TM 9-4910-677-14 & P
Pairs of cylinders can be placed
under load by shorting out the plugs
o f the other cylinders after opening
t h e throttle valve and locking it in
p l a c e . If two cylinders are able to
r u n the engine against the compres-
s i o n of the others, it indicates that
t h e ignition is not missing, that the
carburetor high speed and power
circuits are working and that com-
pression must be at least in fair
condition.
Compare
the
vacuum
r e a d i n g s while running on different
pairs of cylinders.
On 4 cylinder engines, set the
The vacuum gauge i s a n a c c u -
t h r o t t l e so the engine will run about
rate means of adjusting ignition
900 RPM with all cylinders operating.
timing.
With the motor at a fast
Then run on the following pairs with
i d l e , advance the ignition timing so
a s to get the highest possible steady
t h e other cylinders grounded out.
r e a d i n g on the
vacuum gauge. N o w
Run on cyl. 1 and 4.
Run on cyl. 2 and 3
retard so that the needle on the
g a u g e just begins to drop. Now the
On 6 cylinder engines fix the
question of detonation or
ping
throttle at about 1000 RPM w i t h a l l
e n t e r s t h e p i c t u r e . I f the cooling
cylinders operating Then run on
system is functioning properly, the
the following pairs with the remain-
spark plugs are not too hot, the com-
i n g cylinders grounded out.
b u s t i o n chamber is not insulated by
a carbon mixture, the carburetor
Run on cyl. 1 and 6
Run on cyl. 2 and 5
mixture is right, and the mixture
Run on cyl. 3 and 4
from the carburetor has the proper
b e r , this is the proper timing posi-
On 8 cylinder- engines fix the
running
tion. Unfortunately, in many in-
throttle at about 1500 RPM
s t a n c e s , o n e or more of the above
on all cylinders.
Then run on the
mentioned conditions exist, and we
following pairs with the other cyl-
m u s t set the ignition timing to com-
i n d e r s grounded out.
p e n s a t e for them. To do this, short
Run on cyl. 1 and 8
Run on cyl. 3 and 6
out the cylinders by clipping the dif-
Run on cyl. 2 and 7
f e r e n t leads to the spark plugs until
Run on cyl. 4 and 5
the motor is working on two cyl-
inders. We are now able to dup-
The manufacturers of V-8 en-
licate a load tendency of the
gines use several different methods
motor to detonate or ping. Re-
tard the timing till the motor does
not ping, and the timing is now
system (see illustration) has been
set to eliminate detonation.
developed