carb jetting

Well, i replaced pilots in my xj650 with #41's and the mains with #126's.I put one washer under the main jet needles and had no noticable difference in performance.Maybe i did something wrong?

 

#126 main fuel jets are Wa-a-a-y-yyyy too large for a 650 engine, no matter what kind of modifications you've made to the intake system (pods, etc.) or exhaust (4-to-1, etc.).

 

Ok so give us a little clue as to what size we should put, bc i did the same to my 650 maxim...

 

Guidelines To Live By:


WHAT ABOUT RE-JETTING FOR PODS, ETC?:

It's a question we get asked often and unfortunately, one that we cannot answer honestly about your specific bike besides with "it depends".

Which is a nice way of saying "you're about to enter the seventh circle of hell......."!

Carb jet tuning required by aftermarket modifications is somewhat of a black art, part science, part skill, part luck. It depends on the current state of tune of your engine, your altitude, the mix of aftermarket parts on your bike, etc........a lot of variables.

The best advice we can offer is: Just Say No. Don't do it! Leave everything stock!

But, since most people---with good reason, I might add---don't always listen to our well-intentioned advice, then the next best recommendation we can offer is: "if you want more power get a bigger bike!".

And since that doesn't cut it with many owners, either, for the remaining stalwarts out there who insist on "experimenting" with aftermarket intake and exhaust systems, here's the best information that we've come across to give you some GUIDANCE, which you should take as just that, and not as ANSWERS, because it isn't!



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.

- 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 XJ750RJ 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.

XJ750 Seca Stock Hitachi HSC32 Carb Jetting:

#120 Main Fuel Jet
#40 Pilot Fuel Jet
#50 Main Air Jet
#225 Pilot Air Jet
Y-13 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: #120
+ 6 additional sizes
= a #126 main fuel jet size
---------------------------
Subtract: -2 main fuel jet sizes for Altitude of 2500' Average

= #126 calculated from above
-2 jet sizes for altitude adjustment

= a #124 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: #40
+ 2 additional jet sizes (since we went up +6 main fuel jet sizes before the altitude compensation was factored in):

= a #42 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 #124 Main and #42 Pilot is A GOOD STARTING POINT.

******************************************************

Note that the above calculations do NOT take into account any possible changes in the sizes of the air jets nor the main needle size, which are additional variables and opportunities for tuning excellence (or frustration!).

For further insights and understanding, the Holy Grail (meaning: the whole miserable, un-varnished truth of what a real chore carb tuning is going to be, written by people who actually know what they're talking about, rather than by people who are trying to sell you something) can be found at:

www.factorypro.com

and then click on the "Product Support/Technical Support" link at the top of the page, then on the "Motorcycle Tuning Tech" link, and then the "CV Carb Tuning" link........and then read, weep, study, and do....if you still dare to! As you will soon realize, it's takes quite a bit of effort to properly tune a carb to its optimal performance----another reason why we don't suggest using pods!

 

What exactly do you mean by mis-match condition?

I have stock carbs, stock 4 into 1 exhaust (although I don't know if muffler is original, looks it). I want to rejet at sea level with pod style filters.

What is a mis match condition?

 

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.

None of the XJ-series bikes came with a stock 4-into-1 exhaust, so it has been switched. If you have the stock intake system (airbox and filter), then you currently have a "mis-match" condition!

 

None of the XJ's came with a stock 4 into 1? So my exhaust..other than my megaphone cut out isnt factory? its a 4 into 1. (bike is a 81 650 Midnight Maxim) Im going to keep my straight pipe exhaust set up and im putting pods on. I try to follow that guideline up there but im so darn confused it hurts lol I had a few guys tell me that I dont need to buy anything new I can just make the factory jets larger...This sounds a little to redneck for me. Can anyone do the math for me and just tell me what I need to purchase, itd be HUGELY appreciated. I know theres no PERFECT answer but a pretty darn close one would be nice.

 

Xj's are 4-into-2. Different shapes, angles, and cuts, but were all 4/2.

Here's the math: sell the 4/1, take the money and put it toward a nice original 4/2 exhaust. You can put original size jets in, and after checking/adjusting shim clearances, and carb cleaning/syncing/colortuning It'll look better and run better

 

Using pods and an aftermarket exhaust, initial recommendation would be for #114 mains and #41 (or #42) pilots, and tweak from there. Remember after new jets are installed you must re-synch the engine in order to evaluate the changes properly.

 

Or, if you're bent on the 4/1, listen closely to Len!

 

Lol well idk what I'm gonna do. Exhaust studs busted off. Ez out in front of the stud. Looking for a new motor.

 

Why not just a replacement cyl's, or take it to a machine shop to extract them

 

I'm looking at getting new top end. Found a complete YICS motor for 300. But it's looking like top end is half that. I need to stay as budget friendly as possible lol

 

Ok

 

Sorry for any redundancy but if I was to have a 4 into 1 exhaust w/ (4) pod filters. That would be a aftermarket match condition? Or is it only a match condition when intake and exhaust setup is stock? or any aftermarket item in equation causes a "mismatch condition"?

 

It's an even worse, more mis-matched condition