Cam shaft, rockers, ignition, etc.

Camshaft housing

The 1934 model camshaft housing can be recognised by the support surfaces being designed with hollow-shaped sides, so that they match the engine block around the attachment bolts, whereas the drain hole for the return oil has a rectangular shape. 
The earth connection between the camshaft housing and the distributor was very poor with the earliest models, so from No 1526 an earth connection at the side of the camshaft housing was fitted by using a finger screw with a retaining spring.  

From No 2051 the distributor was fitted with a graduated adjustment scale, with its bolt serving as an earth connection.

48.

Nearly all early camshaft housings have been modified in the course of time, either by having a threaded hole for the earth bolt or alternatively, the ignition adjusting screw being used for the same purpose. 

Rockers

All rockers are the same up to No 7500. The part of the rocker protruding into the camshaft housing is 5 mm wide. This width was increased to 7 mm from No 7501 onwards; simultaneously the contact profile with the cam was heightened and more curved.

Rocker guides

Rocker guides are the same until No 10300. The guides have a fixed cover without a gasket around the rockers. It can be seen from the patent description of March 1933 that initially a felt gasket or similar was planned. As far as we know, this gasket was never fitted to motorcycles put into mass production.
In 1934 the seal between the rocker guides and the camshaft housing is a ring shaped paper gasket. Subsequently, this did not appear to be a good solution because, when the nuts were firmly tightened, they had the tendency to bend, leading to oil leakage.

Camshaft

The camshaft is fitted in the pressure lubricated bearing bushes of the camshaft housing and is the same for all models, with the later camshafts marked “NIMBUS”.

Camshaft gearwheel 

The camshaft gearwheel remained unchanged from No 1301 until No 3000.
The original valve timing diagram was unsatisfactory and therefore the key between the camshaft and camshaft gearwheel was replaced by an offset key. This resulted in a slightly changed valve diagram. 
Starting with No 3000, the position of the key slot was changed and the mark “2” was applied. Thereafter a normally shaped key was used again. 

On the camshaft, four cams forming a square are fitted, including centrifugal weights, for ignition advancement. The square with the four cams is locked in position

49.

by means of a flat brass securing plate with long brass pins. Starting with No 2051 the locking plates were made of steel sheeting with a step at both ends. Simultaneously shorter pins were fitted.

1934 camshaft housing with camshaft.

The following article from INGENIØREN (the engineer) of September 29, 1934 is probably the best contemporary technical description of the 1934 Nimbus we have.

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[Translation of page 51]

MACHINE TECHNIQUE

Editor: P.A.Christensen, M. Ing. F.

THE NEW NIMBUS MOTORCYCLE

[Photograph]

The new Nimbus

Motorcycle production ceased in 1927, and A/S Fisker & Nielsen have recently commenced production again and will bring a newly designed model to the market this year.
Production ceased at the time, because the production lines for other products required all the factory capacity, which necessitated segregating the production of motorcycles from the remainder of the production. 
During the last year, a spacious, modern factory was built, specifically for the manufacturing of motorcycles and the capacity is currently 10 machines a week, but are projecting an increase up to 1000 machines annually.
The new model reveals that everything, even the smallest detail, has been thoroughly tested and because the factory manufactures all parts “in house”, eliminating spending energy on purchased items during design, thus allows the company to move more efficiently as opposed to other motorcycle manufacturers. Striking characteristics of the machine are its simple, pure lines, its robust construction, as well as the easy accessibility of the separate elements in case of repair. The factory has not been afraid of adopting new designs, e.g. the front forks and the cylinder block with four cylinders in line, cast as one unit.

The engine

            This is an air cooled 4 cylinder engine with overhead valves and with a bore of 60 mm, a stroke of 66 mm and a cubic capacity of 746 cm³.  Compression ratio is 1:5 and the engine develops a power output of 18 hp at 4000 rpm.  As shown at the longitudinal section, the four cylinders have been cast in line as one unit and the block, which is made of cast iron, is provided with wide, continuing cooling fins cast as a whole with the upper part of the crank case.  The crank shaft runs in bearings, fitted in the block, allowing all internal parts to be assembled, prior to fitting the oil pan. The pistons are made of an aluminium-silicon alloy and have no slits and are fitted with four equal piston rings of which the bottom two serve as oil control rings. The gudgeon pins, which are fully floating and fitted with brass end caps, are directly fitted in the piston body, whereas the gudgeon pin bearing bushes

 51.

[End of translation of page 51]

[Translation of page 52]

[Drawing]

                                                                       CROSS SECTION OF THE REAR WHEEL

                                                                                  BRAKE

LONGITUDIAL SECTION OF THE ENGINE AND GEAR BOX

Longitudinal section of engine, gear box, rear Wheel and brake

are made of bronze. The big end bearing is made of white metal, cast directly into the piston rods. These are drop forged in a rectangular profile and have a longitudinal bore. The crankshaft runs in two heavy single track deep grooved ball bearings and drives the dynamo, which is fitted at the front of the engine, by means of conical gear wheels which, in turn, drives the overhead camshaft through conical gear wheels. The cam shaft runs in two bronze bearing bushes fitted in the separate housing and operates the valves by means of rockers, which are fitted in bearing bushes with external flanges, fitted in the camshaft housing.  At the pivot point, the rockers are ball shaped, and are supported in the cylindrical bore of the bearing bush, thus forming a seal against leaking.
            The cylinder head is made of cast iron (with pressed in valve guides) and, like the cylinder block, cast as a whole with wide, continuous cooling fins. The inlet manifold is cast in one piece with the cylinder head and the connection with the carburettor is placed in the middle. The exhaust ports are situated at the side, where a common exhaust channel discharges the exhaust gasses. 
            The compression chamber is fully machined and has a semi spherical shape, which, together with the correct positioning of the valves, creates a suitable combustion chamber with a small surface. The sparking plugs are fitted oblique between the valves, which are positioned at an angle of 30º. Each valve is provided with two springs, which, in closed position, exert a force of 10 kg. The angle of the valve seat is 45º, the lift is 6 mm. The valve timing diagram can be seen below:

[Drawing at the RH side]

Valve timing diagram

The lube oil system

            The lube oil pump, which is bolted to the cylinder block, is a gear wheel pump, driven by the vertical dynamo shaft, through a claw coupling.
            The speed of the dynamo and consequently that of the lubricating oil pump is 1,5 times that of the crank shaft, but notwithstanding this high speed, the lubricating oil pump has shown to function satisfactorily up to 6000 rpm of the crank shaft. The pump receives oil from the oil pan (containing 2.5 litres of oil) through the oil strainer and is fed under pressure through a special connection to the front side of the crank shaft and from there through this shaft to the 4 big end bearings and then through the hollow piston rods to the gudgeon pins, where the oil will be pressurised. through by the difference in acceleration force between the two dead centres of the crank shaft stroke. The cylinder wall and the main shaft bearings are lubricated with oil which is swung from the big end bearings. The lube oil pressure is 1.7 atm. and is controlled by a pressure relief valve. The lube oil is fed through a special duct to the two

52.

[End of translation of page 52]

[Translation of page 53]

[Drawing]

Cross section of the engine.

cam shaft bearings and the rockers; the excess oil flows back to the oil pan. The gear box is lubricated automatically with lube oil from the engine. The oil consumption is about 0.1 litres per 100 km.

Clutch and gear box
            The clutch, positioned in the fly wheel, is of the dry single plate type, which is declutched by means of a thrust ball bearing and the force, exerted at declutching – 120 kg – is taken by the foremost crank shaft ball bearing.
            The gear box is bolted to the cylinder block and the crank case, with gear ratios of 2.43:1, 1.53:1 and 1:1. The high and low gear wheels are continuously engaged and fitted with claws for connecting the sliding pinion.

Carburettor and dynamo
            The carburettor is fitted with a main jet, containing an adjustable needle which, together with the pilot jet, guarantees the correct mixture. Additionally, there is an acceleration pump, which supplies additional fuel and gives increased acceleration when the throttle valve is opened quickly.
To reduce oil leakage, the oil and air vapours from the crank case are discharged via a pipe to the carburettor under 20mm water gauge vacuum. This also has the effect of lubricating the inlet valves.
An oil separator has been fitted to prevent too much oil from being extracted. The petrol consumption is about 4 litres per 100 km. (approx 70 mpg)
            The dynamo is of the third brush type with, as the name implies, an adjustable third brush. It is capable of supplying 35 Watt at 6 – 8 Volts.
            Battery ignition has been fitted, which is controlled automatically by swing weights, fitted on the conical cam shaft gear wheel. The contact breakers and the condensers are fitted in the distributor housing, mounted at the end of the cam shaft and the ignition coil, cast in the Nokait distributor cap, forms the cover of the distributor housing. 

Transmission and frame
            Transmitting the engine’s driving torque to the rear wheel is accomplished by means of a floating shaft and conical gear wheels; the transmission, which has a ratio of 4:1, gives the motor cycle a speed of 120 km/h at 4000 rpm.  
The brakes, which operate independently, are drum brakes with brake shoes and are self adjusting.
            The frame is made of manganese steel and consists of two parts. In the upper part the fuel tank is placed (pressed in two halves and welded together) and in the bottom part the engine hangs, fastened with four bolts.

[Drawing]

Top view of the front section of the motor bike

            The front fork consists of two outer conically milled fork tubes, fitted to the head tube by means of the yoke and the flat steel handle bars. The fork tubes contain the movable front fork, consisting of two tubes, tightly connected to each other through the front wheel. The movable tubes contain long spiral springs. 
          The handle bars are pressed of 2 mm steel sheeting with the mid section containing the externally illuminated ammeter and speedometer. The hand grips can rotate and are fitted on extensions of the handle bars. The right one controls the gas, the left one the, lighting. The light switch is fitted under the handle bars and has four positions, locked in place by a spring loaded ball: off, parking lights, low beam and high beam.
            The weight of the motor cycle is 170 kg. with accessories. Wheel base is 1400 mm, seat height 710 mm and the tyres are 26” x 3½”.

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[End of translation of page 53]

Ignition coil, distributor housing, brace

The ignition coil is combined with the distributor in the brown, bakelite housing. There is a graduated scale at the bottom of the flange of the housing for the adjustment of the ignition and has the characters “T” and “S” for advance and retard ignition respectively. The ignition is adjusted by rotating the ignition coil relative to the notched mark at the bottom of the camshaft housing.   

The 1934 ignition coil is identified by the sharp transition between the housing and the flange of the ignition coil and by the coil housing and cover being flush. Because of problems caused by water intrusion, the cover has been provided with a collar, starting with number 2050.  

The earliest ignition coils had their 6 volt leads directly screwed onto the coil’s housing. This could however result in cracking of the housing when fastened too tightly. The problem was solved, probably by the end of 1934, by fitting a strong brass angled profile for connecting the leads.  

The ignition coil is attached to the distributor housing by means of a nickel plated brass brace. The brace swings in the distributor housing by means of two pins with thinner ends, which can slide in long slits in the ignition housing; the amount of rotation is limited, in relation to adjustment of the ignition.    

The slits in the 1934 housing are approx. 44 mm long, their effective length is approx. 34 mm because the distributor housing has a soldered brass strip on the inside which serves as a stop. It is unclear whether or not the earliest ignition housings were provided with the soldered strip. This was probably not the case.
The long slits allowed the ignition coil to be rotated approximately 36º. 

The earth connection of the distributor housing runs through the pins of the ignition coil brace. This proved to be an unreliable connection and therefore starting with number 1526, an earth connection was made by means of a 4 mm finger screw, on the side of the camshaft housing. Simultaneously, the slits in the distributor housing were lengthened to 20 mm, which limits the angle of rotation of the ignition coil to approximately 20º.   

After having ridden with the early types of distributor housings for some time, they became stuck and were difficult to rotate when adjusting the ignition.
Deep scratches, caused by a screwdriver, can often be seen on the side of the distributor housing, being the result of the crude method of dealing with this problem. The problem with the earth connection wasn’t solved until a plate for the adjustment of the ignition was fitted, starting with number 2051.

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The distributor’s rotor is made by “Bosch, type Z.V.T. 23/1 Z”. 

The make of condenser is “Hydra”.

Distributor housing, ignition coil and brace

H.T. Leads, spark plug caps, sparkplugs

The H.T. leads are made of blank wire, insulated with rubber, wrapped and lacquered black. They are fitted with brass terminals and brown, bakelite caps.

The H.T. leads are secured with a piece of oiled cardboard, wrapped in linen insulating tape and lacquered black.   

The H.T. leads are secured in the caps with a slotted bush, which at the same time connects the cap with the spark plug. The caps have been unchanged during the entire production period, but as of No 7500 the bushes were replaced by a bush containing a sprung wire serving as a lock. The H.T. ignition leads are attached to the camshaft housing with a single brace without screw.

The spark plugs are “Lodge, type H.D. 14”.

 55.

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