Carburetor Needle Adjustment

brown bull

New Member
Nov 16, 2008
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pittsburgh
Hello, it's great to be part of this forum. I bought a used motorized bicycle and I'm going through all the parts to understand how it works and what can be improved. So far I replaced the stock plug and plug wire with parts from sickbikeparts and I installed a killswitch and idler pulley with parts from dax.

I'm trying to troubleshoot an issue where the the engine bogs down at full throttle. I took out the carburetor needle and moved the c-clip to the notch furthest away from the point of the pin. The engine would hardly run. Then I put the c-clip one notch closer to the point of the pin (which was where it was before) and the engine ran fine, with the same bogging at full throttle. I'm planning to move the c-clip one notch closer to the point of the pin to see what happens, but I was wondering what exactly I'm adjusting by doing this.

I appreciate your feedback! Also, I've read some posts about adjusting the needle, but it's unclear to me whether "up" and "down" refer to the position of the needle or the position/notch of the c-clip. This is an important detail because they're inversely related.

 

misteright1_99

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Mar 21, 2008
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Treasure Coast, Florida
Moving the clip up lowers the needle so less gas is mixed with the incoming air (leaner) mixture, and moving the clip down raises the needle allowing more gas to be mixed with the incoming air (richer) mixture. Watch your plug and that will give you an idea where the correct position is.
 

brown bull

New Member
Nov 16, 2008
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pittsburgh
Thanks for writing. My plug gap is at .028 inches and the plug looks pretty dark, which I'd think means a rich mixture, but the bike hardly ran when I leaned the mix by one notch.
 

misteright1_99

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Mar 21, 2008
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Treasure Coast, Florida
How old is the engine? What oil ratio are you running, and what kind of oil? Are you sure you dont have any intake leaks? Does the bike stall right away when you turn the choke on, after warming it up?
 

brown bull

New Member
Nov 16, 2008
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pittsburgh
The plug is brand new and when I pulled it to check the gap I noticed it was already pretty dark, indicating a rich mix, right? The gas:oops:il is whatever the previous owner mixed and he told me he was using 25:1.

I replaced my intake gasket and put an o-ring at the intake/carb junction. My old intake gasket was not intact, so it should run better with the new gasket.

Thanks!
 

brown bull

New Member
Nov 16, 2008
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pittsburgh
Hi, Things are running better now with the new intake gasket, Carburetor-Intake Pipe O-Ring, and Fresh Gas & Oil 32:1 mixture.

Today I was not having an issue with the bogging at full throttle, but I was having a hard time keeping the engine idling when coming to a stop or slowing down. Eventually I figured that I had to turn the idle adjustment screw all the way IN to keep the engine idling. Turning the screw in makes the mix leaner, correct?

What should I adjust next? It's probably not right that that idle screw needs to be totally turned into the carb and that screw should be for fine tuning only.

Thanks!
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
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up north now
That screw does not adjust the MIXTURE.....only the idle speed. If it has to be in all the way, that's O.K. They all are different.
 

brown bull

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Nov 16, 2008
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pittsburgh
Hi Joe, what does that screw do, then? It must adjust either air or fuel at idle, right? What if I needed to bring up the idle and the screw was already all the way into the carb; how could I bring up the idle?

Is there a manual anywhere online about how the engines work?
 

misteright1_99

New Member
Mar 21, 2008
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Treasure Coast, Florida
Hi Joe, what does that screw do, then? It must adjust either air or fuel at idle, right? What if I needed to bring up the idle and the screw was already all the way into the carb; how could I bring up the idle?

Is there a manual anywhere online about how the engines work?
All that screw does is slightly hold the throttle slide up to keep idle up. You could try cutting the spring on the screw that MIGHT let you turn it in a little bit more.
 

seca40

New Member
Nov 15, 2008
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Albuquerque
After reading this thread so I could try to figure out why my bike won't idle, I went to adjust my idle screw only to find out that it has abandoned ship. D'OH!

Could someone please help me out with some details on this little mutineer? Is it pointed? What size, thread is it? Could someone post a pic of one out of the carb? Got an extra? Any help anyone can offer is greatly appreciated.


Thanks.
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
11,843
236
63
up north now
After reading this thread so I could try to figure out why my bike won't idle, I went to adjust my idle screw only to find out that it has abandoned ship. D'OH!

Could someone please help me out with some details on this little mutineer? Is it pointed? What size, thread is it? Could someone post a pic of one out of the carb? Got an extra? Any help anyone can offer is greatly appreciated.


Thanks.
Pointed, fine thread (don't know) size (don't know). I think you'd be better off if you contact some of the sellers of these kits and ask one to send you an idle screw...should be really cheap.

How good does it run with the needle out? It should be a little leaner sucking air in through that hole?
 

seca40

New Member
Nov 15, 2008
131
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0
Albuquerque
I actually didn't notice any difference in the way it ran with the screw out. I've only ridden it about five miles though so the finer points of tuning might be lost on me. Thanks for your suggestions. I made a replacement. It worked like a charm, then that little bugger jumped ship too.:rolleyes: I made a replacement for the replacement and wrapped it with teflon tape. We'll see how long that one sticks around.
 

jasonh

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Jun 23, 2008
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Longmont, CO
What if I needed to bring up the idle and the screw was already all the way into the carb; how could I bring up the idle?
If the screw is all the way in, you can cut the spring like suggested, find a replacement, or use the cable adjusters on top of the carb and on the throttle to put a little preload in your throttle cable. This will pull that slider up a bit more. Once you use those adjusters, the idle screw does nothing.