Road Star Clinic - Bulldogs Carb Jetting Primer

Bulldogs Carb Jetting Primer - How to Dial it in

Written by David Tise (Bulldog)

Friday, 11 June 2004

David is a Technical Moderator, for Baron's Custom Accessories Technical Forum

I thought I would explain basic carb and jetting functions so everyone could better understand how it all works.

These carbs have two primary fuel circuits; the slow/pilot circuit and the main circuit, which have several parts or components that will effect the mixture at different throttle positions. There are 4 throttle positions that are used to determine what component is effecting the mixture. They are idle/off idle, 1/4 throttle, 1/2 throttle and 3/4 to full throttle.

It is important that you remember that the main fuel circuit is only part of the entire fuel delivery and any changes you make to the slow circuit will affect the main circuit as well, because the slow/pilot circuit(PMS) is delivering fuel from idle to full throttle. So you must start with the slow circuit and work your way up to the main circuit.

The Pilot/Slow Circuit (PMS)
The pilot/slow circuit has two major components that affect the mixture. They are the the pilot/slow jet and the air mixture screw(PMS). The pilot/slow jet has a fixed diameter hole that controls the amount of fuel being delivered through the slow circuit. The mixture screw is an adjustment screw to control the amount of air being mixed with the fuel from the slow jet.

The Main Circuit

This circuit has 4 components that effect the mixture from about 1/4 throttle to wide open/full throttle. These parts are the throttle slide cut-away, jet needle or needle, the needle jet and the main jet. The throttle cut-away effects the amount of air but can be left alone. The needle primarily controls the mixture from 1/4 throttle to 3/4 throttle. The needle moves up and down inside the needle jet as you open and close the throttle. The diameter of the needle is what affects the mixture at 1/4 throttle. The taper of the needle and the clip position effect the mixture at 1/2 throttle up to 3/4 throttle. At 3/4 to full throttle, the mixture is controlled mostly by the main jet.

Starting From The Bottom
The first throttle range to dial in is the idle/off idle range. Any time there is a weather change or you are riding in a different area you may need to adjust the mixture screw(PMS) for a clean and crisp engine response. The two main settings are the air mixture screw(PMS) and the pilot/slow jet. The correct setting is the one that allows a crisp engine response with the air screw about 2 turns out. If you need to turn the air screw out more than 4 turns, then it is too lean and you need a larger pilot jet.

1/4 Throttle
The mixture in this range is mostly controlled by the diameter of the needle. A lean mixture will cause the engine to hesitate and a rich mixture will cause it to sputter while keeping it at a steady speed.

1/2 Throttle
This range is controlled by the clip position and taper angle of the needle. The correct clip position is often all you need to dial in this range. The correct position is the one that will allow the engine to rev cleanly from 1/4 throttle to 3/4 throttle without hesitation or sputtering. Raising the needle clip will lean the mixture and lowering the clip will richen the mixture.
If you find that you are all the way to the top or all the way to the bottom positions and the jetting is still off, you need a different main jet

3/4 to Full Throttle
This range is controlled mostly by the main jet. A correct main jet will allow the engine to rev cleanly all the way until it drops off the power band with out sputtering or hesitation. A lean mixture will have power in the lower RPM's but will get sluggish and/or hesitate in the higher RPM's. A rich mixture will rev ok but sputter in the higher RPM ranges

I hope this information is helpful to everyone.
_________________
Bulldog
Baron's Tech Forum Staff

Ride Safe & God Bless

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DISCLAIMER: This information and procedure is provided as a courtesy and is for informational purposes only. Neither the publishers nor the authors accept any responsibility for the accuracy, applicability, or suitability of this procedure. You assume all risks associated with the use of this information. NEITHER THE PUBLISHERs NOR THE AUTHORs SHALL IN ANY EVENT BE LIABLE FOR ANY DIRECT, INDIRECT, PUNITIVE, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, OF ANY NATURE ARISING OUT OF OR IN ANY WAY CONNECTED WITH THE USE OR MISUSE OF THIS INFORMATION OR LACK OF INFORMATION. Any type of modification or service work on your motorcycle should always be performed by a professional mechanic. If performed incorrectly, this procedure may endanger the safety of you and others on your motorcycle and possibly invalidate your manufacturer’s warranty.

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Comments (3)

HOW TO REJET MY CARBS ?
Written by MATRIX00957, on 04-01-2008 09:28
DO ANYONE HAVE PIC OR IMAGES OF INSTALLATION PROCESS OF RE JETTING OR A LIST OF PROCESSES ON REJETTING . THX DAVID

Written by Scotty, on 01-12-2008 03:37
Dead on. I have repaired autos since I was a kid. But, I've never got so much understanding out of an article before. I know I'm not an authority on carb jetting now, but now I am able to understand why my ride exhibits a hesitation (or sputters) when riding and something else at an idle. Thanks

Mikuni hsr
Written by Dune, on 12-28-2005 22:15
Do you know anything about the Mikuni 42 mm? Im having issues dialing it in.

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