High Idle, Choke Stuck?

Hi I recently got a 2001 Katana 750 and I'm having some trouble with
the idle. When I first got it, it ran fine, but the battery soon died
(possibly because it was low to begin with). I charged the battery but
when I tried to run it again it seemed like it was struggling. It
would idle between 1000 and 1500 rpm but would jerk forward as if it
was about to stall when I would ride slowly in first gear. I thought
it was because of the cold so I opened the choke when I started it
next, and it the jerking seemed to go away. But when I tried to close
the choke, nothing happened. The choke lever seems fine and moves
without a problem, but the engine is now constantly idling at 4500 rpm!
Is it possible that somehow the choke got "stuck" open? I don't know
that the battery dying has anything to do with the choke being stuck,
but everything was running fine until that happened.

Thanks,

Prashant

 

You have the typical dirty carburetor problems that mystify those new
owners who are not acquainted with the care and feeding of constant
vacuum carburetors.

Constant vacuum carburetors work very well when they are clean inside,
but the tiny holes in the jets and internal passages get plugged up
easily.

The gasoline in the float bowls evaporates when the motorcycle sits
unridden for a few months. When that happens, the gasoline turns into
gum and varnish, blocking the idle jets and passages.

Maybe the rider first notices that the engine idles too slow and finds
the master idle knob under the carburetors and adjusts it so the
engine idles at the right speed, but he doesn't notice that he now has
to play with the throttle grip to get the engine to start.

Then he lets the motorbike sit unridden some more and more gasoline
evaporates
and plugs the jets even more. When the idle jets and idle ports are
totally plugged up, the engine will only run on the choke, which isn't
a choke at all, it's a rod or cable operated valve that opens when the
rider moves the "choke" lever to the ON position.

The little valve opens to allow gasoline to be sucked directly out of
the float bowl. But the rider discovers that the engine will only run
on the choke and when he opens the throttle it jerks and stalls because
the idle jets and idle passages are all plugged up

But fresh gasoline has alcohol additives that are designed to clean out
the idle mixture passages and jets. The alcohol starts working, but it
works too slow.

The best way to clean the carbs out quickly so you can ride is to put
about 4 ounces of Berryman's B-12 Chemtool Choke and Carburetor Cleaner
in a full tank of gasoline and go for a slow ride while the xylene,
acetone, and methyl alcohol clean out the passages.

If you cannot buy B-12 in a place where people named "Prashant" live,
try a carburetor cleaner made locally that doesn't have "petroleum
distillate" in it. That sort of fuel system cleaner is for fuel
injection systems.

As you ride your motorbike with the B-12 or the carburetor cleaner in
the gasoline, you will notice that the engine idles too fast and you
will want to turn down the idle RPM using the master idle knob
underneath the carbs.

Turning the idle knob down causes the carburetor butterflies to cover
up the transition ports that give the engine extra gasoline for
acceleration when you first open the throttle to speed up.

The transition ports aren't supposed to be uncovered at idle RPM, but,
when the rider first noticed the engine was idling to slow and he
turned the idle RPM up, the transition ports were uncovered. But, since
the idle mixture passages were already getting plugged up, the engine
RPM didn't run away.

If you are not the first owner of this motorbike, you should take a
mirror and look at the underside of the carburetors, just forward of
the float bowls. If you can see a slot headed screw in a hole there,
the anti-tamper plugs were probably drilled out by a previous owner,
and amateur tuners always make the mistake of turning the idle mixture
screws too far counterclockwise, thinking that this will make the
engine idle faster.

When they go too far, the idle RPM actually slows down, and they turn
the master idle knob to raise the idle speed to the correct RPM. But
this uncovers the transition ports that I previously described, and the
engine idles at a very high RPM.

 

Or maybe a problem in the mechanical linkage between the
choke lever and the choke, like he says.

Since the problems began when he tried to turn on the chike,
I'd definitely look for a problem in the choke cables and/or
return springs. Frayed cable or non-functional return spring
is a reasonable suspect.

You been sipping the B-12 again ?

 

If the choke is stuck closed due to linkage problems the engine will
eventually drown in gasoline and stall out. Naturally he should check
the choke linkage but the problem symptom is classic dirty carbs,
like B-12 said.

Ted

 

That motorbike doesn't even have a choke.

Go look at the parts diagrams on the website of your choice and find a
spring associated with the starting enrichener mechanism.

You been sipping the EDTA?

 

Dirty carb problems are not limited to constant vacuum carbs.

Any kind of carb works well when it's clean but then works like crap when
it's passages get all varnished up.

It does not work at all if the passages are completely plugged up.

No, the best way is to put a 50-50 mix of gas and berrymans in
the float bowls, as you have detailed before, and cross your fingers
and hope that the mixture passages are still passing some small amount of
fuel.
If they are not, and this does not work, then take the carbs into a place
and have them ultrasonically cleaned. Do not screw around with drilling
plugs and such and passing wires through orifices.

Then before putting the carbs back on, make sure that they are visually
synchronized, as the former owner who let them get all gummed up may
have screwed up the synchronization in a desperate effort to get things
to work.

There are 2 kinds of carburetor cleaner out there. The first kind is the
kind you dunk the entire carburetor into to soak it. This is not the kind
that you pour into the gas tank and is not what you want. What you want
is more along the lines of "fuel system cleaner"

Most "fuel system cleaners" particularly those sold in mass-market stores,
are in fact of the "petroleum distillate" kind.

Ted

 

You're saying there's a choke mechansm with a mechanical
linkage and no load mechanism to cause it to return to off
when you put the choke lever to off ?

Pehaps that's the problem.

 

I seriously doubt that the plunger spring is the source of the OP's
high idle RPM problem, even though there are four springs, one on each
plunger.

The sliding operating mechanism grips the end of each plunger and the
plungers all have to move together. So, even if there was a problem
with one spring, that plunger would still be pushed shut by the
movement of the mechanism.

The most likely diagnosis is that his idle passages and idle jets are
clogged up and that he has turned the master idle knob too far and the
butterflies have uncovered the transition ports making the fuel air
mixture far too rich for the load at idle.

 

Plungers in an enrichener circuit ?

I would agree with your diagnosis except for his statement that a
whole new set of problems and symptoms started after he tried to
use the choke and continued after he put the choke off again.

That leads me to suspect something not really returning to
an "off" position. Most likely a cabling issue.

 

That's what the little cylindrical thingumbobs are called.

When the owner lets his motorbike sit until the idle circuits plug up,
the symptom is that the engine runs on the choke, but won't run when
the throttle is cracked open.

The, when it runs a little better, he turns the master idle knob,
uncovering the transition ports.

But the transition ports are plugged up. As the engine runs more,
alcohol additives in the gasoline begin to clean out the idle circuits.

Fuel begins to flow out of the transition ports. The idle RPM runs
away.

 

It's fine if you want to assume he fiddled with the idle afterwards
but that's not what he said:

That sounds like a mechanical binding problem to me. To get to
clogged jets, you've got to read stuff into his problem description
which he never told you.

Taking his description at face value, which is usually a good
starting point, it says mechanical linkage rather than fuel flow.