jetting a fj1200 dynojet kit

Hello I am a little confused about jetting my bike. I already install
the new needles, springs and the main jet on my bike. I have uni air
filters and i am planning to upgrade to an after market exhaust, So far
is running ok. In the instructions of the dyno jet it talks about
drilling the plug underneath the fuel mixture screw. Also it says if
you see a screw head continue to adjusting procedure. I did not drill
anything I simply turned the mixture screw 3 turns. I will be putting
an after market exhaust on my bike and I an not sure if i will need to
install the stage 3 jets. Can anyone tell me if I need to drill
anything? Will a stage 3 jet make any difference with an aftermarket
exhaust? thanks.

 

Welcome to the Dynojet Sucker Club. ;-) Confusion and ignorance and
gullibility qualifies riders to join.

Dynojet makes their money off of the confusion and ignorance of their
customers.
It's a profitable business---for them. :-(

They also profit from wannabe racer's dreams of "keeping up" with the
rich guys they see riding the latest greatest machinery. :-(

A Dynojet kit cannot magically release some "hidden power" in your
engine that the designers never noticed. :-(

A carburetor jet kit helps only if you RIDE the motorcycle harder. If
you ride harder, you will attract police attention and get a
"performance citation" for your enthusiasm.
:-(

You would have been better off with a Factory Pro kit. Too bad Dynojet
already has your money. :-(

Street riders don't need different needles. The jet needle has its
greatest effect at 75% to 90% throttle, and street riders seldom use
that much throttle. :-(

You should have left the airbox in place, it's designed to overcome the
midrange flat spot inherent with inline four cylinder engines that have
4-into-1exhaust systems.

The flat spot between 5000 RPM and about 8000 RPM will GET WORSE if you
install a loud aftermarket pipe. The Dynojet technicians admitted this
to me AFTER THEY HAD MY MONEY. They said that I should just "ride
harder, and shoot through the flat spot under acceleration at full
throttle. :-(

That strategy doesn't work on public highways or in tight twisties in
the canyons. :-(

OK, so you don't have a clue about how the idle mixture screw and the
idle jet works together. There is NO MAGIC NUMBER of turns of the idle
mixture screw that works for everybody. Nobody can tell how many turns
out you need, but I'll tell you WHY there is such a wide range of
adjustment, from 1/8th of a turn to 3-1/2 turns.

Mikuni and Keihin round jets have an orifice hole based upon 1.0
millimeters.

The idle jets in any given constant vacuum carburetor will range from #
30 to # 45.

# 30 means that the orifice hole diameter is 0.30 millimeters in
diameter. # 45 means that the orifice hole is 0.45 millimeters in
diameter.

If you work the Area = pi X radius squared formula that you learned in
junior high school, you will see that the area of the jet is directly
proportionate to the square of the radius of the hole times pi.

So a # 45 idle jet's area is 225 % of the # 30 idle jet.

What does this mean in practical use? It means that, given enough
vacuum, the engine can suck 2.25 times as much fuel through a # 45 idle
jet as it could suck through a # 30 idle jet. :-/

It can drown itself in gasoline from the idle jet, if the idle jet is
too big and you open the idle mixture screw too far. You'll see black
smoke and soot coming out the exhaust pipes if you adjust the screws
wrong or put idle jets that are too large into your carburetors. :-(

How does this affect idle mixture screw adjustment? Mechanics find that
they may have to open the idle mixture screw (by turning it
counterclockwise) as much as 3-1/2 turns to get enough idle mixture to
run the engine if the idle jet is a # 30.

OTOH, mechanics find that the have to turn the idle mixture screw all
the way closed, and back it out only about 1/4 to 1/2 a turn when they
use a # 45 idle jet.

How do you know when the idle mixture screw is opened too far? Amateur
mechanics expect the idle RPM to increase as the turn the idle mixture
screw further and further counterclockwise.

They open the screw so far that the mixture becomes so rich the exhaust
sound becomes dull and thudding, the idle RPM slows down, and they
compensate by turning the master idle knob to increase the idle RPM.
:-(

Then they are startled and dumbfounded by the fact that the idle RPM
hangs up around 4000 to 6000 RPM after they suddenly open and close the
throttle. :-(

There is a pattern of three transition ports just downstream of the
throttle butterfly.

The transition ports are NOT controlled by the idle mixture screw, they
are controlled by the position of the throttle butterfly, which closes
off the transition ports at the correct idle RPM setting.

The uncontrolled flow of gasoline through the uncovered transition
ports combined with the slight opening of the butterfly to allow the
engine to idle far too fast. :-(

So the amateur mechanic has to know these simple facts when attempting
to sort out the MESS that they have gotten themselves into by
installing a Dynojet kit.

Don't let your ego control what you do and how much money you spend
foolishly until you understand what speed tuning is all about.

Stage 3 kits and aftermarket exhaust systems are for motorcycle RACERS
who ride on the race track. If you were a motorcycle racer, you
wouldn't be installing a Dynojet kit, you would be buying a completely
different set of carburetors (Mikuni radial flat slide or Keihin flat
slide carburetors). And you'd be buying a loud competion-only exhaust
system.

There is a hole (or two holes) in the bottom of the slide so that air
with flow through the slide quicker and allow the slide to rise
quicker. This makes the engine more responsive to twisting the
throttle, but it may make throttle control "choppy", and that may make
the engine speed up and slow down too quickly for confident riding in
traffic. :-(

The larger jets are for riding on the RACE TRACK, not on the street. If
you ever actually get out on a race track and ride at large throttle
openings for half an hour at a time, you need larger mainjets to keep
the engine cool by wasting gasoline.

If you live somewhere with wide open spaces and you can ride with the
throttle wide open for ten minutes at a time, you're out in Texas or
Nevada.

Most riders NEVER hold the throttle wide open for more than 20 seconds,
if that.

But, speaking as an experienced tuner, I wouldn't use Dynojet's HOME
MADE jets.

I would buy genuine precision machined Mikuni or Keihin main jets. If
you are racing, the biggest main jet you can use in a 1200cc engine is
about a # 135. If you are a street rider, a # 125 is big enough for
most riding.

 

Air/fuel adjustment screws on carburetors are sealed at the factory (and
have been for some time) for emissions compliance reasons. If you see
screw heads, then someone else has already drilled off the covers.

A stage 3 jet kit will make plenty of difference, whether it's the
difference that you want is another matter. Good luck with it.

later,
Johnj

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.

 

The facts on jetting also are that if the home-made Dynojets (which B12
has said are made by drilling out normal jets) are in fact made by drilling
holes in an existing jet, the flow through them will probably be LESS. The
reason is that the drilling produces a hole with some knurking and scratches
in the hole wall, these imperfections interrupt the flow of gasoline and
cause
turbulance in the jet and so reduce flow through the jet. Factory-made
precision jets like the ones that come from Mikuni and Keihun have holes
that are made on a milling machine and the hole walls are perfect, also the
transition into the hole is a precise amount, and they test flow through the
jets as well.

Ted

PS I've never used the Dynojets myself.

 

Thanks for the replies, i guess dont have to drill anything. I cant
wait to get my new exhaust.

 

I didn't say that Dynojet drills out somebody else's jets and resells
them...

I could have been nicer. I could have written "proprietary" instead of
"home-made". But the Dynojet proprietary jets are crude, compared to
the specially engineered high precision reverse venturi jets that
Mikuni and Keihin supply.

If you section a Mikuni or Keihin main jet across the long axis, you'll
see what is meant by "reverse venturi". There is a funnel-shaped entry
to the orifice, and a funnel-shaped exit that smooths the flow of fuel
through the jet.

OTOH, the Dynojet main jet is just a straight hole drilled into a
threaded piece of aluminum. There's no flow enhancing venturi in the
jet at all.

The last step in manufacturing a Dynojet main jet is to stamp the tiny
numbers onto the jet. These numbers don't directly correspond to Mikuni
or Keihin's numbering system.

When you look at a Keihin or Mikuni precision jet, the incredibly tiny
computer style numbers are apparently *milled* onto the jet.

Gawdonly knows how they do *that*...