10
2. Check the carburetor's mid speed needle / spray bar. Set the mid speed needle / spray bar (small
screw in the center and on the side of carburetor body) so that its outer most edge sits flush with the
carburetor body side. Once this is set, do not move it again. Only make engine tuning adjustments
with idle screw, high speed, and low speed needles.
3. Follow the procedure for starting the engine.
4. To set the idle, turn the idle screw clockwise (higher RPM) or counterclockwise (lower RPM). Basically,
the idle needs to be set at the lowest possible RPM before the engine stalls.
5. With a running engine, run the car back and forth in a straight line (full throttle achieved during each
passage) in an open parking lot. Repeat and note the sound of the exhaust. Do not hold the throttle
open when the engine is without load (car off the ground), or serious engine damage may result.
6. Review the sound noted in STEP #5. If the exhaust does not reach a high pitch note, then the high
speed setting is rich. Turn the high speed needle (long needle extending from carburetor body, pointing
up) 1/8 turn clockwise, and repeat STEP #3.
7. Repeat STEP #6 until the engine reaches peak setting (turning in the high speed needle will not yield
a higher pitch, and it may even cause the engine to bog at full throttle; however, turning out the needle
will cause the engine's full throttle RPM to drop a little). For normal operations, the optimum setting
for high speed needle should be 1/2 to 1/4 turn counterclockwise from the peak setting. Continuous
engine operation at peak setting may result in overheating and engine damage.
8. To set the low speed needle (small hex screw inside the carburetor's slide/lever), the engine needs to
be broken in and high speed needle needs to be set first.
9. Repeat STEP #5 every 12 seconds (2 seconds at full throttle and 10 seconds at idle).
10. The engine RPM at idle should remain constant, drop to a lower RPM after a few seconds, and remain
at that low RPM without stalling.
If the engine stalls after a few seconds at idle, then the low speed setting is too rich.
Turn the low speed needle clockwise 1/8 turn.
If the engine RPM stays the same or races at idle, then the low speed setting is too lean.
Turn the low speed needle counterclockwise 1/8 turn.
11. Repeat STEP #9 and #10 until proper engine low speed setting is achieved.
When running the PRO-70BX, the optimum carburetor setting is rich (high speed needle set at 1/2 to
1/4 turn counterclockwise from the peak performance setting), and as such, it is normal for the engine
to sound rich and expel a lot of smoke. Tune this engine so that it is at maximum fuel delivery (both
high and low speed) without stalling. At this optimum setting, the engine should continue to operate at
normal temperature range (low 100s C or mid 200s F); thus, consistent engine temperature and performance
over extended period of time. At peak carburetor setting (not optimum setting), engine performance will
be much better initially, but it will soon overheat and become inconsistent. Never operate this engine at
peak or lean carburetor setting.
Summary of Contents for EK4-S2
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