The Early S7s : 1946 - 1949

A stunning 1948 S7

This page has been reproduced by kind permission of its author, Peter Bird.

The early Sunbeam S7 is very similar in general appearance to the S7 deluxe but, to the mechanic, the restorer and to the owner using the bike, it is very different indeed.

It would probably cost double or more to correctly restore an early s7 basket case compared to a comparable s7 deluxe basket case (assuming a similar number of parts are missing or would need replacing).  That's because so few parts are the same between the model years.  Just a few hundred of the 2100 early bikes made remained here in England and so replacement parts have been largely unavailable for them since the late 1950's. 

In the 1950's motorcycles were bought and used as transport. If a bike failed and parts were unavailable or very expensive then the bike was junk.  It wasn't until the late 1950's and into the 60's that motorcycling became a lifestyle choice and performance and handling became important factors. Sunbeams were out of fashion and so, in part because hardly anybody knew how to repair or maintain them (and fewer still were interested), the early Sunbeams were widely acknowledged to be unreliable, quirky in an annoying and unfashionable way, slow, and heavy.  Worse, they were no longer being supported by Sunbeam/BSA service and part's departments.   Even today, the early bikes are often thought to be the 'development phase' for the much improved S7 deluxe and S8 models.

But in very recent years, the early bikes have become sought after - as being 'purer' and 'collectable' – for their classy elegance, wonderfully inventive quirkiness, and uniqueness. Undoubtedly, a nice one has an elegance that the deluxe cannot quite aspire to - because the proportions and details are different in many subtle ways.

It isn’t clear how or why many of the differences came about. Some were improvements and some were for economic reasons but there are so many that it is hard to list them all. Almost every casting, thread size, and detail is different but, for those working on their early bikes, I hope the follow collection of exploded and cutaway drawings will be helpful to you.

All technical drawings and cut-away or exploded diagrams are (c) Stewart Engineering, Poole, Dorset.

They may not be reproduced or otherwise commercially used, in whole or in part, without prior written permission.

Below are a few notes that owners and users of an early S7 motorcycle may find helpful.

Putting the early s7 on its centre stand.  Place your foot on the centre stand's side pin, in much the same way as you would an s7-deluxe or s8.  On the deluxe the stand will flip to the ground, however on the early bike - you have to push the stand to the floor and keep your foot on it as you pull the bike back and lift upwards - using the frame tube by the back wheel as a handle.  Perhaps because of this awkward stance the early bike feels still heavier than the deluxe...

In fact, the easiest way to get the s8 or s7-deluxe onto its centre stand is to flip the centre-stand down, and then simply balance the bike as you move around to the back, to then lift it vertically using the two mudguard stays as handles. As you lift the centre-stand will flip forward and lock.

However, the early s7 stand is on a ratchet - which has the great advantage that the stand can be pushed down to the ground (with your foot) and at the same time (with your foot still on the stand) just rolled back. The inertia will lift the bike just a little.  You can do this with both the front & rear wheels still on the ground but with the stand's legs to balance it. This is  the side-stand mode.

It saves a lot of lifting and it means that on softer ground the tyres are taking most of the weight of the bike so the stand is less likely to dig in. In effect you roll the bike a little way up its suspension travel but not off the ground.  It's also very much easier when you are at a rally and parking in a field because you can position the stand on a plywood pad and pull back onto it (to push the pad into the grass), which is very much easier than lifting a deluxe right up in the air.

Getting an early s7 off the centre stand.   Most of us are used to pushing or rocking the bike forward while being ready with the front brake. In contrast, with the early bike, release the ratchet then place your foot on the tread pad, just beside the tool box, and pull the bike (with the handlebars) backwards. Then, with your foot still on that ratchet release pad, ease the bike forward and the centre stand will flip up automatically. But be warned that it all happens very fast. When you pull the bike back with a jerk it will not move far but with the ratchet released it will almost automatically roll forward so make sure you are balanced and be ready with that front brake.

The two good things are:   a) with the wheels still on the ground and the stand only a couple of clicks on the ratchet, there is very little roll forward, which makes balancing much easier and everything very much more dignified.   In the limited space of a garage this is useful.  And b) because once the ratchet is released the centre stand automatically flips up so there is little chance of you riding off with the stand dragging on the floor.

Starting the bike from cold. This is same on any Sunbeam with the early type Amal 276 carburettor.  Turn the fuel on at both taps, to get rid of any air bubble in the pipe and to equalise the fuel in either side of the petrol tank.  But do not tickle the carburettor.  Then turn the left hand tap off again.  Apparently the original S7s only had one fuel tap.   Personally, I use the left hand tap as the reserve because it is easier,  when running out of petrol and the engine is coughing and spluttering,  to reach down with my left hand while my right hand is still in control of the throttle and brake.

The top of the carburettor has a plunger in it.  That is the choke lever.  I mark this by filing a shallow hollow in the top of the plunger handle. This mark should be facing towards the engine.  Gently twist the top of the plunger until that mark faces forward and gently push the plunger down and twist the top until the mark again faces towards the engine.  Continue to gently push the plunger right the way down until you feel or hear a light click. This is a hairspring clip inside the carburettor clicking into a grove in the plunger.  The choke is now fully closed.  You can now release the lever, which should stay down.

Before turning the ignition on, check that the gear selector is in neutral and gently kick the engine over three or four times.  Only then switch the ignition on and note that both the oil warning (green) and ignition (red) warning lights are glowing.  Now, with 1/8 throttle, kick start the engine over with a confident prod, rather than a leaping swinging kick needed for many British bikes.  If it does not start then repeat with a little more throttle.

NOTE:  The throttle does not close automatically when you let go. You need to wind-it-back closed again.

As soon as the bike starts then wind the throttle back to achieve a fast tick-over. Then turn the choke plunger's mark to face forward again and the plunger should spring up half way.  Adjust the throttle again to smooth fast running.  That's it.  Before you ride off in anything but very cold weather, turn the choke plunger's mark to face the engine again and gentle pull it right the way up until you feel the click.  I've cut a second slot in the plunger to hold it all the way up, in the off position.

NOTE: no tickling (flooding the carburettor) is normally needed.

Excessive tickling (flooding) the carburettor is no good for any engine - as the excess petrol washes the lubricating oil off the cylinder walls and piston.  When necessary it should only be done sparingly.  It's always better to turn the engine over with the ignition off several times before trying to start it from cold.

Before you go any further check that:

a) both warning lights have gone out;

b) there are no sudden oil leaks around or under the engine  (i.e. have you left the oil filler cap lid off or its clip is not in the right place);

c) the ammeter is reading a charge (blip the throttle to see that the ammeter needle moves).

 To stop the engine is the same as an s7-deluxe or S8.   I.e. turn off the fuel taps, close the throttle and switch off the ignition.  Note. It is particularly important to turn off the fuel to the 276 carburettor - as they are prone to dribble profusely.   A fire blanket and fire extinguisher, one either side of your garage, is worthwhile.

The float chamber (bolted to the side of the carburettor) has a valve pin (which looks like a narrow inverted cone) passing through the middle of the float. As the float rises it lifts the cone into a valve seat to close it off.  But, aside from the valve seat, the only location to this pin is the top end of the pin which crudely locates into a tapered hole in the underside of the cap. Sometimes it gets snagged.

The vibration of a running engine normally helps prevent this but a bike where the fuel has evaporated away or which has been parked on a side stand is prone to this valve not coming up straight and closing properly.  So, where possible don't use the side-stand (not originally fitted to the early bikes anyway) when starting the engine.

 If the carburettor does leak all over the place: don't smoke anyway near the bike and:

1.      turn the petrol off at both the petrol tank taps;

2.      turn the ignition off;

3.      if at home or in the garage, slip a catch tray under the bike;

4.      unscrew the cap of the top of the float chamber just a few turns and gently re-tighten;

5.      turn the fuel on again and it should have stopped leaking;

6.      wipe up the spilt fuel and, if applicable, allow ventilation through the space;

7.      move the bike away from the petrol fumes before trying to start it.

Starting the bike when Hot.  Firstly, try the same sequence as when cold but without the choke.  If that is no good after four kicks then apply the choke and try again.  Only if that is still no good after another four tries then you can try tickling (flooding) the carburettor but only until the very first signs of fuel seeping out of the tickler. There is actually a tiny hole in the knurled side of the cap so watch out for petrol first coming out of that. This fuel level will then be ½" (13mm) above where it needs to be. So now the petrol will leak out of the left hand side of the carburettor, just above the chromed collar, until the fuel drains back down to the correct level.

Riding the bike for the first time. Firstly, remember that the throttle's return spring only closes the carburettor's slider - and not the throttle grip. You need to open and close the throttle at the handlebar grip manually.  This will be the same when you ride the bike and also when you give right-hand signals.  So even slowing down and cornering is much more a conscious action.

The front brake and clutch are exactly the same in operation as on a S7-deluxe or S8 but for the inverted levers. These are just as easy to use as conventional levers and you'll soon get used to them.

However, the early s7 is a heavier bike, which takes more braking to stop but the front brake lever has less leverage and so it barely seems to work. The back brake is not much better.  Conversely, the acceleration of this bike is better than the s7-deluxe so you will need to take extra care and anticipate all corners and other traffic slowing.  

Also be aware the rear brake light on these early bikes are just 1" diameter and has a very low wattage bulb.  Ok in the countryside at night, but desperate for town traffic or even under street lights. 

The headlight main / dip switch is the left hand twist-grip.

The handlebars are narrower than the S7-deluxe or the S8 and, while this is quite comfortable and certainly much easier to see over your shoulder, it does make the steering feel altogether different. With the bike being heavier (especially on the front end) the steering is slower too.

Don’t allow the handlebars to drop hard onto the steering lock.  There's barely any clearance between the nuts which hold them on and the petrol tank.

The camshaft of these bikes gives very much more responsive power at lower speeds.

Sump :

Sump studs on the early S7's are 1/4" BSF. These and any other which have nuts of 3/16" Whitworth = 3/8" AF spanner size (for example the plates on the back of the engine) should be sequentially tightened to No Tighter than

(I tested a brand new high-tensile ¼" Whitworth thread and tightening a nut to 10 is fine but the stud itself sheared off at less than is enough and leaves a little margin for error.)

Sump Nuts - Tightening sequence :