Pod Filter and Exhaust Re-jetting

I've noticed its kind of difficult to find information about rejetting for pods and pipes even if you do find bits and pieces. I found this article at motorcycles.wikia.com/wiki/Rejetting and thought I would post to help end the endless questioning about it.

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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

can a person drill 'em , if he uses some sense ?

Some do - but to do it at all properly requires a drill press and vise to hold the jet straight and secure - as well as accurate drills.

Some will even solder the jets solid and re-drill them when a smaller size is needed - but I'd have to be pretty desperate to do that...

Gary - K7GLD wrote: Some do - but to do it at all properly requires a drill press and vise to hold the jet straight and secure - as well as accurate drills.

Some will even solder the jets solid and re-drill them when a smaller size is needed - but I'd have to be pretty desperate to do that...

HEY...... :laugh:

The drill sizes to get 2.5 increments are non standard even if you could find them they would be a lot more expensive than just buying some jets.

Well I stumbled upon this jetting thread on a google search. For some reason, the site search always shows no matches, but a google search including the term "Kzrider" will pop with any relevant threads here or any other site too. For an old thread, the link still works to the original info. I'm pretty sure the info is also still good, since the bike and myself haven't gotten any younger since it was posted either.

At any rate, I noticed yesterday that my new SS exhaust is turning blue, and at the same time is scorching my clutch cable to the point that the jacket is melting and it is stuck internally.
I presently have 122.5 main jets and 37.5 pilots. Perhaps go up +4 on the mains per the article?

I would suspect the pilots are the lean ones. Plug chop?
Steve

SWest wrote: I would suspect the pilots are the lean ones. Plug chop?
Steve

I need to pick up some new plugs, so I can do the chop and be able to change them on the road. Do you really suspect the pilots, considering the small amount of time my bike spends in the 1/4 throttle use spectrum.?
I live ~1 mile from I-70 and most of the time my bikes spends anywhere near <1/4 throttle is at the 2 lights between my crib and the interstate.

Has anyone tried the shim under the needle as outlined in this video?


His article:
motorcyclemd.com/carburetor-jetting-for-new-exhuast/

The pilots has a effect all up and down the RPM range. How's WOT?
Steve

At the risk of sounding like a broken record, once you have the jetting in the ballpark, look into booking time on a dyno to get your air-fuel-ratios across the rev range. That $100-200 will get you zeroed in on exactly what you need to do to get optimal performance.

SWest wrote: The pilots has a effect all up and down the RPM range. How's WOT?
Steve

WFO is great, as always. Perhaps too good, as can be witnessed by the last ticket. :woohoo:

The shim under the needle will also fatten it up across the board a lot cheaper and easier. The guy in the motorcycle MD video swears by it.

With all due respect and thanks to moveover for the suggestion, "booking time" on a dyno while buying a box full of jets will not be an option. Thanks for your input.

Carb fuel curves (credit to motorcycle MD web site):

old_kaw wrote: . . . noticed yesterday that my new SS exhaust is turning blue, and at the same time is scorching my clutch cable to the point that the jacket is melting and it is stuck internally. . . .

I believe stock bikes include a clip to hold the clutch cable away from the exhaust header.




www.z1enterprises.com/catalogsearch/result/?q=92037-093

www.partzilla.com/product/kawasaki/92037...ff7bb94b83480d8fb9e9

Good Fortune!

A pic of your plugs would be interesting. I also have a touch of blue on my SS header but plugs do not show a lean condition.
Edit: maybe not quite as blue as yours