Honda Nighthawk/Carburetor Cleaning


1985 CB700/CB750SC Nighthawk S Carburetor

Rough idling, hard starting or other performance problems may be due to dirty carburetors. If the bike has sat for a few years or the gas tank shows signs of varnish and gum deposits the carb should be suspect.

When servicing carbs be aware that:

1) Parts are fragile and become more so with age.

2) Parts made from rubber or plastic can be damaged by carb cleaners.

3) Gasoline is highly flammable.

1984-1986 SB700&750

Recommend removing gas tank to do this job.

Ref 4-3 Rubber connecting tube between the air box and carb intake will be stiff due to age. Loosen both 10mm AF bolts at top rear of airbox and lever box backwards along the slots for 15mm until the bolts are fully back then tighten them there to give clearance for boot removal. Unclamp clips then push the rubber flange inward at the air box end. The connecting tube can now slide off the carb intake and out. WD40 applied to the joint may help.

Ref 4-3 The “Intake” pipe snootboot going to the cylinder head is extremely difficult to remove from the carb as the rubber is stiff.

Working the carb assembly up and down side to side may get results. Heating the rubber with a hair drier helps but rquires extreme caution, make sure no sources of fuel are present. Attempts to lever the carb off should be done with care to avoid damage to the linkage.

Using a Dremel tool to resize boot

Removing the eight allen hd cap screws holding the intake flange to the head can be done if all else fails. To attempt this you will need a 1/4" hex socket, a universal, with a selection of extensions. A ball end hex is also useful. Once off, resize the boot opening so next time you don't have to tear half the bike apart. A Dremel tool with a small drum works well.

Ref 4-4 Check each vacuum piston for movement (black cylinder) visible from the intake side of the carb. Place a finger below the piston and gently lift. Make note of any pistons that appear seized.

Ref 4-4 The vacuum chamber cover screws may be difficult to remove. Avoid stripping the heads by using a proper fitting impact driver.

Carefully lift the cover while separating the rubber diaphragm. Remove spring and lift out piston with attached diaphragm. The piston itself is probably not seized; chances are that the jet needle is stuck. Free it by spraying a small amount of carb cleaner around the end of the jet needle visible through the intake end of the carb. Allow to soak and gently work the needle buy rotating the piston from above. Remove the piston when free. Remember carb cleaner kills rubber; keep it well away from the diaphragm.

Ref 4-5 As you lift the bowl the gasket should stay in the groove on the bowl side. Carefully separate the gasket as the bowl is lifted.

Ref 4-5 to 4-6 Check to make sure the needle valves move freely before attempting to disassemble. Frozen needle valves can be lifted by a collar at the top of the valve where the plastic float is attached . Avoid applying force directly to the plastic float. Avoid using carb cleaner to loosen the valve as the pointed end is made from rubber.

Use a proper fitting screwdriver or where possible a socket to remove the jets.

Ref 4-7 Choke (bystarter) valve removal. Remember orientation of the spring on the choke shaft (bot of 4-7)

Ref 4-8 after loosening the choke shaft locking screws, check the shaft to make sure it slides with finger pressure. If still frozen, slightly wedge open each of the 4 arm clamps and apply some WD40. The dark colored nut on top of each choke valve is made from plastic and will shear off under excessive force. Do not attempt to remove the choke shaft by hammering the end.

Ref 4-13 Pilot screw must be removed for cleaning the low speed circuit. Do not over tighten when counting the turns in. The factory setting is two turns out.

Ref 4-10 Carb Cleaning –“Clean all carburetor passages with compressed air” who ever wrote this manual must have been dreaming.

1) Scrape varnish off as much of the float bowl as possible. Spray with carb cleaner and let soak.

2) Fill a dish with carb cleaner and drop in the brass jets and choke plungers.

3) Visually trace all of the fuel and air passages in the carb body.

The fuel passages are: a) High Speed (main jet) b) Low Speed (slow) c) Choke

Jet towers, float valve opening/seat and choke valve opening should be thoroughly cleaned by soaking in carb cleaner. Working the cleaner with a Qtip and a small bristle brush works well. Next spray carb cleaner into each of the fuel passages. Let sit for 10 minutes and use compressed air to blow each passage out out. If you haven’t figured out the circuit, you may get a face full of carb cleaner at this point. Repeat procedure until the carb cleaner is clear when blown into a white pape towel.

4) The needle valve requires special attention to the small spring loaded rod at the top of the valve. Gently push on the rod and see if it moves into the valve body. If it does not move freely, set the pin end of the valve in a shallow tray of carb cleaner with the rubber end up and out of the cleaner. Soak until the pin is free.

5) Small fuel tubes inter-connecting the carbs contain plastic and rubber components. Any carb cleaner leaking into these should be blown out promptly.

6) Carb cleaners may attack the plastic floats. Exercise caution. <is there any way to adjust float levels by bending the plastic?>

7) After a thorough cleaning, hold the jets up to the light for inspection.

8) Blow out all air passages.

Last modified on 17 December 2010, at 15:03