Honda Nighthawk/Fuel System

1984-1986/SB-700/750SC

CB700/750SC Petcock. At the top is the brass fuel tube with the intank filter removed and used to draw fuel in the "ON" position. , Left side is the fuel supply to carb assembly, far right is vac hose to carb, at bottom is the fuel cup

The Petcock (fuel valve) seems to be an on going issue. This device serves a number of functions which may cause confusion when troubleshooting. They are:

1) Manual fuel ON valve setting for normal operation

2) Manual fuel RESERVE valve setting for reserve operation

3) Manual fuel OFF valve setting

4) Automatic fuel shutoff valve which overrides the manual settings. This valve is operated by engine vacuum via the small rubber hose. The valve opens when the engine is started and closes when the engine is stopped.

5) Primary and secondary fuel filtration. One protrudes into the tank, the other is in the small fuel cup at the bottom of the unit.

Before proceeding, make sure there is a fire extinguisher handy

If the bike has been recently running, turn the petcock to OFF and unscrew the small fuel cup cover on the petcock. Inspect and clean the small circular fuel strainer. Reinstall strainer and cup.

To determine if the petcock is working properly, turn the petcock to the OFF position. Disconnect the fuel line from the petcock and the smaller vacuum line from the carburetor. While holding an empty can under the petcock, turn the valve to the ON position. There should be no gas flow. Suck on the vacuum line or use a hand vacuum pump. Gas should start to flow into the can. Release the vacuum on the line and gas should stop flowing.The above test should also work when the petcock is in the RESERVE position.

If there is no gas flow . Turn the petcock to the OFF position and Ref 4-16 remove the gas tank The manual suggests servicing the petcock while still attached to the tank. It is better to remove the petcock completely and check the filters and the fuel valve diaphragm. If the tank is full, drain out most of the gas through the fill hole. Ref 4-17 in the manual(bottom of page) loosen the petcock lock nut and remove petcock. There should be a tube like fuel strainer attached. If not, check the tank for the strainer and remove it. Remove the O ring. Drain the remaining gas from the tank. Ref 4-16 (bottom of page) carefully remove the cover, spring, and both diaphragms. If the bike has sat for a while, chances are that the diaphragms are stuck in the closed position from gas residue. Exercise care in freeing them. Remove the fuel cup, and the small circular strainer.

Set the petcock valve to ON and inspect the tall fuel tube which sticks up into the tank for gum and varnish residue. Clean with carb cleaner and blow out with compressed air. Set the petcock valve to RESERVE inspect the opening at the base of the tall fuel tube and clean as above. During use, recommend using the RESERVE setting occasionally to prevent the buildup of gum in the passage.

Part of the petcock is assembled with rivets. Some internal components may be rubber and could be affected by prolonged soaking with carb cleaner.

Do not use carb cleaner on the rubber diaphragms. Inspect for tears and wear, Replace if required.