What size Jets?

I have a 1980 yamaha maxim 650 and it just has the Headers, and Air filter Pods. What size jets have you guys ran or do you guys suggest?

Thanks

 

Chacals handywork.



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 ANY 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 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 main or pilot air jets, the main needle or main needle jet size, and thus are additional variables and opportunities for tuning excellence. These are areas which are largely unexplored by most tuners, but logically should allow for additional fine tuning or additional rage and frustration.........


Normally, changing to an aftermarket exhaust does NOT require re-jetting, (or minimal re-jetting) as almost ALL of the airflow restriction in the airflow path thru the engine (meaning: ATMOSPHERE > AIRBOX > FILTER > AIRBOX BOOTS > CARBS > INTAKE MANIFOLDS > CYLINDER HEAD PASSAGES > EXHAUST SYSTEM > BACK OUT INTO THE ATMOSPHERE) is within the intake side of this air flow path, and primarily within the stock airbox/air filter. Your stock EXHAUST system can already flow more air volume than the stock INTAKE system allows.

Thus changing only the stock EXHAUST system, with no changes to the intake side of the heads, normally makes NO DIFFERENCE IN TOTAL AIRFLOW, and "no difference in total airflow" means "no difference in TOTAL FUEL FLOW" either, and thus bigger jets are not needed.


But once you start freeing up the INTAKE side of the entire system, you will produce more system airflow, even with a stock exhaust system (because, the stock exhaust system has the capability to flow more air through it than what the stock intake system allows).

This is why re-jetting is usually needed even if you keep the stock airbox and the stock exhaust, but use a K&N low-restriction filter, or even if you drill holes in the stock airbox, or leave the filter lid off.

All such actions free up the intake side airflow restrictions; the stock exhaust will move this additional airflow, and without providing addition FUEL flow to match the increased airflow (within limits, an engine will gobble up the maximum amount of airflow that it can; an internal combustion engine is actually just a self-powered AIR PUMP) then the engine will run "lean"---meaning not enough fuel to match the amount of airflow that the engine can (and now will) gulp.

Most pod type filters allow for vastly increased airflow, and thus require fuel re-jetting, and although no one really talks much about it, probably also require AIR JET changes to match the additional fuel flow, but since no one likes to deal with two parameters at once, it becomes a "tuning nightmare".


BUT, when you read all of the common symptoms of people who use pods, you quickly come to the conclusion that it's not possible to reproduce the stock "smooth in all rpm ranges" engine response. The reality is that you SHOULD be able to match it pretty darn closely, even with the increased airflow through the system, but ONLY changing the fuel jets isn't going to accomplish that. There are also air jets in the system, and they are there for a reason, as well as needle tapers and vacuum piston responsiveness issues.


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! HINT: if reading through it makes you think to yourself "sheesh, this sounds like an incredible amount of effort!", well, you're right! That's just some of the joys (and pitfalls) of getting to play "tuning engineer", which is what you're going to be doing. Yamaha probably has 10 of those types of guys on staff, and millions of dollars of test equipment, both physical and computer-aided, that allowed them to get the mixture settings just right---from an overall drivability AND power output standpoint----and now, since you're changing the airflow parameters thru the engine, you'll have to figure it all out "from scratch", but WITHOUT the benefit of 10 trained engineers and all that test equipment and experience.

That's why we warn you that setting up a bike for pods can be quite a bit of trial-and-error procedure. You can make the calculations according to what is shown in that guideline and then order the jets that the "formula" recommends, and that should serve as a good STARTING POINT............you may (or may not!) have to do more tuning and trial-and-erroring substitution of different jet sizes, etc. to get it performing to you satisfaction, with the recognition that you may ALWAYS end up with a situation that has some kinds of trade-offs.....lazy at the lower end but runs well at mid/upper-ranges, or runs well at the lower end but a "flat-spot" at some other rpm range, etc. Unfortunately there is no magic formula........you might want to read through the factorypro.com article that I list at the end of that section, and you will get a better understanding of what is involved to get the carbs set-up properly in a non-stock configuration.

As one of our favorite experts says about pod filters: "Get a Rubik's Cube instead.....it's less trouble and actually has an eventual solution!"

 

umm...anyone have stock air box for sale? lol

 

Eh,

This place is a little bias against them. There are quite a few people here, as well as on youtube, who seem to run them absolutely fine. Ive heard of people running them without any modifications at all.

Have you taken a look at the current pod discussion? There are people that are trying some ideas to make our tuning woes less. Its really just a matter of rolling the dice, and: If it isnt broke, dont fix it.

Is your bike not running right? If its running fine, then I wouldnt worry about the jets. You can turn your mixture screws up to adjust for lean conditions. If you end up maxing out your mixture screw, then I would jump a few sizes.


-Kaya

 

I bought a dyno jet kit to re jet. It comes with instructions, and trouble shooting as well as an adjustable needle. You can do it! It is not that scary at all. The main jets are the important area where you need to jump up and probly shim the needle. Increasing the mixture screws will only help idle and off idle. I got mine good with factory pilots and pods with a colortune. I believe the main jets are 136, dyno jet number.

 

I took apart my carbs and looks like my jets are 110. I will be up getting larger gets and from reading what you all have posted you said +4 for pods and +4 for open headers so I suppose I will start off by getting 118 jet sizes?

Thoughts?

 

Jet sizes are in increments of 2.5, 8 sizes from 110 would be 130. Expect to change jets a few times to get it dialed in. Check the plugs for color and go up or down in jet size till its right. Running lean will overheat a motor and rich wastes fuel and fouls plugs quick.

 

Are you sure?

It looks to me that he has the right idea shooting for 118.



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.



Right?


-Kaya

 

Yep I'm sure. Try to find 118 jets you'll only fine 117.5 for Mikunis. When they say go one size larger jet they mean got from a 110 to a 112.5 then 115 then 117.5 then 120 and so on.
Here is a list of available jet sizes
http://www.pjmotorsports.com/mikuni-jets.html#main jets
Hitachi jets are available in steps of 2 so a 118 is available for them.

 

Actually, I don't think you are right. I asked chacal about it and got the following answer:

Thought I should mention it so that people don't put it way too large jets.