Carb tuning

[Octo Ro]

So I've been trying to tune my carb and honestly I don't know too much about what I'm doing. Up to half throttle works fine, but after that it is pretty unresponsive. I've changed the main jet all the way from what came with it down to like a 66. Still can't do full throttle. For some reason though, i can go full throttle for a little bit when I'm just starting up my bike but not for very long. I've tried adjusting the fuel air screw to make it more lean and more rich. Neither have yielded the results i want. I can only go about 30 rn, but I know it should be hitting at least 40. Help is much appreciated. The carb is the same from the gt90 conversion kit. Don't know what it's called. If you know the name of it, lmk plz

 

[Chainlube]

So I've been trying to tune my carb and honestly I don't know too much about what I'm doing. Up to half throttle works fine, but after that it is pretty unresponsive. I've changed the main jet all the way from what came with it down to like a 66. Still can't do full throttle. For some reason though, i can go full throttle for a little bit when I'm just starting up my bike but not for very long. I've tried adjusting the fuel air screw to make it more lean and more rich. Neither have yielded the results i want. I can only go about 30 rn, but I know it should be hitting at least 40. Help is much appreciated. The carb is the same from the gt90 conversion kit. Don't know what it's called. If you know the name of it, lmk plz

Post a picture so we know which carb you're setting. First GUESS, float height adjustment.

 

[Octo Ro]

Post a picture so we know which carb you're setting. First GUESS, float height adjustment.

 

[Octo Ro]

Sorry if this is a retarded question but should I use a non windowed piston?

 

[Street Ryderz]

So I've been trying to tune my carb and honestly I don't know too much about what I'm doing. Up to half throttle works fine, but after that it is pretty unresponsive. I've changed the main jet all the way from what came with it down to like a 66. Still can't do full throttle. For some reason though, i can go full throttle for a little bit when I'm just starting up my bike but not for very long. I've tried adjusting the fuel air screw to make it more lean and more rich. Neither have yielded the results i want. I can only go about 30 rn, but I know it should be hitting at least 40. Help is much appreciated. The carb is the same from the gt90 conversion kit. Don't know what it's called. If you know the name of it, lmk plz

Reed setup's require more fuel and larger jetting and a 66 jet is way too lean for that carb! Turning the air screw has no effect on anything but the idle it does not change how much fuel is available it just allows more or less air to be blended with the idle circuit. Any performance engine needs a pipe that allows the engine to breath properly and without one is very hard to tune!

 

[Street Ryderz]

If all else fails your intake timing can be off.

See this picture, the window in the piston is all wrong and will give excessive duration. Alot of windowed pistons sold are wrongly done.
It should be a no more then 20% of the port window and placed just above the bottom of the skirt.
Reed's need 140-160 degrees too fully realize their power. Too much 200*+ like some of these pre-windowed pistons will give a bad case of the blubbers on a stock NT carb and I can bet it's not better with a Mikuni. I learned this the hard way, when I first did a reed conversion, I dremeled a window just the same and it would only run if I gave it a little throttle at a time. Once I put a normal piston in I got all response back with a little top end loss due to the smaller intake timing.


View attachment 102976

An nt carb is quite small for a reed setup on these engines and a case of the burbles is from cross talk not excess duration, there is no such thing as 200+ or even 200 degrees of intake duration when there is only 180 degrees of up stroke to work with and on our engines the dwell top and bottom of the stroke only leaves 172 degrees available which doesn't mean you can get the full 172 degrees because the piston has to climb some before it actually starts to create a vac and that's dependent on if there is pressure above the piston acting as a blocker for the open port's other wise it doesn't start to create a vac until the transfers are covered which then means the actual duration is much lower!

 

[Octo Ro]

Reed setup's require more fuel and larger jetting and a 66 jet is way too lean for that carb! Turning the air screw has no effect on anything but the idle it does not change how much fuel is available it just allows more or less air to be blended with the idle circuit. Any performance engine needs a pipe that allows the engine to breath properly and without one is very hard to tune!

It was doing the same thing when I had a way bigger main jet too

 

[Chainlube]

I still say sticky float needle, doesn't get enough fuel to do anything other than idle.

 

[Octo Ro]

I still say sticky float needle, doesn't get enough fuel to do anything other than idle.

Do I just clean it to fix that?

 

[Captain Rainmaker]

Are you using a genuine carb or an unlicensed clone? The knockoffs can be pretty craptastic when it comes to getting them to run right.