This is (or should be) an annual part of your maintenance ritual. Curiously it is mentioned in the factory service manual for all fuel injected Vulcan cruisers but omitted in the owner's manual (mom*). As you ride, or more accurately each time you're finished riding and turn your engine off an intake valve may be left open. A little bit of unburned hydrocarbon and other gas wafts out of that valve and drifts out to the throttle body opening. Carried with that gas are tiny specs of carbon that settle on the throttle body bore right behind the brass butterfly valve. Eventually enough carbon settles to form a tiny ring on the wall that begins blocking idle air. Many riders begin noticing the problem as inconsistent idle and just crank the idle adjustment knob up a little. That will only help mask the issue for awhile and eventually enough carbon will build up so the engine won't idle at all. The fix will just take a couple of minutes.
The photo at the top of the page shows what the Vulcan throttle body looks like. Two throats with a brass butterfly in each. The photo (right) I'm using as an example only because it's very dirty and I couldn't actually find a filthy motorcycle intake picture (guess we're a little more scrupulous than some car owners). A throttle body doesn't actually have gasoline flowing through the throat like a carburetor does ( injectors are adding fuel to the mixture further along the line) so there's nothing to wash off carbon accumulations. All the throttle body throat is doing is controlling airflow so as you can see it can get really filthy.
What You'll Need
- 1 - Can of throttle body cleaner or any carbon buster (Seafoam is good)
- 1 or more swabs to remove the carbon ring. An old toothbrush is the best tool for this but you can use a gun cleaning swab, Q-tip on a long handle or even a little piece of cloth rubber banded to the end of a pencil.
How To Do It
- Remove the air box cover (right side) or air cleaner if you have an aftermarket intake mounted on the right side. Once done you'll be able to look straight into the dual throttle body throats.
- Open the throttle and look carefully (use a flashlight if necessary) and you'll probably notice a very thin carbon ring on the throat wall right where the butterflies contact the sides. That carbon blocks idle intake air making the warm idle very unstable. If the ring gets large enough it can cause the engine not to idle at all.
- With the throttle completely open (use a throttle lock or helper) grab your swab of choice.
- Spray some throttle body cleaner into the throat and on your scrubber. Go to work on the ring until it's gone. Check the edges of the butterflies to be sure no carbon has attached to the edges. If it has, very CAREFULLY wipe it off. You don't want to bend the soft brass.
- Put the cover or air cleaner back on the bike and start the engine. Sometimes they'll start a little hard if you got carried away spraying cleaner into the throttle body. If you've been adjusting the idle knob to keep the bike running you'll probably discover (once the engine is warm) that the idle is way too high. Crank it back down to the recommended 950 (just below where you get an annoying fast vibration in the handlebars) and you're good to go another year.
Note: If you have a fuel injected car the same thing is happening in the car/truck throttle body. Fortunately in most idle air comes from a different source but the carbon ring should still be cleaned out periodically. If your car/truck idle doesn't seem as smooth as it once was, the throttle body bore is the first place to look.
*MOM=Motorcycle Owners Manual