View Full Version : Adjusting Idle Mixture Using Propane

Mike Hart
07-14-2002, 02:02 PM
Adjusting Idle Mixture Using Propane
Posted to email list by Wally "wdfrie" on Mar 10, 2001
Edited by Mike Hart
Use this procedure at your own risk

FWIW here is how I adjust the idle mixture.

Leaking small amounts of propane into the intake velocity stacks of an idling motorcycle can be used to adjust the idle mixture screws, because if the idle mixture is too lean, the engine RPM will increase, If the mixture is rich the RPM will decrease, and a properly adjusted mixture has little effect, or a slight increase in RPM. When I adjust my carbs I try to get about 25 RPM increase, or a slightly lean mixture. Adjusting the idle mixture is the last step of an entire tune up and should only be attempted after the following has been completed. Cam chain adjusted, Valve lash adjusted, Carbs synchronized, float height correct, and carbs are clean and operating properly. The first time I tried this my original idle imitated a John Deere tractor.[list=1]
Remove the air cleaner cover.
Remove the seat and gas tank retaining bolt and block the tank
with a 2x4 to raise the rear of the tank.(tank must be full to keep
gas flowing)
Remove the plastic covers over the idle mixture screws.
Connect synchronization gauges to carbs.
Start and idle the motor to warm it up (I use a large fan to
keep air moving past the engine) (see NOTE below}
Synchronize carbs
Light a regular propane hand torch and adjust the flame as
small as possible without going out. My torch will burn with a
flame about 1'. Draw the size of this flame on the side of your
toolbox. The next time you adjust the carbs you will have a
picture of what the flame was like.
IMPORTANT!!!!! Blow out the flame but leave the propane torch leaking propane. Direct the end of the torch into the velocity stack of each carb, and note the change in engine RPM. Point the propane away when not actually adjusting.
Adjust the idle mixture screws so propane leaked into the each carb results in about 25 rpm increase.
Synchronize carbs
Readjust the idle mixture screws so propane leaked into the
each carb results in about 25 rpm increase.
Turn off the propane and listen to the smooth idle of your motor.
Report the result of the procedure back to the group, or any
improvements you can suggest. I hope someone with a colortune
can complete this and report the difference between propane
and color tune adjustments.

NOTE: do this in a well-ventilated area! A buildup of propane and a backfire isn't something you want to see.