Sergio, I saw your query on dwell and offer the following specs.
Lucas 4 cyl dist has (like others) a theoretical 90 of dwell angle
available but of course we cannot obtain that much in actual practice.
When the point gap is set correctly per book, you will have about 74 deg of
dwell on the dwellmeter.
When a new set of points are installed, there is a little bit of wear which
settles the rubbing block into the cam surface. This closes up the gap a
little but the more important affect is that this gap closing RETARDS the
timing setting a degree or two.
Used points don't do that of course. Therefore owners are well advised to
lube the cam with a proper cam grease (sparingly) then recheck timing after
say 100 miles of driving.
In the olden days when we had fiber rubbing blocks, this was a serious
problem and the sad fact was that a tuned engine was only in tune for a few
days, then the timing gradually retarded and HP was lost. With current
Teflon or nylon rubbing blocks (and a nice smooth cam lobe) there is
virtually zero decrease of point gap.
On this same subject.......the stock Lucas dist and points will fire
accurately to at LEAST 6500 engine rpm if the spring tension is about 18-20
oz. On race engines, I use electronic triggering ignition moduals and the
Lucas points only have to handle tiny amounts of amperage. In this case I
use only nylon rubbing block points which have been used, not new.
Increase spring tension to approx. 22 oz. and well lube the cam before
every race event.
Think about this following fact.......points are a switch ! A perfect
switch closes with zero ohms resistence ! If your old used points have
zero ohms when closed...what good is it to replace them?
For the reason that old points have shown reliability and obvious proper
assembly, they should NOT be replaced by points which are not PROVEN to
have such reliability.
The ONLY reason to replace points is....when the rubbing block has worn so
far as to prevent obtaining proper dwell/gap specs. Or of course, if the
spring tension is lost or some other damage is observed. Since the spring
doe not conduct amps. it should never get hot and lose temper. I have seen
this happen however, when improper points were installed or there was
something else which caused amps. to pass through the spring itself.
Due to bushing wear and lack of lubrication, these Lucas units can be found
in really wobbly conditions which cause erratic timing at low rpm. This
timing variation tends to even out above say 2500 rpm but even then the
problem can relate to points failing to follow the can accurately at very
high rpm.
A new bushing and perhaps a new dist. cam can make a big difference in
power and smoothness...and the cost is very low.
Useing a powered timing light, study your timing mark at idle. Look for
bouncing and movement on the pulley rather than a nice steady mark
illumination action.
On another ignition related subject....engine detonation (knocking or
pinging) wrecks a Rootes engine faster than just about anything I know of,
short of oil and coolant loss.
Ignition timing advance is determined by the distance from the spark plug
tip to the furthest part of the combustion chamber. The 82mm bore we use
has been determined to require a 36 deg (crankshaft deg.) spark advance in
order to have the flame front cover the entire piston crown at high rpm.
This flame front has essentually a constant travel speed which means that a
"proper mixture" when exploded, travels at a nicely known predetermined
speed. Therefore.....to safe way to obtain max HP and also to prevent
detonation from wrecking the pistons, is to set timing at full advance and
let the timing at idle rpm fall where it may.
Mark your pulley at 36 deg BTDC then with vacuum OFF, rev engine to 2500
engine rpm (this is full centrifugal advance speed) and turn the dist to
set the flash at 36 deg.
If the cent. weights are free and lubed, then idle rpm will give you at 10
deg. BTDC at idle and this is just fine. The main thing being that your
engine will never see more than 36 deg BTDC at high rpm.
Street engines have a vacuum advance device to overadvance the timing
during light load cruise and on deceleration (both are conditions of high
manifold vacuum). This device contributes to better fuel mileage and should
be used on all engines , even on race engines it is OK.
When you dump open the carb. throttles, the manifold vacuum instantly drops
to zero and the vacuum advance modual never moves at all. Therefore, when
the engine is under any serious load, it will be running ONLY on
centrifugal (mechanical) advance and the vacuum unit will never come into
play at all, until you lift off the pedal and allow manifold vacuum to
instantly build up and cause the unit to advance the timing.
We generally remove the vacuum unbit when we build up a race distibutor
ONLY because we dont want to have the fuel economizing affect and also we
dont want the extra linkage and presence of the device in case it just
might cause a jam or problem. I have NEVER seen one cause a problem but
then race mechanics do lots of things to avoid problems they fear but have
never seen !
There are many more things and tips related to ignition systems but the
above should prove of value to many of you readers here and these facts
also apply to other marque engines, not just to Rootes.
Dick.T.
This archive was generated by hypermail 2b30 : Tue Sep 05 2000 - 09:55:38 CDT