Honda C90 - Carburettor fuel supply

1st Jan 2020
Tags: carburettor


The engine needs a constant supply of fuel – the parts involved are marked on this picture of the dismantled carburettor:

  • fuel tap
  • float bowl
  • float
  • float valve
A fully dismantled PB25A

Fuel supply

The fuel is fed to the tap by gravity from the tank where:

  1. It passes through a filter and up through the hole marked with the arrow A in the picture below.
  2. From there the fuel enters the top part of the carburettor, which contains a hole corresponding to the one below, and along the horizontal channel marked by B
  3. Finally the fuel enters vertical channel C and pours into the bowl below.
note the top half of the carb is upside down in this picture

What stops the bowl over filling? The float and float valve.

The float works like the ballcock in a toilet cistern: as fuel enters the chamber the float rises and the needle is pushed up into a brass seat installed in the carburettor body, and this stops the flow of fuel.  When the fuel level drops the float falls, pulling the needle away from the seat so more fuel flows into the chamber.

Here is the float valve, float and the rod the float pivots on:


and here they are fitted in place:


Float valve

This part is sometimes referred to as the float needle or float needle valve but, as we shall see, there are other “needle” parts in the carburettor, so to avoid confusion we’ll just stick to float valve.

The original Keihin float valves have a tip made from Viton (a trade name for chemically resistant synthetic rubber). Viton is often used to make o-rings and the like since it is soft and malleable allowing it to deform and make a good seal even on imperfect surfaces. There is a little spring loaded pin on the end of the valve that acts as a shock absorber (unlike the valve in your toilet, this valve needs to work while being continually jiggled and bounced around on the bike).

Having a vague idea that the poor running of my c90 might require some new carburettor parts I had already ordered a cheap IMPEX repair kit. Here is the float valve from the kit (top) compared to the one that was already installed on the bike (bottom):


As you can see the repair kit part is not very good – the rubber tip is slightly off centre, has a rough texture; is not seated properly on the metal base; and the hole for the little pin was not drilled correctly so it points off at a jaunty angle. Finally the chrome coating is flaking of in places.

This is a part that does wear over time, and an indentation can develop where the tip contacts the seat. You can just see a small ring is developing on the float valve that was fitted to my bike.

Incidentally, I am not sure the float valve on the bike is original – the jet needle and main jet are replacements (from a Keyster repair kit) so this might have been replaced at the same time. Someone on ebay is currently selling Honda ones for £15 so I have bought one to do a comparison.  As it turned out the replacement was identical to the one already on the bike so presumably the one on the bike was original – I installed the new one, kept the original as a spare and binned the hopeless IMPEX version.

For such a tiny part they are very expensive, but the consensus is that it is best not to skimp on it given the headaches caused when it does not perform correctly (more on that here).

The level of fuel in the float chamber has an important effect on how the engine runs and we will look at how that is controlled in the following post.



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