I agree with Mike. The most common cause of momentary hesitation when the
throttle is depressed is a weak accelerator pump in the carb.
The function of the vacuum advance is to add ignition advance when the
vacuum increases. As the throttle is depressed, the vacuum decreases and
advance decreases. During full throttle acceleration, there is little
and the VA does nothing. If the VA leaks and is non-functional, you won't
feel it under full throttle. But if it is non-functional, you will feel a
general lessening of part throttle power and what fells like "drag" during
cruise. I've had this happen on a couple of other cars. After checking the
carb accelerator pump first, check the VA for proper function. After 50
years, it is not uncommon for the fabric diaphragm to fail. We should all
keep a spare.
Adjusting the vacuum advance with an Allen wrench is a nice feature, but
this style vacuum advance mechanism doesn't apply to ForwardLook era cars.
Bending the arm is highly NOT recommended. It could affect the alignment
the arm and cause an increase in friction or sticking, or tearing of the
diaphragm. If some play in the travel is removed, it would only be an
However, Mopar vacuum advances in the 50s were adjustable. The total
mechanical travel for the total degrees of vacuum advance was pre-set by a
notch limiting travel on the arm. The total degrees can be changed by
a different P/N diaphragm. These were available over a range of 8 to 30
degrees, in 2 degree increments. Make sure you get the correct P/N if you
replace it. The spring inside the diaphragm controls the rate of advance
with the change of vacuum. It is easily removable through a cap on the
where the vacuum line attaches. There are about 8 different springs. The
spring could also be shimmed to shift the vacuum at which the VA starts to
move to a higher vacuum level.
While we are on the subject, the mechanical advance has a lot of variables
too. There is the initial timing at idle, ranging from 2 to 10 degrees.
There are mechanical advance cam/plate assemblies limiting total
advance ranging from 12 to 32 degrees. And there is a wide variety of
mechanical advance springs.
The possible combinations of all of the above factors numbers in the 10s
thousands. Most of which would be bad combinations.
Yes, there is another spot for oil inside the distributor. Underneath the
rotor and on top of the distributor shaft, there is a flat oil wick that
supplies lubrication to the joint between the shaft and the cam/plate
mechanical advance assembly. I would use a 10W motor oil. Since you
done this for at least 10 years, apply about 5 drops or at least enough to
saturate the wick. If you do this every few thousand miles, then use only
or 3 drops. These types of periodic and usually overlooked lubrication
points are all explained in the "Lubrication" section of the Shop Manual.
The breaker plate also rides on greased ball bearings when actuated by the
vacuum advance. And the mechanical advance weight pivot points need
lubrication (I've had these stick too). After 50 years, it might be a
sticky. See the "Electrical" section of the shop manual.
56 Dodge D500
From: Forward Look Mopar Discussion List
[mailto:L-FORWARDLOOK@xxxxxxxxxxxxx]On Behalf Of Mike Apfelbeck
Sent: Saturday, November 27, 2004 5:06 PM
Subject: Re: [FWDLK] Hesitation is depressing!!
Best not fool with that just yet, all you will be changing is the total
degrees of vacuum advance, not when it occurs. The usual cause of a part
throttle stumble is the accelerator pump, check that first. You could run
it down the road with the vacuum advance disconnected to see if it makes a
difference, be sure and check the ground wire on the distributor plate in
case it is breaking the connection when the vacuum advance moves.
At 12:14 PM 11/27/2004, eastern sierra Adj Services wrote:
Does anyone have any experiece in adjusting a vacuum advance? H.'s got a
"part-throttle-hesitation", a flat-spot, after he's already driving down
the road, & the throttle's depressed.
I understand that the V.A. can be adjusted, using allen wrench, but the
V.A. is a tad "remote" on the pre-B-block motors, so I don't wanna get
involved in a big 'project', if there's not much ease/chance of
adjustment without using a timing gun, or 'scope', etc.
Would it just be easier to replace the V.A.?
BTW, H.'s got an electronic ignition, which has "required" no
maintenance, over the past decade. I just put some oil in the
'oiler-fitting', but haven't driven him, since doing so. Is there
another oil-fitting, inside the distributor?
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