CB80 Idle Hunting .
An annoying phenomenon where the engine revs pitch and fall with monotonous regularity. Its generally attributed to air bottling around the ECU water temperature sensor and air valve under the throttle plate . The prescription is to make sure the air is bleed from the coolant system at the high points after a refill.
Other nominated causes have been associated with vacuum hose leaks, incorrect vacuum hose connection and leaking head gasket.
The idle up circuit can also cause problems. The idle up vsv valve opens when the lights are turned on, the cabin fans starts, the air conditioner is running. etc.
"Your post makes perfect sense Richard.
You have too much idle bypass air. Adjust the screw on the TB until the hunting stops.
The reason it's affected by the fans setting is that additional electrical load opens a VSV valve to add more idle air to help the engine with the additional load from the alternator.
The reason the 3rd bar stops it from hunting is they probably sized the VSV to allow for the air required for 3 bars or, more likely, just the headlights on dip.
The stalling during gearshifts means either you have disconnected the dashpot or the piping for it is connected incorrectly. Look at your throttle stop. You should be able to pull it out a few mm. It should extend automatically when you open the throttle. Check the vacuum diagram.
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Cheers,
Ruaraidh
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A faulty engine coolant temp sender (the green one) on the CB80 engine will cause the engine to hunt at idle, especially when warm. the symptoms are very similar to the hunting caused when there is little or no coolant in the engine, or when the wax-stat in the throttle body requires bleeding.
hope this helps,
Fraser
