The FN carburettor

I've just been on the phone, talking with a friend who is trying to tune a 1912 FN four. Inevitably the conversation came around to the carburettor.

The FN automatic carburettor appeared new for the 1910 model, to replace the fixed-jet instrument that had been used (in variable shapes and sizes) on all FN machines since 1901. The early carburettor was a pretty decent thing in its day, and with the French Longuemare it was one of the  best of the first wave of spray carburettors. Note the absence of moving parts in the early model: throttle and extra-air adjustments were on the manifold.

Early FN carburettor

By 1910, there was strong competition in the spray carburettor game. In the UK, for example, Brown and Barlow were marketing a reliable two-lever carburettor that allowed the rider to control his machine over a range of conditions. In fact this early B&B was, except for the absence of a throttle needle, pretty much to the pattern of the AMAL carburettors used on British bikes through to the 1960s. I guess it was FN's stubbornness and independence that ruled out using a bought-in carburettor like the B&B on the 1910 model FN, but in all honesty it might have been a better bet.

Figures 22 (left) and 23, showing the FN automatic carburettor. New for 1910 FN carburettor

Have you ever seen a carburettor with gears? I hadn't until I dismantled the one mounted on my 1910 four. I know that FN reveled in mechanical detail, but surely the design of this carburettor was over the top. I'm not sure that it even works all that well. I've worked pretty hard on mine, but I still rely on a Bowden Extra Air Inlet to keep the mixture correct over the range of throttle openings.

Figure 24 (left) shows the throttle sleeve, with roller spindle and gear, also air and gas shutters. Figure 24 (right) shows the "peculiarly shaped geared rollers". Yes, it has gears!

How does it work? If you really want to know, here is a description from the fourteenth edition of Motor Cycles and How to Manage Them, with reference to the figures above:

Amongst the automatic carburetters must be mentioned the latest type F.N., which is of a peculiarly efficient and interesting character.

Fig. 23 gives a perspective view of the carburetter as a whole, and shows the position of the float chamber, throttle lever, and inlet pipe union. The mixing chamber is cylindrical, and contains a rotating sleeve throttle, which, in addition to the functions dealt with later on, controls the admission of air to the mixture through two ports. The situation of these latter is more clearly shown in fig. 22. That on the left-hand side admits cold air, whilst the righthand one is furnished with a gauze covered funnel, which abuts on the cylinder head and provides warm air.

The throttle sleeve is illustrated in fig. 24, which, for the sake of clearness, shows it the wrong way up. When in the position of full opening, the heart-shaped hole in the sleeve coincides with the outlet port in the top of the mixing chamber, the sleeve passing between the cylinder wall and the small castellated-ended tube, the use of which will he described. For the control of the air, a portion of the sleeve is cut away in such a manner that when the throttle valve is nearly shut, all the air supply is drawn from the warm air port, then, as the valve is gradually opened, the cold air supply comes into use, until at full throttle only cold air is drawn in. It may be mentioned that the area of cold air port is considerably larger than that of the hot air port.

A variable choke tube practically makes a carburetter automatic. The F.N. Co. have set about achieving this laudable end in the following manner: Mounted to the cover plate, through which the throttle valve spindle passes, are a couple of cylindrical rollers, which run the entire length of the mixing chamber, and butt up against its walls at either end. These rollers are geared together by means of teeth milled in them, and cut away in a very peculiar manner, which may be described as a double compound helical groove. From a middle point along the roller's length, a right-handed thread and a left-handed thread are marked oft, and pass some three quarters of the distance round the circumference of the roller. Between these two diverging lines a semi-circular groove is machined, which, as the lines get further apart, becomes not only wider, but deeper. Fig. 25, shows the appearance of the two grooved rollers, which are meshed with one another symmetrically, so that when they roll together the depth of the grooves immediately on either side of the line of contact is the same. Since each groove is semicircular, it will be seen that there is between the rollers a circular gap, which can be increased or decreased by rolling the rollers one way or the other.

In the choke tube thus formed is placed the jet, which, when the rollers are in one extreme position, practically fills the gap.

One of the rollers is fixed to a spindle, which runs loose in bearings at each end, whilst the other is attached by a grub screw to the spindle shown inside the throttle sleeve in fig. 24.  This spindle carries at its rear end a small pinion, which meshes with a similar-sized pinion mounted on the throttle sleeve spindle.

In this way the opening of the throttle is caused to actuate the rollers, with the result that as the throttle is opened the area of the choke tube is increased, and vice versa. The amount of choke tube area compared to the throttle opening can be regulated to any desired degree by slipping the driving roller upon its shaft. The rear plate of the throttle sleeve is slotted to let the roller spindles pass through, as shown in fig. 24, and only a certain amount of motion in the rollers is therefore possible.

The rollers, when placed side by side, are a snug fit inside the throttle sleeve, and the cutting away is such that in any position there is a sufficiency of unbroken surface of the rollers in contact with the sleeve wall to provide a practically perfect airtight joint.

To render the continuation of the choke tube above the rollers as symmetrical as possible, and thus prevent the mixture being obstructed by any sharp corners, the small castellated tube shown in fig. 22 is provided, and is secured to the end wall of the mixing chamber by means of a stud and set pin. The castellations make as near a fit as is possible with the peculiar contour of the grooved rollers.

The lid of the float chamber is made extremely easy to detach, as is also the jet, and all parts of the instrument are readily accessible. Throughout it is of gunmetal, dull nickel-plated.

Phew! Luckily the rider's instructions were a little simpler, because as an "automatic" instrument, there was just one lever to open when you came to a hill.

The FN automatic carburettor didn't last long. The design was modified for the 1912 model by fitting  an extra air inlet on a collar between the carb and the manifold, effectively making it a "two-lever" instrument. AN admission of failure? When the Type 700 four appeared for 1914, it was fitted with a Zenith carburettor, and post-war AMAC instruments were fitted on all models. Still, I like my carburettor. There are not many carburettors around that are worth taking apart just to show your friends the beautiful workings.

Copyright Leon Mitchell 2000

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