O.S. Engines 21XM - Manual (Page 9)

PROPELLER
The MAX-21XM's special carbon-fibre propeller has a
diameter of 42mm and a P/D ratio of 1.2. If any other propeller
is substituted, make sure that it is properly balanced.
FLEXIBLE DRIVE SHAFT
If, due to accidental damage or wear, the flexible drive shaft
(see p.16) should require replacement, proceed as follows.
(Note: a wise precaution is to have a spare flexible shaft on
hand for such emergencies.)
BEFORE STARTING THE ENGINE
When to replace the glowplug -
Apart from when actually burned out, a plug may need to be
replaced because it no longer delivers its best performance,
such as when:
Never touch, or allow any object to come into contact
with, the rotating propeller and do not crouch over the
engine when it is running.
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Filament surface has roughened and turned white.
Filament coil has become distorted.
Foreign matter has adhered to filament or plug body has
corroded.
Engine tends to cut when idling.
Starting qualities deteriorate.
With aid of the correct sized screwdrivers and hexagon
keys, separate the complete lower end (outboard unit
assembly) from the power-head and remove all parts
necessary to give access to the flexible drive shaft. This
has squared ends that fit into the PTO shaft at the power-
head and the propeller shaft at the bottom end. Note that
the propeller shaft bearing has a left-hand thread.
Fit a plug suitable for the engine (e.g. OS No.8).
Use fuel containing a moderate percentage of nitromethane
unless essential for racing events.
Do not run the engine too lean and do not leave the battery
connected while adjusting the needle. ●
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The rapid changes of hull attitude that occur when a fast
boat is operating in choppy water, especially when the fuel
tank is located at some distance from the carburettor,
means that fuel ''head''at the carburettor jet can vary quite
considerably, causing the engine to run erratically or stop.
Therefore, it is advisable to use a moderately pressurized
fuel system.
1.
Remove the flexible drive shaft and its sleeve and if any deb-
ris is found in the sleeve, wash it out with fuel or alcohol be-
fore greasing the full length of the new shaft and reinserting
it into the sleeve.
Make sure that the ends of the new shaft fit properly into the
PTO shaft and propeller shaft and that the new flexible shaft
rotates smoothly.
Then replace the curved side plate, cavitation plate,etc. and
re-unite the power-head with the lower assembly. Gently turn
the flywheel to check that the propeller rotates smoothly be-
fore tightening assembly screws.
2.
3.
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