Jet reccomendations

[chainbearer]

I have an 81 750E, fairly hot cams, header and supertrapp. I'm hoping to get a suggestion on jet sizes for my mikuni bs 33's. Any help would be appreciated.

 

[81GS750]

What is your location? I'm sure some of the guys with racing experience might chime in, but knowing the lift and duration of your "fairly hot cams" and what you have for air filters, (pods, velocity stacks, air box) might help to know. I pulled this from a post on the XJ site, but it's been a good reference for jetting for me.


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!


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.

 

[posplayr]

Those guidelines seem a little funny. For example on a 83 GS1100E with a 34mm stock carb the main is a Mikuni 110. The Dyno jet kit Stage 3 for pods and 4:1 pipe is either a 132 or 138 DJ size. That is close to 122 to 128 Mikuni main (subtracting 10 from the DJ size to get Mikuni size).

That would be equivalent to 22 to 28 sizes? Are you sure the sizes are not the 2.5 increments from Mikuni? That would be closer to 10 sizes not 22-28 but still way off.

Also i think I have seen guidelines here for 2 steps for 4:1 and 4 steps for pods or 6 steps x 2.5 per step = 12.5==> 12.5+110=122.5 Mikuni main jet which is right what the lower Dyno Jet setup is.

http://roadstarclinic.com/content/view/61/96/

 

[chainbearer]

I am running pod air cleaners with the air box removed, and I was wondering if there are any differences in using mikuni jets over dynojets, or vice versa? I am in Ohio, so not much elevation to deal with.

 

[posplayr]

I am running pod air cleaners with the air box removed, and I was wondering if there are any differences in using mikuni jets over dynojets, or vice versa? I am in Ohio, so not much elevation to deal with.

http://roadstarclinic.com/content/view/61/96/

 

[81GS750]

Those guidelines seem a little funny. For example on a 83 GS1100E with a 34mm stock carb the main is a Mikuni 110. The Dyno jet kit Stage 3 for pods and 4:1 pipe is either a 132 or 138 DJ size. That is close to 122 to 128 Mikuni main (subtracting 10 from the DJ size to get Mikuni size).

That would be equivalent to 22 to 28 sizes? Are you sure the sizes are not the 2.5 increments from Mikuni? That would be closer to 10 sizes not 22-28 but still way off.

Also i think I have seen guidelines here for 2 steps for 4:1 and 4 steps for pods or 6 steps x 2.5 per step = 12.5==> 12.5+110=122.5 Mikuni main jet which is right what the lower Dyno Jet setup is.

http://roadstarclinic.com/content/view/61/96/

Really didn't think much about the differences in Mikuni, DJ, and the Hitachi carbs used by Yamaha. It's a good reference for Hitachi carbs, haven't had a real need for it on my GSs. But I think your right, and I do remember reading posts about the 2 steps for 4:1 and so on. So, My Bad

 

[hashjay]

Ok so I have 80 gs550, chopped some of stock exhaust off around where heel is, and individual pod filters, and live at 3483ft above sea. +4 for pods, +2 for exhaust, -2 for elevation, and -2 for total leaves net main +2, and stock is 92, so........

 

[cowboyup3371]

Hashjay, you might want to put your own post out there with a proper question as I'm not so certain anyone will be able to follow what you are trying to say

 

[hashjay]

The question is, going with the formula below, what size jet does that put me at?

 

[Katarat]

http://www.jockeyjournal.com/forum/showthread.php?t=64769

 

[hashjay]

thanks for that, but the mikuni sizes dont exist as per the gs resource oem carb spec page. Such as a 92.5, or a 122.5, so this chart seems more like disinformation than information.