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Jetting Recommendations
by Denny Zander
Here is a simple set of jetting guidelines that have worked for me. For those considering jet changes, this might help select a starting point.
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
3 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 2.5 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 Colour Tune. (See "Colortuning Carburetors" in the Maintenance Section).
Make sure your carbs are in perfect working order before making jet changes.
Example from my '79 XS1100 F:
Stock main jets: 137.5
Stock pilot jets: 42.5
2 sizes for 4:1 exhaust (Jardine)
1 size for single K&N
1 size for drilled air box
4 (main sizes) - 1 = 3 or 137.5 + (2.5 * 3) = 145.0
1 (pilot jet size) or 42.5 + (2.5 * 1) = 45.0
With this jet configuration I get 32-38 mpg on the open road, Smooth idle, very strong acceleration from off idle to 80+ mph, and steady pull past 120 mph. My "F" has 76K mile on it, of which I have put 44K with this jet configuration.
I have applied this to the '78-'79 carb set with great success. These guidelines should work for the '80-'81 carbs also. The only part I have not tested is pilot jet changing on the second generation carbs.
here are jet pictures for reference
a air jet - b main jet - c pilot jet (some have different # of holes)
d various needle jets (emulsion tubes) - e two styles of float needle valves and float needles
(note: there are several styles of Mikuni main jets - the ones of interest to us are the 'large round' style)
Needle (jet needle)
Happy Jetting
by Denny Zander
Here is a simple set of jetting guidelines that have worked for me. For those considering jet changes, this might help select a starting point.
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
3 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 2.5 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 Colour Tune. (See "Colortuning Carburetors" in the Maintenance Section).
Make sure your carbs are in perfect working order before making jet changes.
Example from my '79 XS1100 F:
Stock main jets: 137.5
Stock pilot jets: 42.5
2 sizes for 4:1 exhaust (Jardine)
1 size for single K&N
1 size for drilled air box
4 (main sizes) - 1 = 3 or 137.5 + (2.5 * 3) = 145.0
1 (pilot jet size) or 42.5 + (2.5 * 1) = 45.0
With this jet configuration I get 32-38 mpg on the open road, Smooth idle, very strong acceleration from off idle to 80+ mph, and steady pull past 120 mph. My "F" has 76K mile on it, of which I have put 44K with this jet configuration.
I have applied this to the '78-'79 carb set with great success. These guidelines should work for the '80-'81 carbs also. The only part I have not tested is pilot jet changing on the second generation carbs.
a air jet - b main jet - c pilot jet (some have different # of holes)
d various needle jets (emulsion tubes) - e two styles of float needle valves and float needles
(note: there are several styles of Mikuni main jets - the ones of interest to us are the 'large round' style)
Needle (jet needle)
Happy Jetting