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Discussion Starter · #1 · (Edited)
Understanding exactly how our CBR carburetors work is critical to getting any motorcycle to run correctly. I need to address some of the fundamentals of how CBR carburetors function.

1.) The fuel screw (some have called it air/fuel screw) is actually called the idle mixture screw. It is a fuel screw and turning it clockwise decreases the amount of fuel allowed to flow through the pilot jet. Turning it counterclockwise increases the amount of fuel permitted to flow through the pilot jet. For practical purposes, It affects ONLY the air fuel ratio (AFR) from idle to apprx 1/8th throttle, and, it and the pilot jet affect AFR only an extremely small and negligible amount after 1/8th throttle. The affect on AFR after 1/8th throttle is so negligible that, for all practical purposes, it can be ignored.

2.) The end of the slide needle protrudes down into the bore of the main jet. Without the needle being raised by washers it will just about fully block fuel flowing through the main jet completely. After 1/8th throttle, the fuel flowing through the pilot jet continues to flow and since the needle is being lifted by you twisting the throttle open, fuel begins to also flow through the main jet as well. Now, it's important to realize at this point, there are 4 things affecting AFR at approximately 1/8 to 1/4 throttle,1. the height of the needle, 2. the bore size of the main jet, 3. the pilot jet size and 4. the fuel screw setting. This "transition" through 1/8th to 1/4 throttle position is the most difficult transition to tune and get right.

3.) As the needle gets lifted even higher by you twisting the throttle more, the pilot jet's role becomes increasingly diminished. Somewhere around 1/4 throttle the pilot jets affect on AFR can be ignored for all practical purposes, but yes, it is still continuing to flow fuel as described above in #1. Beyond 1/4 throttle AFR is controlled primarily by the main jet and the main jets needle height.

Ii hope that this is helpful.
 

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Understanding exactly how our CBR carburetors work is critical to getting any motorcycle to run correctly. I need to address some of the fundamentals of how CBR carburetors function.

1.) The fuel screw (some have called it air/fuel screw) is actually called the idle mixture screw. It is a fuel screw and turning it clockwise decreases the amount of fuel allowed to flow through the pilot jet. Turning it counterclockwise increases the amount of fuel permitted to flow through the pilot jet. For practical purposes, It affects ONLY the air fuel ratio (AFR) from idle to apprx 1/8th throttle, and, it and the pilot jet affect AFR only an extremely small and negligible amount after 1/8th throttle. The affect on AFR after 1/8th throttle is so negligible that, for all practical purposes, it can be ignored.

2.) The end of the slide needle protrudes down into the bore of the main jet. Without the needle being raised by washers it will just about fully block fuel flowing through the main jet completely. After 1/8th throttle, the fuel flowing through the pilot jet continues to flow and since the needle is being lifted by you twisting the throttle open, fuel begins to also flow through the main jet as well. Now, it's important to realize at this point, there are 4 things affecting AFR at approximately 1/8 to 1/4 throttle,1. the height of the needle, 2. the bore size of the main jet, 3. the pilot jet size and 4. the fuel screw setting. This "transition" through 1/8th to 1/4 throttle position is the most difficult transition to tune and get right.

3.) As the needle gets lifted even higher by you twisting the throttle more, the pilot jet's role becomes increasingly diminished. Somewhere around 1/4 throttle the pilot jets affect on AFR can be ignored for all practical purposes, but yes, it is still continuing to flow fuel as described above in #1. Beyond 1/4 throttle AFR is controlled primarily by the main jet and the main jets needle height.

Ii hope that this is helpful.
This "transition" through 1/8th to 1/4 throttle position is the most difficult transition to tune and get right.
That's the area I'm interested in. My 400 NC 29 revs crisp when not under load. I fitted new plugs coz one cylinder was intermittent, now all downpipes are hot but when I pull away it sounds like it's getting too much air and blehs it's way (that's the sound it makes "bleh" like the start of the word blessed) to 7,000 rpm where the touch paper is lit and it screams to 12,000rpm no probs.
I seam to remember it sounding better below 7,000 rpm than it does now.
So I can try wind out the Mixture Screw to allow more fuel, yeah?
The Tank was cleaned with a new filter last year but it's done hardly any miles.
 

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This "transition" through 1/8th to 1/4 throttle position is the most difficult transition to tune and get right.
That's the area I'm interested in. My 400 NC 29 revs crisp when not under load. I fitted new plugs coz one cylinder was intermittent, now all downpipes are hot but when I pull away it sounds like it's getting too much air and blehs it's way (that's the sound it makes "bleh" like the start of the word blessed) to 7,000 rpm where the touch paper is lit and it screams to 12,000rpm no probs.
I seam to remember it sounding better below 7,000 rpm than it does now.
So I can try wind out the Mixture Screw to allow more fuel, yeah?
The Tank was cleaned with a new filter last year but it's done hardly any miles.
 

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Discussion Starter · #5 ·
This "transition" through 1/8th to 1/4 throttle position is the most difficult transition to tune and get right.
That's the area I'm interested in. My 400 NC 29 revs crisp when not under load. I fitted new plugs coz one cylinder was intermittent, now all downpipes are hot but when I pull away it sounds like it's getting too much air and blehs it's way (that's the sound it makes "bleh" like the start of the word blessed) to 7,000 rpm where the touch paper is lit and it screams to 12,000rpm no probs.
I seam to remember it sounding better below 7,000 rpm than it does now.
So I can try wind out the Mixture Screw to allow more fuel, yeah?
The Tank was cleaned with a new filter last year but it's done hardly any miles.
I am not familiar with the NC29 specifically, and I don't know if you have made any modifications like exhaust or air filter/intake, ....but, I do know that what you are calling the mixture screw is a flow control for the pilot jet. An easy way to test if you are lean is to block off about 1/2 of the air coming into the air box with a piece of duct tape or anything. If performance below 7000rpm improves then you know a lean condition was the cause. If you block off 1/2 the air coming into the airbox and performance decreases then you know it was too rich before. Once you are certain that you are lean below 7000rpm I recommend opening the mixture screws and raising the needles with 2 M3 washers under each needle. Unfortunately there is no way to avoid removing & installing the carbs a dozen or so times. If the lean condition persists, then go up one jet size on the pilot jet. If the duct tape test also improves performance above 7000rpm then go up a main jet size or two as well.

I ended up going up 5 main jet sizes from 118 to 135, and 1 jet size on the pilot jet from 40 to 42. My needles were raised with 2 M3 washers each, and my mixture screw is at 1.75 turns open (because my pilot was a 42), and all I did was improve exhaust flow by adding a micron exhaust can. I hope you get it right. Once you do the dozen or so carb removals and reinstalls will be worth it!
 

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Discussion Starter · #6 ·
This "transition" through 1/8th to 1/4 throttle position is the most difficult transition to tune and get right.
That's the area I'm interested in. My 400 NC 29 revs crisp when not under load. I fitted new plugs coz one cylinder was intermittent, now all downpipes are hot but when I pull away it sounds like it's getting too much air and blehs it's way (that's the sound it makes "bleh" like the start of the word blessed) to 7,000 rpm where the touch paper is lit and it screams to 12,000rpm no probs.
I seam to remember it sounding better below 7,000 rpm than it does now.
So I can try wind out the Mixture Screw to allow more fuel, yeah?
The Tank was cleaned with a new filter last year but it's done hardly any miles.
Also see my thread
2 Years trying to get tuning right 97 CBR900RRV

Regards
 

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I recall there was something called the Carburetor Bible. I have something in my iBooks (PDF) called the Haynes Motorcycle Carburretor Manual. I'm of the belief that one can find it with a search online.
 

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I recall there was something called the Carburetor Bible. I have something in my iBooks (PDF) called the Haynes Motorcycle Carburretor Manual. I'm of the belief that one can find it with a search online.
I let it warm up properly and took it for a 20mile run and it doesn't seem as sick this time pulling from 3,500 without a hic up so I'm going to give the bike to mate who knows how to clean and balance carbs, I'm not trying to tune it to max power plus just to the best possible with standard equipment coz it'll not be used for racing.
Thanks Jaybird180 and CBR900RRV for your support and CBR919RRV for the original post which went a long way to helping me visualise what could be happening.
 
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