bike won't come down to idle after carb cleaning

Hi,

I took apart and cleaned the carbs on my 2001 Yamaha YZF600R and now
when riding, the engine won't come back down to idle when I back off
the throttle. This almost never happens in the driveway, only when I'm
actually riding. I'm certain the throttle is returning. Any ideas as
to what else could cause this? If it's an air leak, where are the most
likely areas it could occur, and how can I test for this?
Thanks in advance,
Jim

 

Despite what you say re: the throttle returning, the most likely cause is
misrouted/kinked throttle cable(s). Check it by moving the bars side to
side while playing the throttle. If you are positive that the cabling
is OK (including the 'choke') then start looking for intake leaks,
misaligned slide diaphragms, missing/loose vacuum hoses, etc.

 

I took apart and cleaned the carbs on my 2001 Yamaha YZF600R and now
when riding, the engine won't come back down to idle when I back off
the throttle.

As a carburetor gradually gets plugged up with gumand varnish, many
riders will compensate for the gradual loss of idle rpm by turning the
idle speed up a bit with the master idle knob...

Then, when you clean out the carbs, the clean idle mixture jets allow
more gasoline to flow through them, and the engine idle rpm is too
high...

The solution to fixing that is to
just turn the master idle knob counterclockwise a little bit...

But then there is the problem of
"slow-return-to-idle" that isn't caused by throttle butterfly
position...

It can be caused by a slightly open starter enrichment valve...

That's what CV carbs use instead of a choke. There's a small passage
with a valve operated by the "choke" lever, and when you move the
lever, the valve opens and the carb sucks gas directly out of the float
bowl...

Does the engine slowly return to slow idle after about 15 or 20
seconds?

Did you mess with the idle mixture screws at all? The
slow-return-to-idle symptom you describe occurs when the idle mixture
is a bit too rich...

To do a decent job of carb cleaning, the EPA plugs concealing the idle
mixture screws have to be carefully pilot-drilled, a sheet metal screw
is threaded into the pilot holes and the plug is pulled out with a pair
of pliers...

Then the idle mixture screws must be screwed all the way in until they
bottom lightly, counting and recording the number of turns to bottom...

When the idle mixture screws are removed, watch for the tiny
spring, washer, and o-ring. The o-ring may get stuck in the hole and
then lost during cleaning...

Carb cleaner from a spray can may then be squirted through the idle
mixture orifices...

Holding your finger over various holes to block the flow of carb
cleaner will make the cleaner squirt out of different holes...

Like, there is a passage into the float bowl, and another passge in the
intake bell where carb cleaner will squirt out of if you block the idle
mixture orifices in the carburetor main bore with your finger...

When you re-install the idle mixture screws, screw them in until they
lightly bottom and then turn them back out the same number of turns you
recorded...

You might open each idle mixture screw 1/4 to 1/2 extra turns to get
more off-idle and mid-range throttle response, up to the point where
excess idle mixture richness makes the idle rpm too high with the
throttle closed...

It's annoying to be trying to take a slow corner on closed throttle and
have the engine RPM hang up and drive you into the corner faster than
you want to go...

 

To be honest, I've *never* had this problem except with bikes that have
been laid up for long periods of time.

Bikes that are used regularly don't suffer, IME.

 

With the engine running, spray WD40 at each of the joints, in front and
behind each carb in turn.
If there's an air-leak the fluid will momentarrily seal and you'll hear the
engine sound change.
If it's a bad lead you'll also see some white smoke from the exhaust.

Dave

 

If it's anything, it sounds more like an overly rich mixture than
an air leak. Might also be something dumb like a broken or
snagged return spring or frayed cable.

If it idles fast consistently when its hot I'd look for mixture
problems. If it sometimes drops back to a good idle and sometimes
runs fast then I'd look for mechanical problems in the cable
or carb.

Could also be a problem with ignition advance I guess but it'd
be strange to have this coincide with a carb overhaul.

Did you take apart the butterfly assemblies ? If so, I'd take a
hard look there too.

 

have been laid up for long periods of time.

In the climate where I live, the average temperature from June to
September is 99 degrees F, and the average annual temperature is about
82 degrees F...

In a non-airconditioned garage (and how many of us motorbike riders
have airconditioned garages,anyway?) the ambient temperatures will
reach 140 degrees F, and all the gasoline in the float bowls will turn
to
gum and varnish...

A set of CV carbs with all those tiny passages will gum up in about two
months...

To combat this problem, at least a little bit, I insulated the garage
and installed a fan so the temperature inside the garage never gets
higher than the outside temperature...

 

Ah, right, yeah!

I live in the cold and gloomy UK where the annual temperature isn't
*quite* that high and so it isn't a problem.