xj550 Maxim H Model Main Jets & Re-Tuning

TWO QUESTIONS:

What is the stock Main Jet size for the xj550 H Maxim engine?
Have to calculate properly what jet sizes are needed next - counting from the stock size first.

Mine is a 1981 xj550 Maxim running 4 Pods (no airbox) with stock exhaust system, operating at 2,584 ft. elevation.

PO had installed # 107.5 Mains.
Bike starts right up and accelerates well through each gear but runs into a flat spot at 5200 rpm under WOT / load.
New spark plugs are very Light Gray (not yet white) in color; checked at 110 miles after install. Idles okay at 1000 rpm with a very slight sputter. So its time to jet up the Mains a couple of sizes and probably go up 2-3 sizes on the Pilot Jets as well.

Any size recommendations for the Main Jets? What size Idle Jets would be a good place to start?

 

107.5's are awfully small for that bike, although be aware that some aftermarket jet manufacturers use a different "size coding" method than Mikuni did.............


MIKUNI FUEL AND AIR JETS:

MIKUNI MAIN FUEL JETS:

ee) OEM and aftermarket Mikuni brass MAIN FUEL JETS, sizes as follows, fits all XJ550, XJ650 Turbo, all XJ700-X and XJ750-X, XJ900RK, and XJ1100 models.

NOTE: stock Mikuni XJ series carbs used the following original main fuel jet sizes:

#100 1985 all XJ700-X models
#102.5 XJ900RK US & Canada
XJ550L (1984) on cylinders #3 and #4
#105 1986 all XJ700-X models and all XJ750-X
XJ550L (1984) on cylinders #1 and #2
#107.5 XJ900 1983-84 except USA or Canada
#112.5 XJ550 all models and XJ1100
#127.5 XJ650 Turbo carbs for cylinders #1 and #4**
#132.5 XJ650 Turbo carbs for cylinders #2 and #3**


MIKUNI MAIN AIR JETS:

Alas, the Mikuni BSxx series carbs used on XJ models---unlike their cousins the Hitachi HSCxx series carbs---did not have replaceable main jets. The main air jets on these models are in the carb throat, facing the airbox, and are a small, brass pressed-in tube that is non replaceable, and which we suggest you do NOT try to modify by drilling or enlarging.

And just in case anyone is wondering, here are the sizes of these non-removable main air jets in the various models:

#45: XJ900RK
#70: XJ550 all models and XJ650 Turbo models.
#120: XJ700-X all models and XJ750-X all models*
#140: XJ1100 models*

NOTE: we believe that the #120 and #140 sizes listed in the service manuals (from where the above information was gathered) may be incorrect, as these are gigantic sizes for main air jets. But since you can't replace them anyway, I suppose that it doesn't really matter........



MIKUNI PILOT FUEL JETS:

ef) OEM and aftermarket Mikuni brass PILOT FUEL JETS, sizes as follows, fits all XJ550, XJ650 Turbo, all XJ700-X and XJ750-X, XJ900RK, and XJ1100 models.

NOTE: stock Mikuni XJ series carbs used the following original pilot fuel jet sizes:

#32.5 XJ650LK Turbo (1983)
#35 XJ550 all models, XJ650LJ Turbo (1982), and XJ700-X all models
#37.5 XJ750-X and XJ900RK USA & Canada only
#40 XJ900 1983-84 except USA or Canada
#47.5 XJ1100



MIKUNI PILOT AIR JETS:

Pilot air jets on the Mikuni BSxx series carbs are replaceable and are located under the diaphram cover on all XJ550 models, and XJ650 Turbo models, and in the carb throat, facing the airbox, on all XJ700-X, XJ750-X, XJ900RK, and XJ1100 models.


ef) OEM and aftermarket Mikuni brass PILOT AIR JETS, sizes as follows, fits all XJ550, XJ650 (except Turbo), all XJ700-X and XJ750-X, XJ900RK, and XJ1100 models.

NOTE: stock Mikuni XJ series carbs used the following original pilot air jet sizes:

#120 XJ750-X models
#140 XJ700-X and XJ750-X models
#155 XJ550L (1984) models
#160 XJ900RK all models
#165 XJ550 Maxim models (1981-83)
#170 XJ550 Seca models, XJ650 Turbo, and XJ1100



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. 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.


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

RESULT:

A #116.5 Main Fuel Jet and #37 Pilot Fuel Jet 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! 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.

 

From the factory book:

Main Jet #112.5 Air Jet, Main #70
Needle Jet O-8
Pilot jet #35 Air Jet, Pilot #165
Starter jet #35 Throttle Valve #130
Jet Needle 4GN

Can't help you on tuning for pods though; it's witchcraft.

 

PO told me the bike was operated in Las Vegas, NV which lays at 1,200 ft. elevation.
So the smaller jets may have worked ok there.

Will have to re-examine the Main Jets under a powerful light to see their numbers more clearly.
It could be that I read them as 107.5 when they are actually the oem 112.5 size

In any event; given the spark plug color seen below, would #132.5 Main Jets seem to be a decent starting point for final re-tuning? Or would # 127.5 Mains be more suitable? I realize the plugs will have to be read at each stage of the re-jetting process.
Here's the present color of all 4 plugs.

 

Those plugs look awfully white, but..............

Be aware that plug color reflects what was occurring within the combustion chamber during the last 10-30 seconds before the engine was killed....so if the bike was idling right before the engine was killed, well, that tells you a whole lot about what the idle-rpm conditions are, and not all that much else.

Here's a little insight into plug colors and how to use them as a proper tuning tool:

PLUGS CHOPS, a/k/a the Zen(ith) of Engine Tuning:

To get even more aggressive in your tuning efforts (beyond mere colortuning and synching), you'll need to do what people call a "plug chop"---and you do it carefully!

When you remove your plugs after that 50 mile ride, what you're seeing is the color of the plugs due to the last few moments of combustion inside the cylinder......and those last few moments are usually at idle or just off-idle.

So your plugs are telling you what things are like with a warm engine, at idle conditions....a very limited (and not-so-useful) insight.

Plug Chops are the procedure where you run the engine (under load, meaning real-life riding conditions) and then if you want to read the engine conditions at, say, normal cruise speed, you get up to a normal cruise speed for a minute or two, keep it there, and then push the kill button (killing the engine), coast safely to a stop, and remove and read the plugs.....in this way, you get to see what the fuel mixture conditions are (determined via reading the color of the spark plug insulators) is at normal cruise conditions. You can do the same thing at full acceleration, or wide-open throttle (CAREFULLY killing it, lets say, at 5000 rpms in 3rd gear).

This is a much more accurate way of determining what the fuel mixture conditions are inside a real engine, under real conditions. Much more of a pain in the butt, too, which is why most people don't do it. But if you REALLY want to get things right, that's how it's done. NOTE: depending at what load conditions and throttle position you do the "chop" at (meaning "push the kill button") tells you where you might need to adjust things. The pilot fuel circuit is active from idle up to about 2500-3500 rpms, and at about 3000 rpms the main fuel circuit starts becoming involved, and by 4000+ rpms the main fuel circuit is responsible for about 80%+ of the engine's fuel supply conditions. So where you do the "chops" tell you not only what the fuel mixture and combustion conditions are like, but also, which circuits are responsible for those conditions.

This is what is meant by "tuning" a bike via changing fuel and air jets, needle shimming, etc......like when you change the exhaust system or air cleaners to those little "pod" filters.

 

After a month of riding in 10-17C [50-62F] weather my xj550 max is getting 39-40 mpg in town (I push it a bit hard). The way these 550 carbs are setup they really don't like anything less than 22 C [72F] to run smooth.

Tucson, AZ is going thru its annual imitation of winter in the SW ... about 17C average daytime temps with freezes in early morn.

Next week the lean carbs are getting re-jetted / fattened up ... throttle touchiness may improve to where the bike is more tractable at low speeds. Right now its like using a Light Switch for a throttle, e.g. its "Hang on and pray" time.

 

Properly tuned the 550 will actually pull (not like a big bike but pull) from about 2500 rpm. You're definitely too lean, as you know, so I wouldn't "push it a bit hard" until you remedy that. Don't want to put a hole in one of those fresh new old pistons. Yours is the 2600-mile Max, isn't it?

 

Update:

Stock Exhaust, Pods with very clean carbs.

Since January I've been running the 132 MAIN JETS with good results .... bike runs well, pulls hard although idle quality is a bit down due to the PODS I suspect. Plugs look good, light brown color.

This Week- August 2nd:
Installed a Stanhope Tuned (Carbon Fiber) Stainless Exhaust; the inlet is 1.75" diam. It was a bolt-up installation - EZ.

Blocked off left exh. outlet w/ stub. Bike idles a bit smoother and accelerates with no stumbles. Will not rev easily to redline however. No coughing, so I suspect it needs a bit LESS packing to equal the optimum airflow of the 2 OEM silencers.

Plugs don't yet show a big change so I'll have to do a full plug chop.
Sounds a lot like an old Honda 305 Scrambler with open pipes at 7500 rpm.

August 5th:
Made test run after securing new exhaust system with proper clamps, strap etc.
Bike still wants to pull hard, seems like the midrange is coming on fairly strong now with some loss of bottom end torque.
I stand corrected; the engine wants to rev higher than I'd thought or I was used to. That is; one has to be willing to twist the throttle WFO to get a response.
Overall the new carbon fiber exhaust seems to be better than it first appeared. It certainly dropped some weight off the chassis.