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Join Date: Mar 2006
Location: Kyle, Texas
Cindy, you know I'm no expert, so this is the way I understand it. Some of it from experience, since I had carb problems with both of my KZ's and had to work through them, with help from the carb guru, Wired George. I learned a lot from him, but certainly haven't achieved and am far from expert level!
As an engine warms up, the fuel to air ratio has to change. Cold engines need more fuel to run, thus the choke. But as an engine warms up, it needs more air/less fuel to run properly.
On a warmed up engine, too much fuel will bog it down, slow down the idle and make it run sluggish. Because so much fuel is left unburnt, when it hits enough air, it can cause backfiring through the exhaust. You usually will see black exhaust, and smell gas in the exhaust.
When a warmed up engine gets too much air, it will run lean, hotter, and idle will increase. If it's just badly adjusted, idle will be more stable, but will still rise as the engine heats up. If there are vacuum leaks, the idle will be more variable, as vacuum increases, and sucks more air into the intake. Also, vacuum leaks can cause a piston to get a hot spot and literally melt, if left un repaired.
To adjust a carburetor to the right mix at idle, you should warm up the bike, then while it is idling set the idle adjust knob so that it is idling at 1100 rpms, turn in the idle mix screw on carb one until the idle starts to drop, then turn it out until you achieve maximum rpm, go back in 1/4 turn and adjust the idle adjust knob back down to 1100 rpms, then repeat the process with carb two. This should be the correct idle adjustment, not taking into effect any adjustments for popping on decel from marbling etc.
Of course, none of this works right if there is a vacuum leak, or your choke is not turning off all the way, or if there are other carb problems, like clogs, stuck floats, etc.
That's all I know about it!