Jetting re-visited

I have someone in the process of a complete clean and overhaul of my carb set (from my 1982 XJ750R). At one time I had found 2 different jet size calculators that were simply data input charts that did the conversions for you. Now I can only find one of them, and my computer will not let it run. So, since this is the largest pool of knowledge that I have found to draw from, I need some hints for a baseline.

Here are my Specs:

Stock 120-Main Jet 40-Pilot Jet Y-13 Needle

K&N Individual Pod Filters
Dyna DC1-1 Coils and Dyna wires
Supertrapp Adjustable Backpressure Muffler
General Elevation of Use = 2500-4000 ft above sea level

Obviously I realize that no-one can jet my bike correctly from their computer a great distance away. I have full intentions of doing actual tuning once the carbs are back in, and the project is complete. What I am trying to accomplish is to get it close enough first try that I can tune it out with the adjustments available and using exhaust tuning as well. Thanks again for the help everyone has provided in the past and in the future. Regards, Darryl

 

A computer program won't help you much since it has to be done by trial and error on a dyno or by running it on the street while you monitor the mixture.
Start with the stock sizes if you are using the stock air filter and exhaust. If you are running pod filters and a low restriction exhaust system start with pilot jets a size larger than stock and main jets a size larger than stock but lower the tapered needle a notch so the midrange won't be too rich and go from there.
BTW, the Supertrapp Adjustable muffler doesn't rally do much except let
you set the volume from medium to loud. I have a Supertrapp 4-1 stainless steel system with a Supertrapp Adjustable muffler and I can put on a lot of rings or take them all off and the bike runs about the same. Nobody rally knows how to adjust it, not even Supertrapp, I asked them. I ended up using an open end for best performance but putting on some rings for the best appearance.

 

Thanks for the info regarding the Supertrapp. I bought primarily for looks anyways.

Here is the info regarding jetting that I found. Please review my math at the bottom and let me know what you all think. Thanks again!!

1 jet size for custom 4 into 2 exhaust

2 jet sizes for 4 into 1 exhaust

1 jet size for K&N filter (single inside airbox)

1 jet size for drilling out the bottom of the airbox

2 jet sizes for both single K&N and drilled airbox

2 jet sizes for individual filters

2 jet sizes for no muffler (open header)

1 pilot jet size for every 3 main jet size increase

Add up all the jet size increases and subtract one. (Remember they go in steps of 2.0 for each jet size)

Under a mismatch condition, like individual filters and stock exhaust or 4 into 1 header with stock filter and air box, subtract an additional 1 jet size.

Check plug color often, sync carbs after each jet change, make sure the floats are set correctly, and seriously consider purchasing a Color Tune.

Make sure your carbs are in perfect working order before making jet changes.

Additional info: Decrease main jet size one step per 2000' above sea level. Decrease pilot jet every 6000' above sea level.

--------------------------------------------------------------------------------
Stock = 120 Main and 40 Pilot with Y-13 Needle

4 into 1 with Supertrapp = +2 Sizes Main Jet
K&N Pods = +2 sizes Main Jet
Subtract 1 = +3 Sizes Main Jet or a 126
Subtract One More Size for Altitude of 2500' Average

I figure a 124 Main and 42 Pilot should do the trick after adjustment.

If I am way off, please correct me, as I don't want this to hinder other people who search this forum for the info.

 

That's the statement that always confuses me (i'v read these recommendations elsewhere, too)----main fuel jets go in size "steps" of 2 (e.g. from a #110 main fuel jet your next "step" up is to a #112 main fuel jet, as there is no one that makes or uses in-between sizes such as...in this example...a #111 main fuel jet).

However, pilot fuel jets do go in size "step" increments of 1....from a #40 to a #41, etc.,

So does moving from a #110 to a #112 main fuel jet count as 2 or 1 step(s)? Or is it 1 or 2 jet sizes?

And how do you account for pilot fuel jet sizes....is going from a #40 to a #41 1 size of 1 step or ?????......

 

Your calculations are correct. Bear in mind though that those calculations are based and proven on XS1100's with mikunis. The principle is the same and should put you in the ballpark for finer tuning.

124 Mains and 42 Pilot will get you pretty close to spot on.

One either way on the mains but the pilot will be fine.

You wull find the mixture screw will require less turns out. Possibly only one and a quarter turns for a starting point.

Let us know how it goes..

 

110 to 112 is one jet size if the increment is 2.

In general, the pilot size increments used should be the same as the mains. Eg 1 size from 40 is 42.


SHould never be required to go more than 1 pilot size over stock though.

 

Does this typically hold true for most bikes?

I only ask this because the dynojet kits use main jets much larger than what would be considered normal by the above calculations.

 

>Does this typically hold true for most bikes?
Not really since every bike is different and jet sizes are just an arbitrary number. Different suppliers like Dynojet might measure them differently or use their own completely different numbering system.
The trend lately is to use the diameter of the jet opening in mm as the jet size, for instance a #124 jet is usually 1.24mm in diameter but not always. Dynojet jets do not match Mikuni or Hitachi jets exactly even if they are the same number.
To solve this problem I adopted a measuring system to check the actual size using a set of minature drills and a digital micrometer that reads in .o1 increments of millimeters.
Find the drill that fits the hole the best, measure it's diameter and you have the size!
Then you have the problem of different bikes requiring different size jets to obtain the correct mixture due to differences in intake and exhaust restriction, elevation, atmospheric pressure, temperature, humidity, air density, valve tuning, etc. so in the end you still have to determine if the the jet size is correct by measuring the mixture with an exhaust gas analyzer no matter what the number on the jet says.