From: Bert on
I have spent a week installing a jet kit and have gotten the bike to
start cold and idle well. It pulls off idle but just after a 1/4
throttle is stumbles and backfires. This is the throttle position I do
most of my non-acceleration riding at and it will stumble, backfire,
pop, and miss until I roll on the throttle, even a little, and it
clears right up and pulls hard. As I roll the throttle just past this
narrow point it pulls cleanly and hard all the way to red line. I can
do this in 5th or 6th gear. But it does not like to hold a steady
speed.

It has taken a good bit of work to install this kit. The DynoJet stage
1 kit that was in it had needles that were 5mm shorter then the ones
in the new kit. They would not work at all no matter what the clip
setting so I had to go back to the original needles and got it to
start with the choke, idle and come off choke and idle.

To get the start, idle, and off idle throttle response to work as well
as it does, I have had to adjust my pilot jets 2 turns out from the
recommended baseline starting point. Throttle response was poor until
I was able to establish this good idle and then I was able to sync the
carbs and this made the engine rev very smoothly.

My thinking is to back the idle jets out 1/4 turn at a time and try
that or adjust the idle a little higher. Right now it idles at
1100rpm. But at their current setting it idles and starts perfectly
and I don't want that to change.

It has taken some time to work through the various adjustments. The
needles supplied were 5mm longer than the needles that were in the
original Dynojet kit and they did not work at all no matter how they
were adjusted. I called and emailed Dynojet and got non-answers each
time so I went back to the original Dynojet needles and they are
working.

Installing the new main jet and needles required a lot of adjustments
in the idle jets and main idle settings to get it to run as well as it
is. I have no vacuum leaks and good fuel.

The slides move freely and their are no exhaust leaks. The carbs are
clean, and in neutral it revs cleanly and holds rpm and any throttle
position. The carbs are well synced, carb manifolds and exhaust header
are all well attached with no leaks. Aside from it running so much
better when I am accelerating I am very happy with the progress.

It is takes the smallest throttle movement to clear it up but that is
the throttle setting that is used for most riding at a set speed. If I
accelerate all the time its great, but that gets old.

Thank you for any help.
From: frijoli on
Bert wrote:
> I have spent a week installing a jet kit and have gotten the bike to
> start cold and idle well. It pulls off idle but just after a 1/4
> throttle is stumbles and backfires. This is the throttle position I do
> most of my non-acceleration riding at and it will stumble, backfire,
> pop, and miss until I roll on the throttle, even a little, and it
> clears right up and pulls hard. As I roll the throttle just past this
> narrow point it pulls cleanly and hard all the way to red line. I can
> do this in 5th or 6th gear. But it does not like to hold a steady
> speed.
>
> It has taken a good bit of work to install this kit. The DynoJet stage
> 1 kit that was in it had needles that were 5mm shorter then the ones
> in the new kit. They would not work at all no matter what the clip
> setting so I had to go back to the original needles and got it to
> start with the choke, idle and come off choke and idle.
>
> To get the start, idle, and off idle throttle response to work as well
> as it does, I have had to adjust my pilot jets 2 turns out from the
> recommended baseline starting point. Throttle response was poor until
> I was able to establish this good idle and then I was able to sync the
> carbs and this made the engine rev very smoothly.
>
> My thinking is to back the idle jets out 1/4 turn at a time and try
> that or adjust the idle a little higher. Right now it idles at
> 1100rpm. But at their current setting it idles and starts perfectly
> and I don't want that to change.
>
> It has taken some time to work through the various adjustments. The
> needles supplied were 5mm longer than the needles that were in the
> original Dynojet kit and they did not work at all no matter how they
> were adjusted. I called and emailed Dynojet and got non-answers each
> time so I went back to the original Dynojet needles and they are
> working.
>
> Installing the new main jet and needles required a lot of adjustments
> in the idle jets and main idle settings to get it to run as well as it
> is. I have no vacuum leaks and good fuel.
>
> The slides move freely and their are no exhaust leaks. The carbs are
> clean, and in neutral it revs cleanly and holds rpm and any throttle
> position. The carbs are well synced, carb manifolds and exhaust header
> are all well attached with no leaks. Aside from it running so much
> better when I am accelerating I am very happy with the progress.
>
> It is takes the smallest throttle movement to clear it up but that is
> the throttle setting that is used for most riding at a set speed. If I
> accelerate all the time its great, but that gets old.
>
> Thank you for any help.

I would first raise the needle one position at a time to see if that clears it
up. It sounds like you have a lean condition at transition from Idle circuit to
main circuit.

Clay
From: Aham Brahmasmi on
On May 22, 10:51 am, Bert <robertsobo...(a)gmail.com> wrote:

> To get the start, idle, and off idle throttle response to work as well
> as it does, I have had to adjust my pilot jets 2 turns out from the
> recommended baseline starting point.

How many total turns out are the idle mixture screws from lightly
seated at this point?

The idle mixture screws stop having any function after about 3.5 turns
out.

If you have to turn the screws out past 3.5 turns, you need larger
idle jets.

Most amateur mechanics do not realize how the four idle mixture ports
work on
constant vacuum carbs.

The single downstream port is the one that is controlled by the idle
mixture screw.

The three idle mixture ports arranged in a triangle near the throttle
butterflies are called "acceleration transition ports."

They are NOT affected by the idle mixture screw at all, they are
controlled by the position of the lower edge of the throttle
butterfly.

As the butterflies open, engine vacuum drops off substantially, so
more area is required to pass sufficient fuel to accelerate from idle.

Tuning constant vacuum carbs to start, idle, and accelerate properly
requires the mechanic to understand the totality of all four
interconnected legs of the the idle mixture circuit.

Imagine an H-shaped circuit with fuel being sucked in through the idle
jet, air sucked in through the pilot air jet, the fuel and air being
mixed in the cross bar of the "H", and then the fuel air mixture exits
through the the legs, with only one of the legs having an adjustable
mixture screw.

The amateur mechanic either gets stuck with a lean idle mixture
because he's turned the idle mixture screws in too far to avoid having
the idle RPM run away when the engine is on the choke, or he's opened
the idle mixture screws so far he experiences unmanageably high idle
RPM when the engine is hot.

In the latter case, he finds himself lowering the idle RPM on warm
days and raising the idle RPM on cold days and wondering why he's
alternating between having backfires on cold days and high RPM on warm
days...

Stumbling and backfiring indicate that your idle mixture screws are
turned in too far, but, as I said above, having to turn the idle
mixture screws out more than 3.5
turns out indicates that your idle jets may be too small...