UMS 7-90 cc User Manual page 10

1. Start the engine.
2. Move the throttle stick up until the engine is running at approximately 2,000 rpm.
3. If you are using separate exhaust pipes, use a temperature gun to make sure hot exhaust
is exiting all engine cylinders. If you are using a collector ring, use a temperature gun to make
sure all the cylinder temperatures are similar and ring smoothly.
4. Move the throttle stick up to full (open) throttle. If the engine runs rough or if some cylinders
are not operating, the fuel mixture is too rich.
5. Lower the throttle stick until the engine reaches 2,000 rpm. Lean the high-speed needle
(turn clockwise) 1/16 turn.
6. Quickly raise the throttle stick to full (open) throttle. If the engine stops running,
the high-speed needle setting is too lean.
7. Lower the throttle stick until the engine reaches 2,000 rpm. Richen the high-speed
needle (turn counterclockwise) 1/8 turn.
8. Repeat Steps 4–7 until the engine is running reliably at full (open) throttle.
High-Speed Needle Tuning
10
Once the engine is running reliably at full (open) throttle:
1. Lean the high-speed needle 1/16 of a turn at a time.
2. Wait 2–3 seconds for the engine to respond.
Radial engines do not respond immediately
to tuning changes.
a. If there is an increase in RPM: Lean the
high-speed needle an additional 1/16 turn and
wait for the engine to respond to the tuning
change. If there is no increase in RPM
after the change, turn the high-speed needle
back to its previous position.
b. If there is a decrease in RPM: Richen
the high-speed needle 1/8 turn and wait for
the engine RPM to stabilize.
3. Repeat Step 2 until you determine the
maximum RPM with the fuel and propeller
you selected.
4. Richen the high-speed needle so when full throttle
is applied, the engine rpm will be steady
(not drop off) after reaching its peak RPM. Richening
the high-speed needle allows the RPM
to increase when the airplane is ying.