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Posted by =?ISO-8859-7?B?w8TIy87Q09bY2Q= on August 11, 2008, 9:30 pm
> i have a 1983 yamaha xj750 seca. bike runs great but been having a
> problem with the idle. sometimes it idles fine. But other times it
> will idle up to about 4000rpm and stick up there and slowly go back
> doen or just stay there.
http://webservices.motorsportdealers.com/parts/partImages/YAM/2/02/0765/001=
2.Gif
This drawing doesn't even show the secret idle mixture screws.
Probably nobody has ever adjusted the idle *mixture* screws. They are
probably still hidden underneath small metal plugs just forward of the
round diaphragm cover on top of the carbs. Or they might be underneath
the carbs, just forward of the float bowls.
Whatever. If you see slot headed screws on the carbs, they are the
idle mixture screws.
Home mechanics who fiddle around with the idle mixture screws expect
the idle RPM to keep increasing as they turn the screws
counterclockwise, but, when they go too far, the idle mixture gets too
rich and the idle RPM slows down, so they turn the master idle knob
clockwise to get the engine to idle faster.
That uncovers a pattern of acceleration transition ports downstream of
the throttle butterflies and the engine idles too fast when they twist
the throttle grip suddenly and turn it back to idle. The RPM will hang
up around 4000 RPM.
OTOH, maybe nobody has ever messed with the idle mixture screws and
what you're really having a problem with is dirty carburetors and
sticky float valves.
If the float valves stick and the gas level in the float bowls rises
too high, the engine will act just like somebody has fooled around
with the idle mixture screws.
The solution to that problem is to add 4 or 5 ounces of Berryman B12
Choke and Carburetor Cleaner to a full tank of gasoline and go for a
long ride.
B12 comes in a 15 ounce non-aerosol can so you can measure it into the
gas tank. Don't get any on your paint, it contains fingernail polish
remover.
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