Start engine and warm to normal operating temperature (cooling fan comes on). Disconnect vacuum hose from throttle opener diaphragm. See Fig 1. Connect a hand-held vacuum pump to diaphragm.
Apply at least 15-20 in. Hg of vacuum to diaphragm. Engine speed should stabilize and remain at 1500-2500 RPM. If speed is too low, widen slot in dashpot speed adjustment lever. If speed is too high, narrow slot in dashpot speed adjustment lever.
Disconnect vacuum hose from dashpot diaphragm. Connect a jumper wire from positive battery terminal to Yellow/Black wire of emission control box connector. Slowly increase engine speed to 4000 RPM and release throttle.
If throttle takes less than 3 seconds or longer than 5 seconds to return to idle, remove emission control box cover and throttle opener control valve cover. Loosen lock nut and turn adjusting nut clockwise to decrease time or counterclockwise to increase return time.
If throttle opener time cannot be changed, check voltage at throttle opener solenoid valve. Connect voltmeter positive probe to emission control box connector Yellow/Black wire and negative probe to Black wire.
If 12 volts exist, throttle opener solenoid or throttle control valve are at fault. To test throttle control valve, remove vacuum hose to throttle opener solenoid. If vacuum is present, replace faulty throttle control valve.

Fig 1: 1975 Throttle Opener System

Courtesy of AMERICAN HONDA MOTOR CO., INC.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

1975 Models