# Carb talk

Main Jet, Pilot jet, Fuel screw, Needle

Someone please educate me. What exactly do each of these things do? I saw a thread on the EKG needles which seemed like a good idea. If the idea is to get more fuel in the motor couldn't one simply leave the needle alone and go richer on the main jet? What about the fuel screw? Couldn't one leave all the jets alone and simply turn in out more to allow more fuel?

Main Jet: Top end

Pilot Jet: Bottom end

Needle: Midrange

Air Screw: Bottom end also????

I think this is how it goes, but I'm not sure. Can someone correct me if I'm wrong.

Welcome to the mystries of the "carb". A good way to get educated on the subject is buy a generic motorcycle book like "Chilton's Motorcycle Owners Handbook". Very good explanation of fuel systems and so forth. And older version is better. But the gist of it is as follows: (Not a complete explanation)

Think of it as 3 diff. and distinct circuits.

1.Idle circuit (0-1/8 throttle) which is idle and slightly above. This is controlled by your pilot, (slow jet) and fuel screw. So, if you have to go 3 or more turns out on the fuel screw, you probably need to increase your pilot size. How do you know if your rich or lean? One way is to adjust the fuel screw to find the highest RPM at idle, too many turns out, RPM goes down, too many turns in, RPM goes down. If your three or more turns out, get a larger size pilot. There are other symptoms like delayed high idle and so forth.

2.Mid--As you twist the throttle you start to raise the needle. The needle is tapered and is raised out of the needle jet hole as the throttle is opened. Eventually, it is all the way out and then the main jet takes over, which is the next circuit. Various taper needles exist in order to control how much fuel is deliverd when you are working the throttle between the 1/8 (1/4?) to 3/4 positions. Suzuki choose to go with a lean taper needle for the E. Which tames out the mid response. The needle clip position determines whether you are running rich or lean, (assuming the taper is correct). How do you know if your rich or lean? One way is to put in a fresh plug and run the bike at half throttle for a bit, cut the engine and pull the plug. Too black, it's rich, too grey it's lean. You can also get some backfire either through acceleration or deleceration. I believe that decel back fire indicates a lean condition, but I have to check on that.

3.Fast or full throttle (3/4 to full throttle-- Needle is completely up and out of the needle jet hole (at wide open). Main jet takes over. Again you can have lean or rich or just right conditions.

There is overlap between each of the circuits but it helps to diagnose problems by thinking of them as seperate.

[ December 31, 2001: Message edited by: Jim65 ]

GOOD JOB JIM65:

Might add that the main jet can be tested by running the bike at mid RPM. Find a nice straight preferably a little up hill and do some third or 4th gear roll ons to W O T. When at WOT and pulling nicely under load chop the throttle back to 7/8 opening. It helps to mark the grip at the 7/8 opening. When you chop it, if it seems to surge ahead a bit the main jet can be increased. If it burbles or stutters the main jet is too large.This works on all size bikes including Harleys with cv carbs.

Make sure it is fully warmed up including the oil. This is one of those freebies that can be done anytime you think of it during your next ride when you happen to find just the right place.

Make sure the air filter is in good condition.

A tip for finding what part of the jetting circuit you are on: Put a piece of white first-aid tape on the throttle housing to go 180 degress over the top front to back. Make a mark or cut a little notch in the grip and mark the tape at the closed position, Twist throttle open all the way ( motor NOT running) and mark full throttle on the tape. Now mark quarter,half-throttle and 3/4 throttle on the tape. Go ride and when the bike acts funny look down and see where in the throttle range the problem is. Is it at 1/4, 1/8, 3/4 etc.?

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This post has been promoted to a wiki

• Since I've been getting a few requests a week to e-mail the re-jet instructions word document (offered to send it to someone in old post that lives on) I thought I'd throw this post together. It is perhaps the one stop shop for 3x3 mod info: the master 3x3 post
The 3x3 Mod
So what is the 3x3 mod anyway? Well, this mod is basically re-jetting your carb and allowing it to run to its full potential. This mod can be done by even the most newbie of wrenchers out there. 3x3 refers to the size of hole that you will need to cut into the top of your airbox where your snorkel currently is. Don’t worry, you are not carelessly risking getting water into your engine by removing the snorkel. The bread and butter of the mod is the taper of new needle which you will get with your kit.
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Scroll down to the bottom to find the right kit for your DRZ
If you purchased a S kit for SM by accident don’t worry it will work (different jetting specs are used, just ask!)
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bayoubikebruiser’s post is a nice one (Good Job BBB)
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Sample Jetting recommendations (note: these are different for different elevations and different bikes)
SM
- 150 main jet (155mj for full exhaust system)
- 25 pilot jet
- 2nd clip on needle from top
- 2.5 turns out on fuel screw
S
- 140 – 142.mj (bigger for exhaust)
- 25 pilot jet
- 4th position from top
- 2.5 to 3 turns out on fuel screw
Burned’s fuel screw setting method:
with the bike warm and idleing turn the fuel screw in till the idle drops/misses.then go back out till the idle peaks/smooths.
this should happen between 1 and 3 turns out.
if you end up at less than 1 turn you need a smaller pilot jet.more than 2.5 (or 3 turns on a cv) you need a bigger pilot jet.
choose the appropriate size and retest.
The 25 Pilot jet part number for TT is:
N224.103-25
So… now you have the info that you need, get to it! Your bike will sound better and rip a little faster; get ready to grin!