Aftermarket Exhaust Suggestions - No re-jetting Required

Hey all,

Does anyone have experience in aftermarket mufflers for an XJ 750 that will not require re-jetting? I'm currently set up on the stock 4-2. I'm looking to slap on two new mufflers.

I tried a spare aftermarket 4-1 exhaust system (headers and muffler), however I could tell the engine wasn't running smoothly. After a day of riding I pulled a sparkplug and it looked dry AF. Went back to the stock 4-2 but I cant stand the look and its too quiet!

 

MAC sells both a 4 into 2 exhaust as well as a 4 into 1 exhaust . Both systems have removable baffles . I'm running a MAC 4 into 2 system stock airbox with 124 jets ( stock is 120 main) I like it and has a good sound .

 

hd sportster mufflers are supposed to work

 

If you fart too close to the intake you need to rejet. These bikes carbs have very sensitive settings that need to be balanced

 

Based on these responses, there is no clear cut muffler that's just an easy swap without the need to re-jet.

Guess I'll have to wait until I'm ready to assess the fuel mixture when making the change.

 

In reality, if you only change the exhaust, re-jetting is not necessary.......the stock exhaust system flows about as well as any aftermarket system design.

Re-jetting is almost always required when you free up airflow on the intake side of the cylinder head........that is where the restriction is (the airbox and paper pleated air filter element). Anything you do to free up flow thru the intake system is what really leads to the requirement to re-jet (i.e. more fuel is needed since more air can be flowed thru the system).

One caveat though: most of these bikes (at least in the USA) were jetted pretty lean from the factory in order to meet EPA emission regs. You can always fiddle with the pilot mixture screw to richen up the pilot circuit some, but going up 1 size on both the main and pilot fuel jets isn't a bad idea even on purely stock bikes.

 

Hmm, interesting @chacal .

So in the event that I do re-jet the carb, for whatever reason (new intake, exhaust, or just the hell of it), do you have any links for a step by step backyard tune? Is it possible to diagnose your mixture and determine what - if any - adjustment is required.

For example, if I'm running too lean how would I determine the proper jet size?

 

Best method for tuning the pilot fuel circuit is via the use of a "see-thru" Colortune spark plug:

j5) Aftermarket Gunson COLORTUNE TUNING SPARK PLUG KIT....brand new, no display box, includes all accessories and instructions. If you really want to tune your multi-carbs to perfection, you'll have to use this unique "see-through" spark plug that allows you to view the combustion chamber flame color in real-time, thus allowing accurate (rather than guesswork) adjustments to your idle mixture screws. Use it once and you won't believe how you ever got by without it, and will pity those who don't have one of their own. Beware: you'll find that a lot of people now want to be your "friend" and have you come and tune on their bikes, once they find out you have a Colortune!

Also, please note! you can find quite a few of these plugs at a discount at various places, but most of these places are selling an older, now-discontinued "automotive" version of the Gunson colortune plug. While there are no fitment differences between the automotive and "motorcycle" versions of these plugs, just be aware that the automotive versions were designed to be used in a water-cooled engine, and thus have a significantly lower heat tolerance----and if these plugs overheat, the "clear" window either goes cloudy or just simply fractures.

NOTE: the proper use of the Colortune plug requires you to:

a) have your valve clearances within spec,
b) have the engine proper vacuum-synched,
c) have the YICS chamber blanking tool installed (if it is a YICS engine),
d) perform a re-synch of the engine after each cylinder is color-tuned. This colortune - synch - colortune - synch etc. iteration is necessary as any adjustments made to each cylinder (as determined by the results of the colortune operation) throws the engine out of synch.
e) the above procedure is the only way to truly and properly "tune" a multi-carbed engine!


The following video presentations, although definitely not Academy Award quality, show the Colortune Plug in action:

http://www.gadgetjq.com/Vulcan_1500_Idle_Mixture_Test.wmv

and

http://www.gadgetjq.com/Vulcan_1500_Mixture_Test.wmv


and here's a few more:

going from orange (rich) to blue:




showing the blue to orange to blue changes:




showing a colortune being hooked up incorrectly (arcing):





And here's a super-sized colortune plug, don't try this at home kids!:





Please remember, though, that a Colortune is NOT used to set your carbs at anything other than idle rpm's!!

HCP95C Aftermarket Gunson COLORTUNE TUNING SPARK PLUG KIT, for 14mm spark plugs. NOTE: NGK plugs that begin with the letter "B" are 14mm, and are used on all XJ650, XJ700 non-X, XJ750 non-X, XJ900RK, , RL, F, N, and FN models, XJ1100, and all XS1100 models.
$


Although you can tune the pilot circuit by "ear/sound", it takes a bit of experience to do it properly and consistently. The main difficulty is that it can become an almost "endless loop" process, because each cylinder that you adjust throws off the other 3 cylinders, and so the process has to be repeated again, and again, and again......

Of course, a proper running synch of the engine is just as important as the fuel mixture settings, so the best advice is to use a Colortune plug AND a 4-tube synch gauge simultaneously so that the "endless loop" of adjustments can be whittled back to just 1 or 2 iterations (and then it purrs like a sewing machine).

Main fuel circuit tuning is typically accomplished by observing spark plug insulator colors after doing numerous "plug chops".....in effect, the spark plug insulator colors give you the same type of information as the colortune plug does, but in a more-difficult, much less easier manner.

After that, there are also adjustments that can be made to main jet needle heights (shimming needles) and/or adjusting the fuel levels in the bowl (as the fuel levels have an effect on rich/lean conditions), but now you're getting into the realm of race-tuning, which you really need to just go find someone with a cycle dyno and plan to spend a bit of time trying out different configurations. And all of this is complete overkill for street-driven bike (unless you're the "can't sleep at night" kind of person who requires something akin to perfection in certain endeavors in your life).

That's why most people choose the colortune/synchronization route and just follow these basic guidelines:

WHAT ABOUT RE-JETTING FOR PODS, ETC?:

Before we even start down this bumpy road, let's get this out of the way: changing the mufflers (only) will generally not require re-jetting, as the stock mufflers are pretty darn free-flowing as it is. The major airflow restrictions (by design) in these engines is on the INTAKE side (the airbox system and its pleated paper filter). Changing to a high-flow air filter (like a K&N) will definitely require increased fuel jet sizing.

That being said, all of these bikes were jetted a bit lean from the factory, in order to satisfy EPA emissions requirements of that era. So even a stock engine can benefit from slightly larger fuel jets; for example, a stock XJ650 non-Turbo USA model, increasing fuel jet sizes to #112 on the mains (from the stock #110 size) and to a #41 on the pilot (stock is a #40 size) can be beneficial. Note that the pilot jet size is not as critical as the main jet size, since the pilot circuit mixture screw allows you to “open up” the pilot circuit independent of the jet size.


But what about when you do make other intake/exhaust system changes? 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!

Definitely not plug-and-play:

https://www.youtube.com/watch?v=Vt6HtsggcqU

And some pod filters make a difficult situation even worse.........read all about it:

http://www.caferacer.net/forum/technical/19639-pod-filter-thread-geeto.html

Yoji himself cautions against it:

http://xjbikes.com/forums/index.php?attachments/mikuni-article-jpg.15101


Also, if you’ve ever wondered what the numbers on your jets refer to:

- on Hitachi fuel jets, the numbers refer to the actual physical size of the passage hole expressed in millimeters, i.e. a #114 fuel jets is 1.14mm in diameter, a #42 fuel jet is 0.42mm in diameter, etc.

- on Mikuni fuel jets, the numbers are a measurement of the volume (cc’s of gasoline) that will flow thru the jet when poured in from a height of 50cm.

- we do not know what air jet numbers refer to…….although we suspect it is a volume of air that can pass through the jet at some level of vacuum. Anyone who has insight into this issue please let us know more!


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!"

 

Excellent write up as a starting point for selecting jets - but a couple of points I'd make:-
The use of a colourtune will only work if the engine is in good condition, valve clearances etc correct. Most of all, it must not be burning oil. Furthermore, it can only work when you can see it - so that means idle and early progression. Pretty much useless on the road type range then. Wideband lambda is the only answer sadly.

Secondly, the rules of thumb are guestimates only - but you have to start somewhere right?

Oh, and there is a third point not often mentioned - if you go for podfilters one thing that really changes is the pressure the carb sees from the outside. This increase in pressure raises the piston relative to where it was previously, the effect is likely to be a weakening of the mixture - at mid to high range anyway. Fitting a larger jet only fixes the top end, so the weak mid range remains. The answer is to fit stiffer springs - but how stiff? You're on your own, but each coil on the spring is one part of the total number. So for a 20 coil spring, cutting off 2 is a tenth stiffer, or thereabouts. The trouble is cutting coils off actually shortens the spring, reducing the preload - you might consider a spacer, or stretching the spring to original length - again good luck.

 

how about a weight on top the slide

 

I've seen a lot of varying opinions on this, but trust @chacal.

I have a XJ650 RJ where a PO replaced the original exhaust pipes with 4 into 1 and an Kerker muffler. The pipes are in rough shape and will need replacing. If I keep the intake stock (air box, OEM filter, original carb jets) and replace with a new 4 into 1 and a new muffler, there is no need to re-jet?

 

I’ve heard the same about Harley mufflers
And I have a set here. Maybe I’ll try them at some point

 

They work well and sound good

 

So where can I get an aftermarket exhaust? Cant find anything for my 550 on google, the stock exhaust is ugly, I only want one muffler!

 

Look at MAC exhaust products.

 

ebay has them, nobody said they were cheap

 

be careful with the Harley mufflers. I used a set on mine and i had to get custom spacers and extensions to clear the rear brake.

What i used was a 2010 Harley fxs 1800cc slash cut with heat shield. I got them for $40 so i couldn't pass them up. They are super long, so i cut a chunk off and shortened the baffle and removed the packing. I sealed them back up with a small tac weld. If i didn't add the spacer about 6" if i remember correctly, the rear brake would hit the muffler and wouldn't work.

IF you aren't prepared at the possibility of doing all that, then buy whats made for the bike.

 

I didn’t have any problems really straight bolt on once i made a bracket. Disregard picture without pipes wrong one

 

you used a different muffler than i did. if it was just a bracket job, do you mind telling the OP what muffler you used so he knows.

 

look for an FZ600 exhaust that was an exhuast i had on one of my 550'S

https://www.yamahapartsnation.com/oemparts/a/yam/5004295bf8700209bc78910d/exhaust