tech6

Oh how I hate to jet a motorcycle. It seems to take forever trying
endless combinations of main jets, needles, needle jets and pilot
jets, not to mention slides. It doesn't help that I don't do it very
often so I don't really get a chance to hone my skills. It doesn't
help that manufactures change the jetting from year to year on
apparently identical machines except for the color of the paint,
making the starting point a moving target. It doesn't help that I
don't own a Dyno on which to try the various combinations, forcing me
to test ride every combination out on the street. It doesn't help that
for every Amal needle there are hundreds of Mikuni needles to choose
from or that for every needle jet Amal makes, Mikuni makes ten in
between in size. It doesn't help that needle selections include
single, double, and even triple tapers to say nothing about the
difference between top and bottom diameters. It doesn't help that Amal
and Mikuni and Kehin and Delorto and Bing and Jicov all use random and
completely different jet/ needle jet/ needle/ slide numbering system.
It doesn't help that every carb manufacture seems to use different
size air bleed jets (air correction jets) which make using mathematics
to compare jet sizes between models or brands virtually meaningless.
It doesn't help that even Amal (and all the other carb makers, for
that matter) jets are not interchangeable between different models. I
just hate jetting!

I seem to have the best luck by just staying with the setup that came
on the bike but even that doesn't work when manufactures change
jetting specs between years for apparently equivalent bikes. My
friends Triumph Daytona is a prime example. Over a five year period
the Concentric main jet changed from 200 to 160 to 180 to 170 to 150
with the UK model being different than the US model. What's up with that?

When I changed to Mikuni's using a kit for my Hornet you'd think you
would get the right jetting with the kit. Opps, said the kit provider,
someone put the wrong jetting in the card file. Here try this. Oh,
that didn't work out? You do know every bike is different! After an
iterative approach over a multi year period I got close for 3000 ft
(my local altitude) but I ride it most often at lower elevations and
even at 3000 feet it is slightly lean. Each carb jet/needle
modification takes time because the spaces are so close around them
that you have to take them off the manifold even to change the main
jets.

Now, my Ducati is a wonder for jet changes. When I changed to a less
restrictive (read loud) exhaust system I was warned to raise the
needle to richen it slightly. What a hassle that was. Off came the gas
tank, the airbox, and tons of other stuff like smog hoses, to even be
able to see the carbs. And of course both carbs are mounted on a
single mount with linked throttle shafts. The idle screws are behind
brass plugs. Fortunately the needles at least had grooves and a clip
that I could change. So many new smog bikes don't even have that
adjust ability.

And what about deciding how and what to change. Yea, I know that if
the bike seizes, it is to lean and if it blubbers so bad that it won't
move off, it is too rich. Everything between those two points is
trouble for me to decern. Yea, I know that you tune the main jet for
maximum top speed but with 100+ mph bikes where can you run them? A
dry lake, you say, but most are hundreds of miles away and at 3000
feet. Ever done a high speed run and chop on the freeway? What did you
tell the freeway cop that stopped while you had your plugs out holding
them to the sunlight to see the "color"? What about setting the idle?
Should be easy: tune for peak idle speed. But what to do if the idle
speed peak doesn't seem to occur at all? Do you need a richer or
leaner pilot jet? What to do with an Amal Concentric which doesn't
even have a changeable idle jet. In fact the idle "bushing" that is
pressed in can't even be seen on carb disassembly. It is insert at the
factory and the insertion hole plugged with a unobtainable plug that
only removes by destruction. Oh, and if you have turned the idle
mixture (air screw) to a point outside the recommended 1 1/2 to 2 1/2
turns out, which direction of screw turning corresponds to needing a
richer idle jet and which to needing a leaner idle jet? It seems no
two carb models or manufactures have the same meaning for rich or lean
for clockwise screw rotation. Some of the newer smog carbs even limit
the amount of adjust ability that the idle mixture screws actually do.
What about slide cutaway choices? What do those numbers mean? Are
bigger numbers on the slides richer or leaner and what exactly does a
change in slide cutaway even do for you?

So you see I hate jetting bikes! Science seems to be meaningless for
carbs and their jets. You know every bike is different!