Forum airhuile

MAIN FUEL JET SIZE CHANGES NEEDED PER TYPICAL MODIFICATION:

Typical Exhaust Changes:

+2 main fuel jet size for custom 4-into-2 exhaust
or
+4 main fuel jet sizes for 4-into-1 exhaust
or
+4 main jet sizes for no muffler (open headers)


Typical Intake Changes:
+2 main fuel jet sizes for single K&N filter (inside a stock airbox)
or
+2 main fuel jet size for drilling holes in the airbox with stock filter
or
+4 main fuel jet sizes for individual pod filters (no airbox)

Additional changes:
- Add up all the main fuel jet size increases and subtract 2 sizes.
- Decrease main fuel jet size by 2 sizes per every 2000' above sea level.
- Under a mis-match condition, such as when using pod filters with a 100% stock exhaust, or 4-into-1 header with stock filter and air box, then subtract 2 main fuel jet sizes.

PILOT FUEL JET SIZES CHANGES NEEDED PER TYPICAL MODIFICATION:

Pilot fuel jet size changes are related only to the change in main fuel jet sizes according to the main fuel jet size formula described above. Note that this pilot fuel jet rule is for the main fuel jet size change BEFORE any main fuel jet altitude compensation is factored in:

Increase the pilot fuel jet size +1 for every +3 main fuel jet size increases.

Additional changes:

- Decrease pilot fuel jet size by 1 for every 6000' above sea level.

PRECAUTIONS:

- Make sure your carbs are in perfect working order before making jet changes....meaning fully cleaned internally and rebuilt, operating properly in their stock configuration, proper sized air jets and needles, etc. Otherwise, you'll like find that all of your efforts are going to be a HUGE waste of time.

- Check plug color often and adjust as needed, 2 main fuel jet sizes at a time and 1 pilot fuel jet size at a time. Bright white plug insulators are a sign of an overly lean fuel mixture condition and WILL cause damage to your engine over time, up to and including engine seizure!

- Synch the carbs after each jet change.

- Make sure the floats are set correctly

- Seriously consider purchasing a Colortune Plug Tuning kit.

- You may find it necessary to make changes to the size or shimming of the main jet needle. There are no guidelines on what or how to do these changes, this is true trial-and-error tuning!


EXAMPLE:

A 1982 XJ550RJ Seca using an aftermarket Supertrapp 4-into-1 exhaust and a single K&N air filter in the stock, unmodified airbox. Bike is primarily operated at an altitude of 2600 feet above sea level.

XJ550 Seca Stock Mikuni BS28-series Carb Jetting:

#112.5 Main Fuel Jet
#35 Pilot Fuel Jet
#70 Main Air Jet
#170 Pilot Air Jet
4GZ11 Needle


MAIN FUEL JET SIZE CALCULATIONS:

Changes made:

Exhaust:
4 into 1 with Supertrapp = +4 Sizes Main Fuel Jet

Intake:
K&N Pod Filters = +4 sizes Main Fuel Jet
----------------------------
Equals: +8 main fuel jet sizes above baseline
Subtract: -2 main fuel jet size per formula above
----------------------------
Equals: +6 main fuel jet sizes due to modifications, thus:

Stock main fuel jet size is: #112.5
+ 6 additional sizes
= a #118.5 main fuel jet size
---------------------------
Subtract: -2 main fuel jet sizes for Altitude of 2500' Average

= #118.5 calculated from above
-2 jet sizes for altitude adjustment

= a #116.5 main fuel jet size.


PILOT FUEL JET SIZE CALCULATIONS:

The formula is: +1 pilot jet size increase for every +3 main jet sizes increased.

Stock pilot fuel jet size is: #35
+ 2 additional jet sizes (since we went up +6 main fuel jet sizes before the altitude compensation was factored in):

= a #37 pilot fuel jet size.

Note that no altitude compensation is needed on the pilot fuel jet since our elevation is less than 6000' a-s-l.


------------------------------

RESULT:

A #116.5 Main Fuel Jet and #37 Pilot Fuel Jet is A GOOD STARTING POINT.



Sorry forgot to mention your carbs if you dont already have the specs...



Katana (GSX600F)

1988-89 (US Models)

Type: BST31SS

Main Jet
Cylinders #1 and #4: 137.5
Cylinders #2 and #3: 135

Main Air Jet: 1.0mm

Jet Needle
California: 4CZ-5-1
Expect California: 4CZ-4-1

Needle Jet: P-8

Pilot Jet
California: 37.5
Except California: 32.5

Pilot Air Jet
California: 155
Except California: 150

Pilot Screw setting: Preset

Starter Jet (choke): 45

Float Height: 14.6mm (+/- 1mm)


1988-89 (UK Models)

Type: BST31SS

Main Jet
Cylinders #1 and #4: 137.5
Cylinders #2 and #3: 135

Main Air Jet: 1.0mm

Jet Needle: 4CZ-3-3

Needle Jet: P-9

Pilot Jet: 40

Pilot Air Jet: 160

Pilot Screw setting
1988: 1-7/8 turns out
1989: Preset

Starter Jet (choke): 45

Float Height: 14.6mm (+/- 1mm)

Gicleur de ralenti fonctionnant sur toute la plage ???

au final les réglages carbu ne sont pas du tout identique

Certes le gicleur de ralenti serai bon a augmenter un peu vu qu'effectivement l'apport d'air est plus grand, qu'importe l'ouverture des gaz, m'enfin c'est pas une priorité,

le circuit de ralenti débite de 0 à l'ouverture totale du carbu, il débite en permanence un débit d'essence qui joue surtout entre 0 et 1/4 de gaz.

Diaboloz a écrit : - Pour les GP je suis parti sur 150 (base 127.5)mais c'est trop juste dès que j'ouvre en grand ça coupe, je pense repartir sur 160/170