Theory Question: Carburetors

yuckfoo

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Lets assume no spark problems and no fuel problems and no air leaks. The only thing we need to adjust is the carb. Let's further assume that your carb came from the factory with the jet, needle and idle screw set in such a way that the carb wouldn't run properly. You can change jets all day or ... you could change the needle clip all day ... or.... you could change the idle all day... but your carb is still never going to be right. No amount of changing one of those things is going to make up for the other two being drastically wrong. So my question is, when you have no experience with carbs ... how do you get one or two of those things (jet, needle clip, idle) close enough to good that you can adjust the remaining things?

If someone said "here fix this, the idle is wrong" that I could do. Same for the jet or the needle clip. But if you don't know what is right or wrong it becomes impossible to decide what to do first.

I'm assuming that If three things are drastically wrong, changing one thing at a time is not going to be much help. Am I wrong? If not where do you start?
 

Karl Snarl

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5 MIN NT CARB, BY JIMMY B. no caps, 'cause im lazy. lol

the wonderful nt carb, sleek, simple and effective, but a huge pain in the you know where. The slide, in the middle of your punisher beast of a carb, is a steel cylinder, with, a spring and washer on top, that hold down a needle with a jesus clip ( JESUS CH... WHERE THE BEEP DID IT GO!! WHO KNEW SOMETHING SO TINY COULD FLY 8 AND A HALF MILES TO A RAP BATTLE!!). the job of the needle and slide is to meter fuel and air. the slide moves up with the cable, bringing the needle. the needle is tapered. the needle is in effect from idle to 3/4 throttle, then the main jet takes over fully. the needle is important, it takes care of the slow side of the fuel delivery. the slide moves up and down with your meat grabbers, as you throttle, letting more air in, when you goose it, less when you slap it back home. when you WOT, you allow the maximum amount of air the engine can take in at a given rpm, the main jet determines all the fuel flow at WOT. back to the slide, the slide has a notch. with a screw nub from the outside, fitting in there. it is the idle screw. it simple moves the slide up or down a bit so the right amount of air comes in at idle, also moving the needle. it is attached to the slide, allowing the right amount of fuel. the needle also has some notches, so you can adjust how much fuel is metered on the low side, remember the main jet is in control at WOT, so it is metering the high side. So when you are rich/lean on idle to 3/4, but good on wot, you use the needle. when you are crabin on top, but can dial the lower, you play with a jet.

5 MIN NT CARBS BY JIMMY B. condensed as far as I could go. lol
 

Karl Snarl

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The speed carb with the square red cover is the same carb, different filter housing.
 

Karl Snarl

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You're forgetting the float, too high and it will always be rich and too low the fuel level in the jet won't be high enough to get drawn into the carb venturi.
I left it out on purpose. These new guys tend to dink the float arms up and then the float is even worse than when they started. Figured if they still can't dial in after that, someone would tell them check the float height.
 

Captain Rainmaker

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Get your tune at idle right, then move on to part throttle and full throttle. Knowing which setting affects what phase of operation helps a lot with this.

On an NT, it'd be something like: adjust screw for best idle, adjust needle for best part-throttle, then select the right jet for best full-throttle. Don't be afraid to go back at any point and re-adjust anything you did previously.
 

yuckfoo

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5 MIN NT CARB, BY JIMMY B. no caps, 'cause im lazy. lol

the wonderful nt carb, sleek, simple and effective, but a huge pain in the you know where. The slide, in the middle of your punisher beast of a carb, is a steel cylinder, with, a spring and washer on top, that hold down a needle with a jesus clip ( JESUS CH... WHERE THE BEEP DID IT GO!! WHO KNEW SOMETHING SO TINY COULD FLY 8 AND A HALF MILES TO A RAP BATTLE!!). the job of the needle and slide is to meter fuel and air. the slide moves up with the cable, bringing the needle. the needle is tapered. the needle is in effect from idle to 3/4 throttle, then the main jet takes over fully. the needle is important, it takes care of the slow side of the fuel delivery. the slide moves up and down with your meat grabbers, as you throttle, letting more air in, when you goose it, less when you slap it back home. when you WOT, you allow the maximum amount of air the engine can take in at a given rpm, the main jet determines all the fuel flow at WOT. back to the slide, the slide has a notch. with a screw nub from the outside, fitting in there. it is the idle screw. it simple moves the slide up or down a bit so the right amount of air comes in at idle, also moving the needle. it is attached to the slide, allowing the right amount of fuel. the needle also has some notches, so you can adjust how much fuel is metered on the low side, remember the main jet is in control at WOT, so it is metering the high side. So when you are rich/lean on idle to 3/4, but good on wot, you use the needle. when you are crabin on top, but can dial the lower, you play with a jet.

5 MIN NT CARBS BY JIMMY B. condensed as far as I could go. lol
I know all of that. The point is if two out of three things are fine, it's easy to adjust the third. If all three are wrong how are you supposed to know where to start?
 

yuckfoo

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Get your tune at idle right, then move on to part throttle and full throttle. Knowing which setting affects what phase of operation helps a lot with this.

On an NT, it'd be something like: adjust screw for best idle, adjust needle for best part-throttle, then select the right jet for best full-throttle. Don't be afraid to go back at any point and re-adjust anything you did previously.
Let me ask you a question: How uncommon is it to use a 72 jet or higher on these things? I'm at about 170' above sea level. My motor finally starts easily now that I put a 72 jet in. One person told me it sounded "fuel starved." A lot of guys are telling me I have an air leak but I haven't been able to find one yet (I've never tested the crank seals, will do that next.) I'm half tempted to try my next higher jet (75) and see what that does.

Thanks for the tip! Idle, needle, then jet. I'll follow that plan.
 

Captain Rainmaker

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Let me ask you a question: How uncommon is it to use a 72 jet or higher on these things? I'm at about 170' above sea level. My motor finally starts easily now that I put a 72 jet in. One person told me it sounded "fuel starved." A lot of guys are telling me I have an air leak but I haven't been able to find one yet (I've never tested the crank seals, will do that next.) I'm half tempted to try my next higher jet (75) and see what that does.

Thanks for the tip! Idle, needle, then jet. I'll follow that plan.
If it's what it needs, it's what it needs. Worst case scenario, if you try the 75 and it makes it pig rich, just go back to the 72.
 

Karl Snarl

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I know all of that. The point is if two out of three things are fine, it's easy to adjust the third. If all three are wrong how are you supposed to know where to start?
You start with whats in it. Then read the plug. The plug will tell you which way to go. Once you've been doing this a while, the seat of the pants dyno will tell you which way to go.
 
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