tech34

Carburetor Jetting Theory
by Lynn Bennett

I have written several articles on carburetors since putting Mikuni's
on my BSA A65 Hornet almost ten years ago. The subject seems important
to me since I have not been able to get the jetting right, yet. But I'm
close. After a recent ride to a local mountain area one plug looked
near perfect while the other was excessively rich. A little
investigation revealed that the choke on the rich side was ever so
slightly on. Why all the trouble? The first set of Sudco jetting was
for the wrong motorcycle. They quickly replaced it but with a leaking
head gasket (between cylinders) the jetting progress was stopped due to
inconsistent running. The rebuild seven years ago fixed the leak but
short rides showed extreme leanness in the control range of the needle
and needle jet. Jetting is such a pain since the space on the bike is
so cramped and getting the carbs off such a hassle. But I think I am
very close now after several needle position changes and two needle jet
changes. But why is the Sudco jetting so off? And what makes everyone
favorite jetting so different? My recent study of this topic has
revealed to me why. The problem is in the details. For this discussion
I'll assume that all understand the basic carb circuits: Idle(pilot)
jet for 0 to 1/4 throttle, needle and needle jet for 1/4 to 3/4
throttle, and main jet for 3/4 to full throttle.

First off, all the circuits overlap in their control somewhat. For
instance, a lean low mid range stumble can be helped by running the
idle mixture rich or an excessively rich main jet can affect the mid
range, causing it to be too rich as well. Jet from the top down, but be
careful as the idle jetting affects the main circuit as well. Get the
main jet right first using full throttle runs, chopping the engine, and
reading the plugs. Then read the plugs after around town riding at
middle throttle settings, selecting the needle profile, selecting the
needle jet, and adjusting the needle position. Adjust the idle jetting
using the air screw (or idle mixture screw) for best idle. If the idle
air (mixture) screw is outside the range of 1 to 3 turns out change to
the appropriate richer or leaner idle (pilot) jet. But look at the
Mikuni choices: for pilot jets they go from 10 to 95, perhaps 50
different needles, 50 or more different needle jets of two distinct
types, main jets from 50 to 700, and about 15 different air correction
jets. By contrast Amal makes one needle that is readily available,
perhaps 4 needle jets (104, 105 , 106 , 107) , pilots from 20 to 40 of
which perhaps only three are applicable (25, 30, 35), the air jets are
fixed, and many main jets which the factory has dutifully already
selected for each model.

What are the details that complicate the jetting selection or help the
super tuner? Needle jet selection is based on the needle diameter and
the taper (or tapers for multi-tapered needles). Needle jets can be
either primary type (a short screen or shield protects the jet opening
from direct venturi air flow) or bleed type (correction air from the
air jet causes the jetted fuel to be mixed into an emulsion rather than
droplets). Primary needle jets are normally used for two strokes that
need to be richer at lower RPMs and bleed types are used for four
strokes. While this is the theory, Sudco can use either type for four
strokes, but not exactly interchangeably. Air jet selection is used to
tailor the engine's different fuel needs for both low and high RPM in
the range of 3/4 to full throttle.

Testing is probably the best way to get the jetting correct. The
variables in testing for best performance are throttle position (1/4,
1/2, 3/4, full, and all the steps in between), load placed on the
engine (a measure of the horse power the engine produces), the engine
RPM resulting to sustain that given load, and the fuel delivered to the
engine during the load application. The Dyno is the measuring tool. The
Dyno runs are done by fixing the throttle position, loading the engine
with the dyno progressively heavier with a resultant engine RPM, and
gathering data of dyno load level verses engine RPM. RPM and load are
used to calculate horse power for a given fuel flow (fixed jetting).

How do the super tuners do it? They use the dyno, running the engine at
full throttle, for example, and loading it down to plot the horse power
at various RPMs. They change the main jet and make a run again. A
succession of runs go from rich to lean. But every RPM might want a
different main jet. The requirements are plotted and the air correction
and main jets are changed to tailor the carb to the engines fuel
requirement curves. More air (bigger air jet) leans the higher RPMs.
This is all a compromise that the super tuner does. It takes many hours
on the dyno. For mid range the test are repeated at perhaps 1/4
throttle, 1/2 throttle and 3/4 throttle, this time adjusting the needle
or the needle jet to get that plot of fuel requirement vs RPM for each
throttle setting. Again, many dyno hours, lots of plots, and hours of
research to find the combination of needle taper and needle jet that
meets those requirements. I don't know how the average vintage bike
rider can do this. He must rely on the Sudco's of the world to do this
but that doesn't always work out, as in my case. And then you have the
old adage that "every engine is different" and will take different
jetting. I guess once you get it to "good enough", stop. Or buy an new
Amal jetted to the factory specs and just ride the thing.

So what are the lessons learned? The list below may help.

1. A main jet change affects the entire 3/4 to full throttle range
uniformly and slightly affects the 1/4 to 3/4 throttle range, so do it
first.

2. An increased air correction jet change leans the top RPM range at
3/4 to full throttle.

3. A needle clip position change affects the entire 1/4 to 3/4 throttle
range pretty much uniformly.

4. A needle jet change has the most affect in the 1/4 to 1/2 throttle
range.

5. Primary type needle jets are much richer than the same part number
in a bleed type jet.

6. A needle change can affect everything in the 1/4 to 3/4 throttle
range since both the diameter of the needle and the taper and length
and start points of the taper(s) are variables across all the needles
available.

7. A slide cutaway change affects the range between 1/8 to 1/2 with a
particular emphasis on the 1/8 to 1/4 range.

8. Too small of a carb venturi will increase the low RPM horse power
but reduce the top end horse power. The opposite is true of too big a
carb with the added effect of poor low RPM throttle response.

9. Adjust the air (mixture) screw for peak idle RPM with a uniform idle
speed. Next adjust the idle speed down to the factory requirement by
lowering the slide with the idle speed screw. Repeat these steps
multiple times to home in on the best setting.

10. If the air (mixture) screw is out of the range of 1 to 3 turns out,
then a change in pilot jets is in order.

11. Jetting specs, even when converted to equivalent units of diameter
like inches, aren't directly comparable between any two different carb
models or carb manufacturer usually because the air correction jets are
different sizes (and air flow differences in the venturi due to carb
size and ventur shapes). For example, the Monoblocks off my A65 and a
Concentric from a Victor, and two Mikuni's, that I happen to have on
hand, have air jet sizes in millimeters of 4.0, 4.6, 2.0, and 2.0
diameter, respectively.

So, maybe this will help you decide whether to put Mikuni's on your
British bike. Here's to good jetting. And check that choke!