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 ]


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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