Reader Query: Some thoughts on early Honda Cub carburettors

Reader Raj recently contacted me for some Honda Cub advice so I thought I’d share my thoughts here in case it might help others with an early C50 with non-standard parts.

Raj has an old C50E – a very rare beast in Malaysia where he lives as he tells me most sold there were C70s. He is now restoring his bike but in doing so found his carb to be a larger C70 type. What type of carb should his Cub have then?

Here’s what I found on my own Cub, a UK issue 1967 Honda C50E with engine no. C50EDO17537:

Left hand side. Note choke flap is missing from this carb.
Right side view
Inlet mesh. Again note lever and choke flap assembly is missing. No idea why.
Slide and needle
Base of slide detail.
Fuel tap mechanism
Identifying stampings
More identifying marks
Underside of mounting flange showing o-ring seal and two access screws

Summary: it’s a fixed jet Keihin horizontal slide carb and fits onto the cylinder head directly without an aluminium swan neck inlet manifold as some Hondas of the period used. There is a thick gasket between the cylinder head and the mating flange of the carb where it fits onto the studs. Visible identifying and other info is:

Slide: marked DP13 / 2.0 underneath

Inlet bore diameter: 16.5 mm

Identifying marks: Keihin CY50E / FB7

Raj’s only guide so far is his mechanic, who believes he is looking for a replacement carb with two unusual lower mounting screws like this image he sent in. Goes without saying, those don’t look factory.

My carb does not have these bolts, so what’s going on?

Here’s what I think:

It seems likely that Honda altered and improved the carb designs as they developed the C100/102/110/C65/C50 line up through the early to mid Sixties – when the development of the Cub line was at its peak before settling into a rinse and repeat pattern in the Seventies.

Based on this schematic diagram from a British Haynes workshop manual covering the early C100/C102 and latterly C50 models covering the period from 1962 until the late Sixties, early Honda carbs were a two piece design:

This diagram shows an early carburettor most likely fitted to the early pushrod engines. On the left, the venturi with the carb slide is one separate part and on the right the float bowl is another. They are both joined at the base of the carb by two screws – just like Raj’s mechanic said. This suggests the carb Raj showed me and that the mechanic knows about is one of these types which I believe were used on early pushrod engined bikes. In fact here’s an ancient pushrod cub allegedly at the San Diego Children’s Museum.

Look carefully – that’s a two part carb joined at the base with two screws, just like the one Raj showed me. Click and zoom – I’ve arrowed the two screw locations.

NB: The non-standard looking screws on Raj’s picture appear to be a field repair using thru-bolts, probably done after the soft metal threads of the carb were stripped at some point.

The later carb design is like mine. Honda changed carb designs quite a bit in the early period of the Cub’s development and once they developed the overhead camshaft engine instead of using old style pushrods in the mid 1960s, it’s likely the Cub carbs were changed too to reflect the altered dynamics of the OHC motor.

Now instead of being two separable parts, the main carb body was cast in one piece with an integral float chamber (still looking very similar to the earlier ones though – it’s the same basic design) but this time without the issue of a potential fail joint at the base of the float chamber. Access to the float chamber was exclusively from above, where a horizontal split remained: a better solution. The screws in the bottom of the carb in this version are not there to join two parts but are simply access screws for the fuel jets/internal galleries.

Assuming my carb is original (and I have no reason to doubt this) and Raj’s C50E has an early overhead cam engine like mine, I suspect the one-piece carb and not the earlier two-piece is probably what he is looking for.

Unless we can flush out any other Honda experts who can chime in with ideas, superior knowledge and parts number support?!

Good luck and happy carb hunting to Raj…

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