1992 ZX-7 won't idle right.

Lately I've been having trouble with a 1992 Kawasaki ZX-7. If I set
the idle high it will rev way too fast in neutral, but if I lower the
idle speed it gradually slows down, begins to run roughly then
dies. Right now it does some of both. It will over rev for a momment
then fall into an good idle then slowly quit. I have the carbs in sync
and I blew some solvent through the jets but that didn't seem to
help. Any ideas on what I problems should be looking for?
thank you.

 

Have you checked the usual suspects:
air filter
spark plug condition
good fuel flow (e.g. by draining fuel into a bottle and seeing if the
flow quits, running bike with fuel tank open, etc.)

And then there are the usual questions:
has the bike been sitting for a while?
Was the change sudden or gradual?
Have you done any work on the bike recently?

Without knowing more, I'd say you probably need to thoroughly clean your
carburetors, though running a couple of bottles of fuel system cleaner
(according to the product directions) likely wouldn't hurt even if the
problem's somewhere else.

--
Later,
John



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

 

Carefully examine the carburetors to see if you can find the slot
headed idle mixture screws. They will be forward of the diaphragm cap
on top of the carbs, or underneath, forward of the float bowls.

To look under the carbs you'll need a mirror. If you can see the
slot-headed screws, some previous owner has drilled out the EPA
anti-tamper plugs to tweak the carbs, and screwed up the adjustment.

They always make the same mistake. They turn the screws too many turns
counter clockwise, expecting the idle speed to increase.

When it doesn't increase, they wind up turning the idle speed knob up
and then the RPM hangs up when they blip the throttle.

This happens because adjusting the idle speed knob uncovers the
transition ports just downstream of the butterflies. The rider gets
confused, he has no idea what happened.

Maybe he puts the motocycle into storage over the winter and the fuel
evaporates out of the float bowls, plugging up the idle ports. Now the
damned thing keeps stalling, but the idle is way too fast when he blips
the throttle.

What the f***?

This drives the amateur mechanic nuts, he can't figure out what's going
on because he doesn't know how the idle mixture circuits are arranged.

It's this: the idle mixture screws affect ONE of four idle mixture
ports. The transition ports aren't affected by that screw at all.

The carburetor sucks idle fuel out of the float bowl through the idle
jet. It gets idle air through a hole in the carb intake or hidden
inside the diaphragm chamber.

Idle fuel and idle air mix in a passage, then split four ways to get to
the carburetor downstream of the butterflies. Only one of the four
ports is controlled by the idle mixture screw, the other three are
covered by the closed butterflies.

The strategy for adjusting the idle mixture screws, once the carbs are
nice and clean, is to set the master idle knob to the specified RPM and
then turn the four idle mixture screws clockwise about 1/4th of a turn
at a time.

The idle RPM should increase, so turn the master idle knob back down
and close the idle mixture screws more and turn the master idle knob
down again.

The idea is to get the fastest smooth idle with the least amount of
throttle butterfly opening. Some Japanese motorcycles have really large
idle jets, so the correct idle mixture screw setting may be only 1/4 of
a turn out from lightly seated.

 

The screws were uncovered. I shot some solvent through them when I did the
rest of the carb but I never tried to adjust them. I simply counted the
turns (3) and put them back in. I'll have to give that a try later. It's
just that once upon a time I fiddled with those things on another cycle
and it took me forever to get it working again. Probably because I didn't
have a clue what I was doing!
Thanx.

 

Three full turns out is probably far too much, making the idle mixture
too rich.

If you go to the Factory Pro website, there is a good constant vacuum
carburetor tuning guide. It's the best I've ever seen. Mark says that
the idle mixture screw usually have no influence on the idle mixture
once they are 3-1/2 turns open, but I have never owned a motorcycle
that needed the idle mixture screws open more than 1-1/2 full turns.

My Yamaha FZR1000 came from the factory with the four idle mixture
screws set at
1/8th, 1/4th, 3/8ths, and 1-1/2 turns open, so I figure somebody really
screwed those carbs up or took a lunch break while working on them.

I was communicating with a guy that owns one of those Kawasaki
ZX-1200's that look like an old Eddie Lawson Replica, and he told me
that his idle screws were open
about 3.0 turns and that surprised me until I discovered that model has
really tiny idle jets, about # 30 to # 35.

The orifice holes in Mikuni and Keihin jets are based upon a #100 hole
being 1.0 millimeter in diameter. So a # 30 idle jet has a hole that's
0.3 millimeters in diameter and a # 40 idle jet has a hole that's 0.4
millimeters in diameter.

Just glancing at the numbers, you'd get the mistaken impression that
the 0.4 diameter hole would flow 25% more fuel than the 0.3 hole, but
the area is actually
1.77 times the area of the smaller hole.

So, if there is such a large difference in flow through idle jets, you
might find other CV carbs that need to have the idle mixture screws
opened a lot further.

and I'll have to give that a try later. It's

Most amateur tuners don't have a clue about how CV carbs work, so I
have probably posted about a million words on Usenet explaining what
the deal is with CV carbs running on the idle jets 80% of the time and
this confusing business about the transition ports was a mystery even
to me, until I read Mark Salvisberg's excellent tuning guide on
www.factorypro.com