My machine will not drop idle when just running

check the MAP sensor (manifold absolute pressure). The ECM (car computer) uses that to keep idle at minimum 750 RPM. It's normal for it to run high when cold. These small 4 banger motors typically run hot, and they won't run right IF they're NOT running hot, ergo the idle is usually high when it is cold so as to cause it to warm up quicker. If the RPM's drop below 750, suspect the MAP sensor, &/or ECM.

which car is it?

problem with 1993 Geo Prizm '99 silverado 5.3 engin will not idle runs at 2500 rpm and drops down to 1000 rpm the back up to 2500 rpm -------------------------

is ther a check engine light

Sleep Mode
Setting the Sleep Mode reduces power consumption while the machine is idle.
You can choose how long the machine is idle (from 00 to 99 minutes) before it
drops into sleep mode. The timer automatically resets itself after the machine
receives a fax or PC data, or makes a copy. The default setting is 5 minutes.
1 Press Menu, 1, 8.
2 Use the Dial pad to enter the length of
time the machine is idle before it drops
into sleep mode (00 to 99 minutes), and then press Set.
3 Press Stop/Exit.

check and clean the carburator... doble check the fuel line in the carb....

Have you checked the water filter to see if it is plugged with weeds. If the engine is over heating it will drop to idle automatically. If the filter is clear there is a small cooling water elbow (90 degree fitting ) below the exhaust ports of the exhaust manifold. If a small pebble or piece of crud gets caught in the elbow your machine will continually over heat. Be sure you have a water filter on the machine and clean/clear it carefully so this is not a problem. If everything is clean/clear and you continue to drop to idle you may have a bad over heat safety switch( does happen but not to often).

Setting the Sleep Mode reduces power consumption while the machine is idle.

You can choose how long the machine is idle (from 00 to 99 minutes) before it drops into sleep mode.

The timer automatically resets itself after the machine
receives a fax or PC data, or makes a copy. The default setting is 5 minutes.

1 Press Menu, 1, 8.
2 Use the Dial pad to enter the length of
time the machine is idle before it drops
into sleep mode (00 to 99 minutes), and then press Set.
3 Press Stop/Exit.

Check fuel flow into the right carburetor (drop the float bowl). If it tapers off, remove the float and needle valve, then try again. If it seems better, put things back together and try the machine again. Hope this gets you running!

Sleep Mode
Setting the Sleep Mode reduces power consumption while the machine is idle.
You can choose how long the FAX/MFC is idle (from 00 to 99 minutes) before
it drops into sleep mode. The timer automatically resets itself after the machine
receives a fax or PC data or makes a copy. The default setting is 0 minutes.
1 Press Menu/Set, 1, 7.
2 Use the Dial pad to enter the length of
time the machine is idle before it drops into sleep mode (00 to 99 minutes),
and then press Menu/Set.
3 Press Stop/Exit.

Check the fuel system, chenge fuel filter. clean the fuel tank.