if they do notice, but I think this engine could run better when going slowly and get much better low-speed fuel economy than it already does.

Peace

Ian

Re: 66cc China Girl runs too rich at low throttle, can't adjust anymore!

dodge dude94 said:

Get an sbp filter. Looks boss and has increased airflow that can help with your rich condition.

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...except the stock air filter allows me to go like snot, I haven't measured it but it's over 50 km/h, because I can easily keep up with cars in 50 zones, so it isn't air filter restriction that's causing it. The 4-stroking and bucking happens at very low speeds and at anything less than a little under half throttle.

I'm contemplating putting an air bleed on the carb. The stock manifold didn't fit the carb (NTTC unit) properly, I have 2 carbs and 2 manifolds and neither set ever fit right, allowing 4 small air leaks at the carb-to-manifold joint.

When fitted with the original jet, these air leaks probably allowed the engine to run properly at part throttle! Now that all air leaks have been fixed (by machining the manifold to fit the carb properly) there needs to be an air bleed from the inlet to the outlet side of the carb, allowing filtered air in after the throttle at high manifold vacuum, instead of unfiltered air getting in through the 'stock' air leaks.

Peace

Ian

Re: 66cc China Girl runs too rich at low throttle, can't adjust anymore!

Thanks for everyone for helping me out with this, yes the engine was running far too rich at small throttle openings, but ran perfectly at half or more throttle. I'd have used an adjustable air bleed to offset the rich mixture at part throttle if I were to continue with this project, it's a really nice engine and ran beautifully.

The cops nailed my nuts to the cross a couple days ago, seven court summons for 'improperly equipped motorcycle' and a pile of paperwork issues. They're rabid about it here, it seems. Looks like I'm in *seriously* deep dung on this one, we must live in such a wonderfully flawless world for the judicial system to get so bent out of shape over a grown adult tinkering with his bicycle... It's great to know the nice officers are hard at work keeping our world perfect and ridding society of antisocial menaces like myself.

Had a great time rebuilding my engine and getting it to run nicely, but it looks like I won't be going any further on it...

Thanks for the fun three days China Girl, time to look for some action on someone else's street corner

D*mn cops

Ian

Re: 66cc China Girl runs too rich at low throttle, can't adjust anymore!

Kcip said:

Hey JohnnyCrash, how did you make out with your summons issues?

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It'll take a while, but the crown attorney just chiseled it down to one charge, driving with improper license.

I haven't used the bike since I got busted.

I have yet to go to court in November; hopefully, the judge will let it slide since the police destroyed my car when an officer ran a red light and clobbered me--my reason for building the bike. I wonder if the cop got a slap on the wrist or anything for destroying private and public property--because I certainly got a kick in the nutz for trying to find an alternative to the now-deceased car. Rust in , Corolla...

Ah, government... Parasites.

Ian