The Keihin carbs on my machine required stripping to the component parts. The plate set, stay p/no.16035-333-004 linkage shaft will need to be removed if you wish to re-plate the throttle cable cam etc. To do this a small pin will need to be carefully drilled out and of course, you will have a replacement to fabricate when you re-assemble. The inserted end of the pin is tapered to a smaller diameter. First job after stripping was to degrease using white spirit. Some scrubbing with a fine brass brush may be necessary to remove stubborn/engrained dirt.

    The float valve and jets were soaked in carb cleaner to dissolve fuel residues and wiped clean. The hinge pin that retains the float assembly and the tang that actuates the float valve should be polished with brass cleaner, this helps to avoid petrol flooding the carburetors. Compressed air is useful to make sure that all the passageways are now clear. Aerosols of compressed air are now available if you don’t have access to a compressor. You are now ready to re-assemble. My gaskets & o-rings were no longer serviceable so have these to hand before starting.

Measure o-rings as shown below.

    Honda gasket set 16010-333-305 contents, top paper gasket, float valve o-ring, jet o-ring, drain screw o-ring, float bowl o-ring and only one petrol feed o-ring per set.

    Keyster repair kit – contents, needle with clip, float valve, main jet #75, slow jet #40 and idle screw. O-rings for drain screw, float bowl, jet and float bowl plus paper gasket for top of carburettor. Try to get Genuine Honda/Keihin if possible or cleaning the originals.

    In the event of the gasket sets being discontinued, twelve petrol feed nitrile o-rings 5.8×1.9mm, jet o-ring 4.2×1.1mm, float valve 5.7×1.3mm, float bowl drain BS008 3/16″x1/16″ or 4.47×1.78mm and large float bowl o-ring 62.5×1.9mm. All these o-rings seem to have a hardness (Durometer Shore A) of 70. Paper gasket material is 0.56mm thick. Not easy to measure, all sizes are as near as I can get using digital calipers, please correct me if you know better.

Known problems:-

    Can be avoided in the main part by scrupulous cleaning of all components, also consider using an in line fuel filter to supplement the in tank filter. Many more difficulties can be avoided by keeping the bikes exhaust and air filtration as standard.

    Make sure the four inlet manifold brackets are in good shape and do not leak air. Problems here will give poor idle, low vacuums & difficult starting, especially from cold. I tried to repair these with 1 contact adhesive – Failed due to it being soluble with petrol, 2 Epoxy resin – Failed due to its intolerance to high temperatures. Plastic padding or JB Weld may have worked but I lost patience, I gave in and bought new from Honda.

    The float valves are prone to sticking, especially after storage, causing petrol to flood out of the overflows. Before stripping the carbs be patient. Waiting a couple of hours, (with the fuel tap off) can in some cases allow the stale fuel residue to dissolve into the fresh fuel and effect a perfect cure! If this failed then polish the float retaining pins and make sure there are no burrs on the float adjusting tang where it contacts the valve. Use very fine wet or dry to produce a smooth surface to the tang and metal polish for the retaining pin. Now re-check the float heights. Float height is quite critical and can cause a number of running difficulties, checking while off the bike is much easier than when mounted. A tyre tread depth guage can be improvised to do the measuring.
    I avoid my floats sticking by turning the fuel tap on for a short while twice a week. This keeps the float chambers topped up with fresh fuel, you’ll be amazed how quick the fuel evaporates from them. Some riders prefer to leave the float chambers empty by turning the fuel cock off a mile from journeys end. I always drain the chambers for long term winter storage.


    When it comes to remounting the carbs you will find there is little room for fingers, this makes the task difficult. To make the aging hardened rubberware push home more easily, warm parts with a hair drier first. WARNING: Do take care when using electrical devices if there is fuel present. My preference is to find an air chamber with the four rubber mounts that are soft and flexible.

    If you don’t have carb cleaner, paint stripper works very well – metal parts ONLY.

    Using suitable vacuum gauges balancing the carburettors to the specified 16-24cmHg is easy, keeping the synchronisation when tightening the lock nuts is another matter. I find avoiding any downward pressure or over tightening helps. To avoid the locknut vibrating loose, I lock it with a blob of yellow paint as did Honda when the bike left the factory. Properly adjusted carburetors is essential for a tick over free of that diesel type knocking sound.

    Using pod filters, a good starting place for rejetting is to try #85 jets.

    Here are two excellent guides Making felt o-rings & Re-building the carburettors courtesy of Tim Boughen.