Honda C90 - carburettor float height

1st Jan 2020
Tags: carburettor

The level in the float chamber effects the ratio of fuel and air passed to the engine: when the level is too high the additional pressure exerted by the higher volume of fuel means that it is delivered faster and the mixture becomes richer, when it is too low the mixture is made correspondingly lean.

Honda supply instructions for checking and modifying the float height:

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The illustration below is from the Haynes manual and shows a float height gauge being used:

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Given that the correct float height is set at the factory, and that there is no obvious way for it to go out of adjustment in normal use, it is surprising that Honda give instructions on how to check and correct it. One possibility explanation is that owners adjust the height to compensate for some other problem, the obvious one being that the tip of the float valve has become badly worn, such that the float has to rise higher before the valve can seal off the flow of fuel.

In this case It might be tempting – in order to avoid buying a new float valve – to bend the float tab to get back to the correct fuel level. It appears Honda eventually thought better of this and started to supply one piece plastic floats that do not allow for this kind of adjustment:

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Anyhow, if you do want to measure the float height there are a couple of ways of doing it:

With a float gauge (I improvised with a small combination square):

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Or you can attach a transparent tube to the drain nozzle to see the actual level in the float chamber – handy as it can be done while the carburettor is still on the bike.

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I can't find a workshop manual that covers the PB25a carburettor, but the height given in the Haynes manual is 10mm.  Note that the carb described in the 1971 manual (PW15 HA1) has a brass float and the float height is specified as 21mm.  Possibly there were other variations in the interim, so you may need to experiment.

Here is an American chap explaining how to do the measurements and also showing various float designs. enjoy!

The next post describes the other parts concerned with fuel supply (the drain, overflow and breather).

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