1/2 throttle sputter

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longduckdong

longduckdong

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Aug 9, 2012
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I searched this problem but only came up with threads about high RPM sputter. Still breaking mine in. Maybe two miles on it. I have some engine sputter at low rpms when the throttle is 1/4 to 1/2 open Runs perfectly at full throttle. Sputtering enough to make it a rough ride but doesn't die. Any thoughts?
 
shwnrttr

shwnrttr

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Dec 2, 2009
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if your running a nt carb try a higher clip setting.(so the needle sits further in the jet) mine did the same thing when i got it. i had to solder the jet and redrill it to a smaller size and change the clip setting on the needle i also had to bend the float tabs so the float would sit lower in the bowl. it took a while to get it right but it ran well through out the rpm range after the tedious tinkering.
 
biknut

biknut

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Sep 28, 2010
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longduckdong said:
If that's the one that came with the kit then yes. Should I just upgrade it and save myself the grief?

thanks
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There's several different carbs that come with kits. NT, Speed, and CNS. The CNS is best used as a paper weight. The other two work well.

If it's a CNS it will have 2 cables coming out the top. One of the cables operates the choke, and the other one the throttle.
 
longduckdong

longduckdong

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Looks like I have an NT.I'll have to dig into it tomorrow but if I recall the needle is set at the lowest setting right now.
 
biknut

biknut

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Sep 28, 2010
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longduckdong said:
Looks like I have an NT.I'll have to dig into it tomorrow but if I recall the needle is set at the lowest setting right now.
Click to expand...
Try it with the needle clip in the second from the top position. All my NT carbs liked that position, and same with the Speed carb too.
 
2door

2door

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Sep 15, 2008
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And give your engine a little time. 2 miles isn't going to tell you much. Wait until you have burned two or three gallons of fuel then start worrying about real performance.

Don't 'baby' the engine during break-in. Run it hard a few times. Don't abuse it by running full throttle all day but idling around at 1/4 throttle will increase the possibility of fouling the spark plug and will increase the break-in time.

As Biknut suggested, move the clip to the second from the top and see if that doesn't help your low speed performance a little. The clip position determines how much fuel is fed to the engine below about 3/4 throttle. The higher the clip the less fuel. Lowing the clip allows more fuel to pass and richens the mixture.
Above 3/4 throtle the main jet alone determines the fuel flow because the needle is lifted out of the orifice.

Tom
 
N

nightcruiser

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Mar 25, 2011
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longduckdong said:
Looks like I have an NT.I'll have to dig into it tomorrow but if I recall the needle is set at the lowest setting right now.
Click to expand...
Having problems with a new NT carb, well, throw it away.... Just kidding. Kind of poking at the guys who constantly tell people to throw away their CNS carbs. They need to get over that, my CNS runs great!
Just goes to show the NT carb needs some tuning too, not just the CNS.

I'm sure you have gotten your answer already, but if you motor runs well at WOT and bogs mid throttle then you should adjust your needle...
 
longduckdong

longduckdong

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Aug 9, 2012
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So I adjusted the needle down and notice no difference. Raised it and created other problems. Any other suggestions? Thanks.
 
dragray

dragray

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Mar 10, 2012
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is your choke open?
sometimes the lever for the choke on the nt carbs will come loose from the choke plate causing the lever to not open or close the choke.
It's possible that your choke plate is partially closed.
 
J

JohnnyCrash

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Jun 28, 2012
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longduckdong said:
I searched this problem but only came up with threads about high RPM sputter. Still breaking mine in. Maybe two miles on it. I have some engine sputter at low rpms when the throttle is 1/4 to 1/2 open Runs perfectly at full throttle. Sputtering enough to make it a rough ride but doesn't die. Any thoughts?
Click to expand...
dong,

My 66cc China Girl does the exact same thing. With the main jet soldered and drilled to provide excellent high speed running, and the needle set to the highest clip position (leanest setting), the engine runs too rich at low speeds just like yours, at up to around 1/2 throttle the acceleration is fine but once the bike reaches a stead-state speed, the engine begins four-stroking (firing every second revolution, an indication of excessive fuel).

This seems to be what happens with the NT style carb when you have the intake pipe and carburetor joints properly sealed, with no air leaks diluting the intake charge at high intake vacuum (small throttle openings). The bike will 'buck' and the engine will run rough, with excessive fuel consumption.

The only cure I can see is adding an air bleed, a small tube with an adjustable screw valve on it that bypasses fresh air from the air filter mounting face at the carb's air intake side directly to the outlet side after the throttle/jet tube. This way, at high intake vacuum, the air bleed can be adjusted to give the right amount of air bypass preventing excessive amounts of fuel from being drawn into the engine.

Almost every good small carb has an idle and slow-running mixture adjustment, either an air bleed like I described or a separate idle jet. These NT style carbs don't have that, and adjusting the jet needle clip position doesn't lean the mixture enough to provide smooth running at low speed.

I'll have to get around to installing an adjustable air bleed on mine... I'm too cheap to buy a different carb :)
 
2door

2door

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Sep 15, 2008
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Littleton, Colorado
JohnnyCrash said:
dong,

My 66cc China Girl does the exact same thing. With the main jet soldered and drilled to provide excellent high speed running, and the needle set to the highest clip position (leanest setting), the engine runs too rich at low speeds just like yours, at up to around 1/2 throttle the acceleration is fine but once the bike reaches a stead-state speed, the engine begins four-stroking (firing every second revolution, an indication of excessive fuel).

This seems to be what happens with the NT style carb when you have the intake pipe and carburetor joints properly sealed, with no air leaks diluting the intake charge at high intake vacuum (small throttle openings). The bike will 'buck' and the engine will run rough, with excessive fuel consumption.

The only cure I can see is adding an air bleed, a small tube with an adjustable screw valve on it that bypasses fresh air from the air filter mounting face at the carb's air intake side directly to the outlet side after the throttle/jet tube. This way, at high intake vacuum, the air bleed can be adjusted to give the right amount of air bypass preventing excessive amounts of fuel from being drawn into the engine.

Almost every good small carb has an idle and slow-running mixture adjustment, either an air bleed like I described or a separate idle jet. These NT style carbs don't have that, and adjusting the jet needle clip position doesn't lean the mixture enough to provide smooth running at low speed.

I'll have to get around to installing an adjustable air bleed on mine... I'm too cheap to buy a different carb :)
Click to expand...
I'm not optomistic about your idea on several levels.

1. This is not a common problem with the NT carburetor. If the float level is set correctly and the clip positioned for your altitude, rich conditions at low speed should not be a problem.

2. How do you plan to "adjust" your "air bleed screw" while riding? Riding one handed while reaching down to find and adjust the screw sounds unsafe. Plus you'd have to do that everytime you go from mid/low throttle to WOT then back again. A handlebar mounted control might help.

3. If your intake manifold is aluminum how are you going to install your air bleed? Please don't say, JB-Weld. :) They aren't thick enough to thread into. If it is steel you could solder or weld a fitting.

As I said, I'm not sure about your idea but don't let that sway you. Give it a try but please come back and let us know how your concept works. Before you install a bleed, first check your float level, maybe even experiment by bending the tangs down a little to lower the float (which will actually close off the fuel flow a little sooner thereby lowering the fuel level in the bowl which might be the cause of your rich condition.
Just a suggestion.

Tom
 
N

nightcruiser

New Member
Mar 25, 2011
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JohnnyCrash said:
dong,
This seems to be what happens with the NT style carb when you have the intake pipe and carburetor joints properly sealed, with no air leaks diluting the intake charge at high intake vacuum (small throttle openings). The bike will 'buck' and the engine will run rough, with excessive fuel consumption.
Click to expand...
You mean the NT Carb isn't heaven sent and perfect? Since people seem to be constantly saying "Throw away your CNS carb and get an NT" I figured they ran perfectly, now I am reading they have some inherent issues? My CNS has 3 jets and doesn't suffer from any of these problems. The NT carb is a bit simplistic and not perfect, and apparently can require some fine tuning just like the CNS? It seems this is the same kind of stuff that has made people give the CNS a bad rap, what gives? Both carbs can be tuned well, but you have to know what you are doing and actually do it...
 
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