An update to Jeff's Q about the stumble resolution
----- Original Message -----
From: <ptegler@gouldfo.com>
To: <Mjsprite@aol.com>
Sent: Friday, February 02, 2001 11:40 AM
Subject: Fw: Vacuum Advance - pulse/stumble/surge
below... are three of the latest emails that were
swapped back and forth on the MG list
you may want to go in and join the MG list on the
team.net site and read the 400+ posts over the last
two years concerning this problem and all the crap
I tried with no resolution. The vac advance curve solved it.
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----- Original Message -----
From: <ptegler@gouldfo.com>
To: "R. O. Lindsay" <rolindsay@dgrc.com>
Sent: Thursday, February 01, 2001 12:49 PM
Subject: Re: Vacuum Advance
Well if you've been following my stumble/surge
threads... you're giving everyone a hint as to what
was going on my '73 BGT.
Here's the deal. You don't repeat this.. yet (to the lists)
And I'll give you a clue on how to determine your answer
correctly. I'm still writing up details for my web pages.
To start with.... check
http://www.teglerizer.com/mgstuff/advance_curves.htm
and also not too bad but a bit confusing to read)
http://www.mgbmga.com/tech/mgb21.htm
for a complete list of curves, part numbers, specs.
On your dist. body, you'll find a flat area that has one of the numbers
listed on the chart stamped into the body of the dist.
This identifies the mech. advance curve.
Around the neck of your vac adv. unit (right near the vac line
attachment) will be tiny numbers stamped into it. The long number
will match one in the chart as well. Make sure the right vac unit
is on the right dist. per the chart. The chart also lists the
'ported' or 'manifold' vac source for setup requirement for
that unit.
I had rebuilt everything to '73 specs. Had a '73 Spec
distributor with the correct vac unit for the dist.
In my case... a 41491 dist with vac unit 54425359 running
10.15.5 vac connected directly to the manifold
(See the teglerizer chart)
Apparently both the vac unit and dist model did not match
the motor/cam config which WAS causing this stumble at cruise.
I was using manifold vac (correct spec for the 18V '73)
model engine I 'thought' I had.
Turns out my engine is a 68-70 model as far as cam is concerned
and has a HC (high compression) head on it.
I put in a spare 41288 dist I had laying around with the correct
matching 54411985 vac unit (5.13.10) and hooked up the vac
source to the port on the carb. (see 41491 specs mentioned above
for vac scale difference)
I initially reset timing to 10... then bumped it up to 14 BTDC
at 600 rpm.
BINGO! I can actually set the idle as low as 550 rpm without dying. Starts
even easier than before (even in cold weather) I was able to
lean out the carbs a bit. and have more power at the low end
(upper end seams un effected by the change over)
So .... to tell what you need (what runs best regardless of what
you think, or what the books say it should be....
you need to determine what you have...and then test
a few variations. I'm sure you can find a friend with a car
that has some setup different then yours and experiment.
I didn't even change dist. caps. It took less than ten minutes to swap
distributors, hook up the vac lines and retime it.
I was actually a little pissed-off that it was such a simple fix
to my problem. In the past I had replaced carbs (brand new ones)
tried two different elec. ign. systems as well as points
ran no vac/with vac played with mech. advance weights...
all kinds of stupid stuff with no change in this problem.
The distributor changed everything. Even my lady friend
says it feels like a different car.
I haven't sent this info off to the list.
Paul Tegler ptegler@gouldfo.com www.teglerizer.com
> ----- Original Message -----
> From: "R. O. Lindsay" <rolindsay@dgrc.com>
> Cc: <mgs@autox.team.net>
> Sent: Thursday, February 01, 2001 11:46 AM
> Subject: Vacuum Advance
> Hello gang,
> The original designs for vacuum advance used a port on
> the carburetor that was connected to a small opening in
> the throat of the carburetor. At idle, that opening is almost
> completely covered by the throttle plate. Little-to-no
> vacuum is then present on the port when the throttle is closed.
> As the throttle is opened, the vacuum rises (depresses?) and
> the vacuum advance module goes to work. My vacuum module
> is a 5-17-10 model. That is, no action until 5psi depression
> (meaning no advance at all when the throttle is closed) and a
> maximum of 20 degrees <crankshaft> advance when the
> depression is at 17 psi. or greater.
> Now for the problem. Newer designs have a vacuum port on
> the inlet manifold rather than the carb. That port shows
> pretty strong vacuum even at idle. My car has BOTH ports.
> Which method of applying advance is BETTER for my engine
> and my application? Emissions are not an issue. Recalling, the
> car is a '70 B-GT to be used for primarily for spirited street
> driving. It is equipped with dual HS4 carbs.
> Thanks for all constructive advice.
> Best,
> rick
-----------------------------
...have to add a bit of 'technology flip-flop' info that
has taken place over the years.
As Barney described...manifold vac goes with retard
ported goes to adv.
But there were years/models that did use a vac adv.
pot on the dist. while hooked to the manifold rather than
ported to the carb throat.
The idea 'at that time' was with max vacuum at idle...
you'd get max advance. No engine load at idle with max advance
meant a cleaner idle burn. As you opened the carb...
(accel) the vac dropped off a bit, so it wouldn't ping.
Then up at cruise... the high manifold vac would
then again adv. for max burn time.
The reason this format is no longer used is one of component
'slop' and timing of cam/dist/spark etc.
If just a wee bit off...the systems start fighting each other
very inefficiently.
If anyone has been following my stumble/surge problem....
this very subject was the root of the problem.
The aging systems were the 'stumble' I had.
I've gone to an older model dist, (different vac adv. curve)
and now use ported vac, and the problem is totally gone
now. A side reward was also better power at the lower
rpm ranges.
So ...just because you rebuilt everything to spec for your model year
doesn't mean it's the best setup for your aging toy.
The cat's out of the bag Robert L. ... :-)
...web pages to come.
Paul Tegler ptegler@gouldfo.com www.teglerizer.com
> ----- Original Message -----
> From: "Barney Gaylord" <barneymg@ntsource.com>
> To: "R. O. Lindsay" <rolindsay@dgrc.com>
> Cc: <mgs@autox.team.net>
> Sent: Thursday, February 01, 2001 2:39 PM
> Subject: Re: Vacuum Advance
> At 10:46 AM 2/1/2001 -0600, Rick Lindsay wrote:
> The original designs for vacuum advance used a port on the carburetor
> that was connected to a small opening in the throat of the carburetor. ....
> As the throttle is opened, the vacuum rises (depresses?) and the vacuum
> advance module goes to work. ....
> .... Newer designs have a vacuum port on the inlet manifold rather than
> the carb. That port shows pretty strong vacuum even at idle. My car has
> BOTH ports. Which method of applying advance is BETTER for my engine and my
> application? ....
> The method ported from the manifold uses a different distributor, which can
> be thought of as "vacuum retard" distributor. The vacuum from the manifold
> is high at idle and holds the distributor in a retarded position. When you
> open the throttle the manifold vacuum drops, allowing the timing to
> advance. You need to use whichever port is appropriate for your
> distributor. Or conversely, for those who do not have both ports, you need
> to use whichever distributor is appropriate for your existing port.
>
> Barney Gaylord
> 1958 MGA with an attitude (and no vac Mallory dizzy)
> http://www.ntsource.com/~barneymg
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hope all this helps!
Paul Tegler ptegler@gouldfo.com www.teglerizer.com
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