air fuel screw.

[99_300ex]

II know not the right section but I'm hoping for a quick response so on an air fuel screw on my 300ex carb if you turn it in does that richen or lean the mixture thsnkd.

[300ex_#387]

Turn it in to richen the mixture and out to lean out the mixture. If you get past 2.5 turns out you need to try the next size smaller pilot. If you have to screw it in all the way try the next size bigger pilot.

[CJM]

In is lean, out is rich...

[Honda4life05]

This just goes to show how much incorrect info is on this forum...

[Exrider434]

Originally posted by 300ex_#387
Turn it in to richen the mixture and out to lean out the mixture. If you get past 2.5 turns out you need to try the next size smaller pilot. If you have to screw it in all the way try the next size bigger pilot.


Wow wow wow. Think about it man, if your screwing the screw in making a SMALLER passage for fuel, your leaning it out. If you screw it out it will make a BIGGER passage, allowing more fuel through, and richening it up.

[300ex_#387]

Originally posted by Exrider434
Wow wow wow. Think about it man, if your screwing the screw in making a SMALLER passage for fuel, your leaning it out. If you screw it out it will make a BIGGER passage, allowing more fuel through, and richening it up.

I am wrong about that carburator. Do the complete opposite of what I said. I had just got done working on my cr125 and they have an air screw instead of a fuel screw. Works in the opposite effect. Here is a good thread from here describing how to adjust it.

http://www.exriders.com/archive/topic/229923-1.html

[chucked]

If the screw is between the slide and the head its a fuel screw, so out is rich. If its between the air filter and the slide its an air screw, out is lean.

[NacsMXer]

Originally posted by chucked
If the screw is between the slide and the head its a fuel screw, so out is rich. If its between the air filter and the slide its an air screw, out is lean.

Yep, that would be right. I think a lot of confusion starts when people refer to the screw as an A/F screw.

It's not a darn A/F screw lol. It's either a fuel screw, or an air screw. They both adjust the air/fuel ratio of your pilot circuit, but do so in opposite ways.

[jessieherrera]

Hello,
The following is how to adjust and tune your carb. I sent you the complete instructions, but you will find the Idle/air screw adjustement procedure #1.
Carb jetting:
First, always run the carb dry when you are finished riding by shutting off the gas at the petcock and let it idle until it stops running. This is the best practice for any small or industrial engine. You may have a Mikuni. Even if you don't many carbs are similar in design and parts. Mikuni carbs are very easy to work with. Make sure that you have no vacuum leaks whatsoever.
Usually, the bigger the carb, the smaller the jets.
With today's gas it is almost impossible to read a spark plug. It used to be that if they were a soft white to a medium tan the mixture was correct. Today, you may want to try this to get the right mixture.
Adjusting your carb/mixture/jetting:
1. The idle air screw always starts at 1 1/2 turns from closed. Let the engine warm up and open or close an 1/8 of a turn at a time. Give it about 5 seconds to settle. Continue until the engine reaches its highest RPM/speed. This is called your best lean idle.
2. The cutaway on the slide regulates your mixture just off idle. The different cutaways enrich or lean the mixture.
3. The slow speed jet regulates the mixture until the needle in the slide takes over. This is initially about 850-1000RPM. Increase the jet size one at a time where you get the highest RPM at that slide or throttle position, not RPM as this will change with jet size. You should feel more power at that throttle position. Your needle on your slide should not be active or at the taper.
4. The needle in the slide regulates the mixture to wide open throttle. The needle has a taper to it which regulates the fuel amount from the main jet. It also has a few notches that will raise or lower the needle. Raising position enriches. You will feel more power at that throttle position.
5. The main jet regulates the mixture at "wide open" throttle. Read below to adjust.
I have found that the best way to jet your engine is by what's called "seat of the pants performance".
Adjust the jet size by enriching the jet until you get the highest RPM/speed at that specific RPM/throttle position. An example would be the slow speed jet at let’s say, 850-1000 RPM though this will change with the richer jet so actually it's better to go by slide position.
The high speed jet you do by accelerating through the gears until you reach top end. Keep increasing the size one size at a time until you get the maximum speed and feel of power. Once that is reached go down one jet size.
With different pipes and types of driving as well as types of intakes jetting will change. Altitude is also a factor. An example of intakes would be velocity stacks or air cleaner inside configurations. Different lengths of these passages affect you RPM and acceleration. High performance demand will always require richer mixtures as moderate driving will perform better at leaner mixtures.
Good luck:)