Question about jeting

Alright i have an 82 xj 750 max, Pods and a strigtht through exhaust, The question is, What are my stock jet sizes and what i am I going to need to put in? Any help will be very apprecated.

 

[opens Chacal's price list]

[cuts and pastes]

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ee) Aftermarket Hitachi brass MAIN FUEL JETS, sizes as follows, fits all XJ650 (except Turbo), all XJ750, and all XJ700 (exc. "X") models.

NOTE: stock Hitachi XJ series carbs used the following original sizes:

#106 (XJ 750RL Seca, 1984 only)
#107 (XJ700 non-X)
#110 (XJ650 models)
#120 (all XJ750 models except 1984 XJ750 Seca)

ff) Aftermarket Hitachi PILOT FUEL JET, sizes as follows, fits all XJ650 (except Turbo), all XJ750, and all XJ700 (exc. "X") models.

NOTE: stock Hitachi XJ series carbs used the following original sizes:

#36.5 all XJ700 non-X models.
#40 all XJ650 models, all XJ750 (USA) models, and XJ750RL (1984).
#43: some Canadian-Euro-UK XJ750 models.

All the other sizes are for "tuning" or re-jetting purposes when changing intakes or exhaust systems.


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 tube of your engine, your altitude, the mix of aftermarket parts on your bike, etc........alot of variables.

The best advice we can offer is: Just Say No. Don't do it! Leave eberything 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 that doesn't cut it with many riders, either, so 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!

We wish you luck with your endeavors!

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

Hi xyz!.....we've never used pods on any of our bikes, they just seem like way too much trouble to me! BUT, here is the "standard" re-jetting recommendations from people who fool around with them....although, please be aware that every situation is different, and you should expect to go through some amount of trial and error (and frustration!) before you get it as close to right as is possible.

By the way, the most important part of the information below is in the section titled "PRECAUTIONS"!:

So here is the info regarding jetting that you might find helpful:


MAIN FUEL JET SIZE CHANGES NEEDED PER TYPICAL MODIFICATION:


Exhaust Changes:

+2 main fuel jet size for custom 4-into-2 exhaust

+4 main fuel jet sizes for 4-into-1 exhaust

+4 main jet sizes for no muffler (open headers)


Intake Changes:

+2 main fuel jet sizes for single K&N filter (inside a stock airbox)

+2 main fuel jet size for drilling holes in the airbox

+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 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.

- 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:

Stock Carb Settings:
#120 Main Fuel Jet
#40 Pilot Fuel 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 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' asl.


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

RESULT:

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

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


For further insights and understanding, the Holy Grail for performance carb tuning information can be found here:

www.factorypro.com

 

Intake Changes:

+2 main fuel jet sizes for single K&N filter (inside a stock airbox)

I have a K&N filter in the stock airbox. Didn't realize I needed to re-jet just for that...

What say the rest of the experts?

 

When ever i look up the jets i need i always get chacal's list which is not a problem except it does not answer my question lol i am not sure how to read his list i just want a # of the jets that i need lol

 

Ratbike:

Here's the straight skinny!
Nobody knows what Jets you need.
There is no one who knows.

Converting to Pods deviates from the way the Carbs were designed to operate.
There is no accurate formula.
There is no place that has done any scientific testing.
I'd say: "Your guess is as good as mine", but I don't presume to guess.

The design is for the four Carbs to share the combined Cubic Feet Per Minute of Air that enters the Airbox through the Fresh Air Inlet.

It all boils down to the one inescapable decision you make without regard for design engineering: a) Look cool ... -or- ... b) Run good.

The bottom line is that you simply cannot have it both ways.
Oh, sure ... there's close.

But, close is No Cigar.

 

http://www.motorcyclecruiser.com/tech/m ... index.html

I found this.

 

The above website and article is a great warm-up act.

Here's the headline performer:

The Holy Grail (meaning: "the 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. As you will soon realize, it's quite a bit of work to properly tune a carb to its optimal performance----another reason why we don't suggest using pods!

 

For those of you who do check the FactoryPro web site thinking that it might be a solution to your woes; be sure to continue on to see what Model Bikes with what Model Carbs they are able to re-jet and fine tune.

For Yamaha Models:
No XJ-Bikes are included.
Only Mikuni Carbs are mentioned.

But, there is a wealth of information that you should read and understand before attempting to re-jet your Carbs.

 

Very true.......Mikuni only (basically) over at factory pro......but the theory is the same for all CV carbs (and Hitachi carbs are CV carbs).

And just remember, Mikuni carbs are quite a bit more "tune-able" than Hitachi carbs simply because many more of the "tuning" parts are available for Mikuni carbs...meaning different size air jets, main needles, main jets (emulasion tubes), etc. than for Hitachi carbs.

 

I wonder if no XJs were included because YICS introduces an unknown variable into the equation making a "backyard redesign" of the intake system even more difficult. If you start flowing substantially more air than designed for, how does YICS react? Can it no longer "keep up" with demand? There is no way to effectively increase the YICS airflow, yet another reason messing with the original design is to tread on new ground...

 

You absolutely can have it both ways. It takes a bit of experimenting, and most of all patience but it can be done. Take it from a "Pod Boy" as Rick has dubbed those of us that have made them work... *lights cigar*

 

didnt somebody post a chart awhile back to determine jetting? it was like you go up so many for pod filters, so many for open exhaust. Or am i just crazy?

 

Ya i am strickly looking for performance, I dont really care what it looks like as long as it makes power i have a goal by the end of the year to run in the quarter so lets make it happan lol

 

Ok guys, help me to understand air filters better.

I have a K&N filter but it's in the factory air box, no modifications to the airbox.

This is not the same as the pods you are speaking of, is that correct?

And if so will the filter i have create a need to rejet for best tuning?


Thanks Rick

 

No....pods are individual, very free-flowing air "filterettes" that attach directly to the carb throats, in the spot where the rubber airbox-to-carb boots would normally be located on a stock system.

A K&N air filter inside a stock, unmodified airbox may not require re-jetting, but on the other hand----it may. Proper plug inspections as well as noting what performance problems that may be caused by overly lean fuel-air mixtures are what you want to be on the lookout for.

Almost any modification that allows more air to flow through the engine will result int he need for additional fuel, too----to balance that increased air flow----and the way you get more fuel into the engine is many, but the most common is via "re-jetting" the main and pilot fuel jets.

 

Yes, it is repostetd in this thread.