All Models (Engine Hot)

Connect a tachometer to the engine. Start engine and allow to reach normal operating temperature (cooling fan on).
Check throttle opener speed by disconnecting hose from the throttle opener diaphragm and connecting a hand vacuum pump.

With engine running, apply at least 16 in. Hg for more than one minute and check that engine speed rises to RPM shown in the following table.

Throttle Opener Speed

NOTE: Engine should rise to specified RPM within 1 minute.

If necessary, adjust engine speed; if speed was too low, widen adjustment slot in dashpot speed adjustment lever using a screwdriver; if too high, narrow the slot with suitable pair of pliers; if speed cannot be adjusted, or diaphragm will not hold vacuum, replace the diaphragm and repeat test.
Reconnect vacuum hose and raise engine speed to 3500 RPM and maintain for 2-3 seconds. Quickly release throttle.
Engine should return to idle in 1-4 seconds. If return time is correct, proceed to "Throttle Opener Check".
If return time is less than 1 second, "T" a vacuum gauge into the lower vacuum line at dashpot check valve. Vacuum should be at least 1.2 in. (30 mm) Hg at 4000 RPM. If so, replace control valve. If less, clean carburetor port and hoses and repeat test in step 5).
If idle return time is more than 4 seconds, clamp hose between throttle opener solenoid valve and control valve, then repeat test. If no change, replace control valve. If test is now within limits, check for voltage at solenoid valve. If voltage is present, replace speed sensor. If no voltage, replace throttle opener solenoid valve.

Fig 1: Widening Dashpot Speed Adjusting Lever

Courtesy of AMERICAN HONDA MOTOR CO., INC.