Tuesday, July 19, 2011

Yamaha Motorcycle Engine won't idle

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

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

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...---------------------------------------------

It is very hard to get these carbs
clean without soaking them in Yamaha carb cleaner.
The jets and passageways will not clean out on their
own but Seafoam is a good start.

The idle jets usually need to be physically
cleaned with smooth wire like a fine guitar
string or they must be replaced.

After sitting so long the carbs will have dried
fuel varnish thoughout. If you do not want to clean
them or remove them for whatever reasons I suggest
filling the float bowls with Yamaha carb cleaner
and letting the carbs sit overnight.

Seafoam may help but often the small idle jets
need physical cleaning.

There is some chance that the carb mounting rubbers
could be leaking vacuum. They can dry out or
even crack which can allow air instead of
fuel to enter the engine.

I would turn the idle screws out a few turns to start
and adjust them individually for the highest idle speed.

Use regular fuel as it has a faster burning rate than
premium fuels. Premium fuel is designed to burn slower
to prevent engine knock or pinging.

Jetting must only be done once it is running
better and warmed up.
Read the spark plug color and if they are
a light color increase the main jet size
until plugs read light brown on a fast run.

I think you just have more carb and jet cleaning
to do but there are rare times when ignition
connectors and battery condition can affect
the electronic ignition.

Make sure you have plenty of fuel getting to the carbs as well.

Blow out the carbs with compressed air and be careful
not to damage any rubber parts.



  • Begin by adjusting the throttle cable. Start the dirt bike and apply enough throttle to keep the engine running. Tighten the tension on the throttle cable using the adjustments at the hand grip and on the carburetor. If the bike idles, the problem has been corrected and you can fine tune the idle setting. If not, proceed to the next step.

  • Remove the hose clamp that secures the fuel line to the carburetor. Detach the line and hold the open end over a cup. Turn the engine over. If no fuel pumps into the cup, remove and replace the fuel pump. If gasoline pumps into the cup the fuel pump is okay. Proceed to the next step.

  • Shut off the gasoline supply valve. Loosen the cap on top of the carburetor and slide the cap up the throttle cable and out of the way. Remove the throttle cable from the carburetor.

  • Loosen the nuts that connect the carburetor to the intake manifold. Remove the carburetor and place it on workbench. Remove the idle adjustment screws from the carburetor body. Remove and replace the air filter cartridge.

  • Remove the bowl at the underside of the carburetor. Use a screwdriver to remove the main jet and needle jet from inside the bowl. Clean the bowl and float with spray carburetor cleaner. Wipe the excess cleaner with a rag. Clean the jets with spray carburetor cleaner and replace them into the body.

  • Reassemble the carburetor and install it on the intake manifold. Connect the fuel line and throttle cable. Refer to the owner's manual for idle adjustment settings. Set the idle screws, turn on the fuel supply and start the dirt bike.


  • Remove the fuel line that connects to the injector. Hold a cup under the end of the line and turn the engine over. If the fuel supply is weak or non-existent remove the fuel pump and replace it. If you have a steady fuel supply the fuel pump is okay and you can proceed to the next step.

  • Turn off the fuel supply valve. Remove the air filter at the end of the intake. Remove the nuts that hold the intake to the engine head and remove the intake. Remove and replace the intake manifold sensor on the underside of the intake. Remove and replace the fast idle mechanism on the side of the intake.

  • Reattach the intake to the engine manifold. Refer to the owner's manual for torque specifications and tighten the intake nuts to the specified torque. Reconnect the line from the fuel pump to the injector. Wipe away any residual gasoline with a rag. Start the dirt bike.

  • If the idle problem is not corrected, have a service technician check the crank angle sensor and the electrical signal to the throttle position sensor.