Maintenance - UMS 7-90 cc User Manual

Low-Speed Needle Adjustment
Low-speed needle adjustment controls the fuel/air mixture at idle and the quality
of the transition between idle and wide open throttle.
Low-Speed Needle Tuning
1. Adjust the high - speed needle for the current conditions.
2. Allow the engine to idle for 10–15 seconds. Rapidly advance the throttle from idle
to full open.
If the engine transitions with no hesitation, the low-speed needle is adjusted perfectly.
If the engine slowly responds to throttle change and gradually reaches peak RPM, the
low-speed needle is too rich. Lean out the low-speed needle (turn clockwise) 1/16
turn and test again.
If the engine immediately stops running, the low-speed needle is lean. Richen the low-speed
needle 1/16 turn and test again.
3. When the low-speed needle is adjusted correctly, check the high-speed needle setting again.
4. When you are satised with the needle settings, you should not need to adjust the low-
speed needle again. The low-speed needle setting is mechanically controlled and does
not change with atmospheric conditions.
The best indication that your engine is tuned properly is the color of your spark plugs when
removed from the cylinder. A nice dry tan color is an indicator that the fuel mixture is correct
and burning well. Anything else (wet plugs, or carbon deposits) generally indicates you are
running the engine too rich.

Maintenance

There are a few things you must do to ensure proper
operation of your UMS radial engine.
1. Use a high quality gear oil to lubricate the outer
parts of the valve train at the beginning of each ying day.
2. Occasionally, you will need to adjust the valves.
Adjust the valves the rst time after one
hour of engine run time. After the rst hour,
monitor the valve lash and adjust the valves
as needed. Adjust the valves when the engine
is cool. The lash is set at 0.05mm–0.08mm
at the factory.
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