The spread of push button electric starting among motorcycles and scooters in recent years has been due primarily to Siba, whose
12V Dynastart, a combined dc dynamo and starter motor, has been incorporated into more and more machines.
The Dynastart is a remarkable unit. It fulfils several functions efficiently and unobtrusively and is renowned for its reliability.
Not only does it spin the engine vigorously for starting; it also serves as a direct-current dynamo, with an output of 60, 90, or 120 watts
according to size, provides a most effective dynamically balanced engine flywheel and carries the fan on engines cooled by forced
But the very dependability of the equipment encourages less conscientious owners to neglect
elementary care and maintenance. This is inexcusable for occasional straightforward attention only is needed to keep the Dynastart in
tip-top trim, while neglect may bring trouble in the long run.
As with any mechanism, a rudimentary understanding of the make-up helps in getting
the best from it. In principle and general layout all three sizes are the same, though
there are detail differences in the brush gear on engines fitted with reverse switching (which
enables them to be run backward), while installation is different on Excelsior engines and
slightly simplifies servicing. (Design for singles up to 250 cc or three-fifty twins, the 90-watt
model is commonest; the 120-watt pattern is for singles up to 250 cc or five-hundred twins.
and the 60-watt for 125 cc singles and two-fifty twins.)
Main components of the Dynastart are the stator with 12 radial pole shoes. and the bell-shape rotating armature with the commutator
formed on its inner vertical face. Alternate pole shoes of the stator are series wound with
heavy-gauge wire for starting; the others carry lighter-gauge shunt windings for generating.
On the stator are four spring—loaded carbon brushes that bear lightly on the commutator.
Their guide boxes painted red for identification, the upper two brushes are insulated by fibre
washers above and below the fixing ears of the box and fibre sleeves in the ears. (When reversing switching is used all four brushes are
insulated). Other components are the contact breaker, ignition coil (or coils). sealed switch-assembiy
box (containing Voltage regulator. cut-out and starter solenoid), condensers on the
switch-box aluminium cover. and the ignition starter switch — in some cases actuated by a push
button and in others by turning the ignition key beyond the "on” position.
The disposition of these parts varies with different installations.
On some three wheelers the switch-asembly box, ignition coil and reversing
solenoid are grouped in what is called the switch-gear box.
Routine maintenance can ensure satisfactory working only if the equipment is not abused.
For instance, on some Villiers Mark 9E installations petroil can drip on to the wiring
harness at the back of the stator and so get in the bellhousing. Since that leads to over-heating and excessive wear of the commutator
and brushes, the rubber grommet sealing the cables at that point must be sound and properly positioned. Better still is to make
certain that the fuel line is free from leaks or make up a drip guard.
Another abuse to be avoided is too long a continuous push on the starter button: this
could, of course, occur inadvertently if the button has a tendency to stick. After several
minutes of this treatment the starter solenoid will overheat. thereby causing an open circuit
and rendering the starter button dead. Incidently the box should never be tampered
with: not only does removal of the cover invaliate the guarantee but it also involves
a risk of burning out the unit
If the starter spins: the engine but the engine does not fire. there can be no fault in the
battery, switch-assembly box, armature or stator. Assuming the trouble does not lie out-side the electrical system (shortage of fuel or
oxer-rich mixture for instance) then it should looked for in the condenser or coil.
Needless to say, it is as important with the Siba Dynastart as with any other electrical system to keep the battery terminals and all
other connections clean and tight. This applies particularly to the battery earth connection
(which is negative in all cases); any trace of sulphation should be removed with a wire
brush and the surfazes then smeared with petroleum jelly. The battery, too, must be kept
in good shape by regular checking of the electrolyte level and replenishment with distilled water necessary.
Periodic servicing is best made a 5,000-mile task: certainly no attention should be needed
at shorter intervals. The first thing to check is the contact breaker. In most cases this is
at the right-hand end of the engine crankshaft and is exposed by removing a small cover.
On Sachs engines, however, the contact breaker is found at the left—hand end of the crankshaft.
In all cases. with the points fully open the gap between them should be 0.020in to 0.022in (obviously on twin-cylinder engines,
both gaps need checking). Should the gap be outside these limits. slacken the larger (clamping) screw and turn the smaller (eccentric) screw
until the correct setting is obtained; it is advisable to recheck after tightening the clamp screw, To ensure that the point faces are clean
it is a good plan to grip a piece of clean paper lightly between them, then pull it carefully out.
If the felt pad is dry smear it very sparingly with high-melting-point or molybdenum-disulphide grease and put a further trace at the
back of the fibre heel on the pivoted arm. Ensure that the felt pad is touching the cam.
Should the arm be sticking on its pivot post it must be removed for greasing; if the fibre
heel is excessively worn through lack of lubrication the arm will have to be renewed.
This is the way to remove the arm. Take the circlip and washer off the post, unhook the flanged
nylon insulating pad at the anchored end of the spring from its hole in the fixed-point
bracket and ease the arm off the post. Take good care not to lose the washer behind the
arm. The greases mentioned earlier are suitable lubricants.
The next item for servicing is the brush gear. On all engines expect Excelsiors this involves withdrawing the armature. for it is
fitted with the open end facing the crankcase. On Excelsior engines the open end faces outward so that the brushes can be reached by
removing the stator, for which no special tools are required.
But in all cases it is most important first to disconnect the battery, otherwise there is a risk of burning out the switch-assembly box.
That done, the right-hand cover should be removed from the engine; the leads may have to be disconnected from the switchbox first.
If the two screws securing the cam to the aimature are then undone the cam will come away and, for correct reassembly, you should
note the small cutaway on the periphery which engages with a pimple on the armature.
The time has then come to use a peg spanner to hold the armature while the crankshaft nut is undone with a 7/16in Whitworth,
a 9/16 BSF or 14mm plug spanner. If. the armature carries a fan, that must be removed to
use the peg spanner; jamming a tool in the blades to hold the armature is a bad practice.
Next the extractor body is screwed into the armature and held with a large adjustable
spanner while the centre bolt is screwed hard against the crankshaft end with a 7/16in BSF
spanner. It may be necessary to jar the spanner with a mallet to free the armature.
Care is essential when withdrawing and handling the armature; when it is placed on
the bench the open end should face upward to avoid risk of damaging the windings. Since the
air gap (radial clearance) between stator and armature is only 0.012 to 0.020in, dropping the
armature may well distort it enough to make it useless.
Every trace of carbon dust should be dislodged from the grooves between the
commutator segments and blown away. If, after a long mileage. the segments are worn flush with the
separators they should be undercut with, say, a suitably ground piece of hacksaw blade.
Following that, any rough edges on the segments should be polished away with very fine emery cloth.
After very long service the commutator face may require skimming to remove grooves
or pitting. This is a job for the factory. but it is up to you to see that the armature is
packed well enough to prevent damage in transit. Turning to the stator. all carbon dust must
be blown out of the brush boxes so that the brushes are perfectly free to slide. ln normal
use brush life is at least 15,000 miles and they are due for renewal if worn down to the
shoulders on their flanks.
Further wear would bring the brush pigtail into contact with the commutator, thus damaging it to the point where the segments short.
The consequent sluggish starting and failure to charge would then be the least of your worries,
for a service-exchange armature costs £9 13s 6d as compared with 14s for a set of new brushes.
When fitting new brushes make sure the fibre washers and sleeves are correctly replaced,
otherwise the Dynastart will function neither as a starter nor a dynamo. (Failure to insulate
one pair of brushes on the reversible unit results in the starter working in one direction only.)
The pigtaii (IE lead) is an integral part of the brush and, after being screwed to the brush
box, should be bent outward just enough to prevent pressure on the windings and a possible
short circuit. After fitting new brushes, the circuits can be checked with a lamp and battery. With the
A, DF and D+ connections removed from the switchâ€”assembly box to prevent false readings
there should be a continuous circuit from the stator pole shoes to the lower
uninsulated brushes but not to the upper brushes. (On reversible units there should be no continuity
from pole shoes to any brush.)
Any sign of oil must be removed and the leakage stopped. If it is coming along the
crankshaft the oil seal in the stator will have to be replaced; but if oil is leaking past the
three stator-fixing screws the stator should be removed and a non-setting gasket cement
smeared on the mating faces and screw threads.
For reassembly the taper faces must be clean and the key correctly ?tted. To prevent its
being dislodged as the armature is pushed home, the. key should be at the top of the
shaft and tilted so that its straight edge is parallel to the taper. The crankshaft nut should
be done up tightly but not hammered (for those possessing the necessary equipment.
650in-lb is the recommended torque). Finally, ensure that battery and switchbox connections are remade correctly.
To remove the stator, complete with right-hand engine cover, from Excelsior engines, it
is necessary (after disconnecting the battery and switch-box leads) only to undo the three
socket-head cover screws. But the cover comes away more easily when the engine is quite
cold. Servicing procedure is similar to that already described. except that the insulation of
the upper brushes is not disturbed when renewing them. since their pigtails are secured
by a separate, common screw and not by the brush-box screws.
Only for commutator skimming or parting the crankcase halves is it necessary to withdraw
the armature from the crankshaft: in that case the extractor and an adaptor (but not the
peg spanner) are needed. First, though, the Contact-breaker cam has to be taken from the
end of the crankshaft after removing a socket-head screw.
When refitting the stator all brushes must be properly home. in their boxes. If it proves
necessary to jar the stator home the brushes will be dislodged and may ruin the commutator, So unless the stator slides home smoothly
it should be removed and the armature centre boss lightly polished with fine emery cloth
until it does. Of course, all traces of emery dust must be blown away.
By the way, if the stator is ever removed from a Villiers Mark 9E engine (for parting the
crankcase), it is important on reassembly to lock the four screws holding the back plate
to the crankcase by punching a little of the surrounding metal into the screw slots; for if
the screws work loose they will tear up the armature windings.
Access to the brush gear presents no difficulty on most two-wheelers, nor on many three-
wheelers. But the engine must first be removed from the 191cc Messerschmitt, after the
various leads and control cables have been disconnected and the three mountings undone.
The Heinkel Cabin Cruiser needs to be placed over a pit or to have the rear wheel supported on a bench.
On both the Heinkel three-wheeler and its sister scooter removal of the right-hand engine
cover breaks an oil-tight joint and allows oil to leak from the sump. If a permanent leak
is not to follow the joint must be properly remade later.
TEST PROCEDURE ON VOLTAGE REGULATORS AND REVERSING SOLENOIDS
With this unit on the
bench, applying a voltage of not less than 7
between terminal 50 and case, the starter solenoid will be operated and continuity should be
read between terminal 51/30 and A. If the solenoid is heard to pull in and no continuity
is read between the two terminals, then the starter contacts need adjusting. It is advisable
to check that there is no continuity between terminals A and 51/30.
To check the cut-out, apply a voltage of not less than 12 from terminal 61D+ and
case, continuity should then be read between terminal 51/30 and 6lD+. If no reading is
obtained. it could be due to: (1) open circuit in the cut-out shunt winding, or (2) cut-out
contacts not making, hence requiring adjustment. With the unit in-situ the open circuit
voltage reading measured from terminal 61D+ to earth. with no load and engine rpm
approximately 2,000, should be between 14 and 15 volts, if less than this the unit should be
returned to the Siba service department for readjustment If no voltage is obtained then by
shorting out lead DF to earth and with approximately the same rpm the voltmeter needle
should show over 20 V; if it does, it proves
that the armature and stator etc. are satisfactory, and that the regulator unit is unserviceable.
The functioning of the reversing solenoid is to change over the polarity of the brush gear and Contact breaker.
On a bench check in the forward position, continuity should be read from terminal HE to
B2, neither should earth. 50R to B1 should read to earth, and from 1 to UV again
neither should earth, To select reverse, apply a voltage to terminal 50R and case when
the solenoid will pull in, continuity will then be read from terminal HE to Bl neither should
read to earth. from terminal 50R to B2 both will be earth and 1 to UR neither should
earth. If the continuity tests break down, it points to the contacts inside the unit not
making, and would account for either not starting or charging.
Again Siba does not advise a unit to be tampered with, but the above checks will prove
whether a unit is faulty or not.