Vacuum advance advances too much
Halterb
Initial timing is 6 degrees at 600 RPM, with vacuum advance specification 13
1/2 to 14 1/2 at 16 PSI. With the vacuum hose disconnected and plugged the
timing checks out at 6 degrees at idle (600 RPM).. Yet, when the vacuum hose is
attached, the advance jumps up to 30 degrees, about 10 degrees too much. I
assume this is the cause of the spark knock I get on light accelleration.
Spark knock has been a problem with this car most of its life. (I've had it
since new.) I replaced the vacuum advance unit a number of years ago, but still
had knocking to some extent. At that time, I didn't think to observe the total
timing advance at idle so I can't say what that figure was. I use Shell 92
octane or Mobile 93 octane gas.
It would seem that if the spring in the vacuum advance unit was weak, the
timing would advance too much--I'm assuming that is the problem. The original
GM part is no longer available, so I had to use another brand. If I replace it
again, I'm wondering how to be certain that the substitute unit is properly
calibrated for this car.
Any comments would be appreciated.
Kevin Mouton
Most after market vacuum advance units are adjustable. If you insert an allen
wrench in the tube where the vacuum hose would connect you may find that there is a
small screw in there that can be turned to limit the total advance.
--
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Kevin Mouton
Automotive Instructor
http://www.eatel.net/~kevinm/homepage.htm
"If women don't find you handsome,
they should at least find you handy" Red Green
DDDservice
>> timing would advance too much--I'm assuming that is the problem. The
>original
>> GM part is no longer available, so I had to use another brand. If I replace
>it
>> again, I'm wondering how to be certain that the substitute unit is properly
>> calibrated for this car.
>>
>> Any comments would be appreciated.
>
>Most after market vacuum advance units are adjustable. If you insert an allen
>wrench in the tube where the vacuum hose would connect you may find that
>there is a
>small screw in there that can be turned to limit the total advance.
>
>
>--
>* WARNING * Reply address may contain anti spam.
>
>Kevin Mouton
>Automotive Instructor
>http://www.eatel.net/~kevinm/homepage.htm
> "If women don't find you handsome,
> they should at least find you handy" Red Green
>
advance systems, the vacuum line is normally attached to port vacuum. If you
are seeing 16in Hg. at idle, it would indicate you are tied to manifold vacuum.
I'm not a Cadillac expert, so I could be all wet here.
Glenn
the fly
The vacuum advance should not be fully advanced at idle. The diaphragm
is supposed to be connected to ported vacuum at the carb throttle body,
not straight manifold vacuum.
jiva
>The vacuum advance on my 1976 Cadillac Sedan DeVille 500 ci goes too far.
>Initial timing is 6 degrees at 600 RPM, with vacuum advance specification 13
>1/2 to 14 1/2 at 16 PSI. With the vacuum hose disconnected and plugged the
>timing checks out at 6 degrees at idle (600 RPM).. Yet, when the vacuum hose is
>attached, the advance jumps up to 30 degrees, about 10 degrees too much. I
>assume this is the cause of the spark knock I get on light accelleration.
>
>Spark knock has been a problem with this car most of its life. (I've had it
>since new.) I replaced the vacuum advance unit a number of years ago, but still
>had knocking to some extent. At that time, I didn't think to observe the total
>timing advance at idle so I can't say what that figure was. I use Shell 92
>octane or Mobile 93 octane gas.
>
>It would seem that if the spring in the vacuum advance unit was weak, the
>timing would advance too much--I'm assuming that is the problem. The original
>GM part is no longer available, so I had to use another brand. If I replace it
>again, I'm wondering how to be certain that the substitute unit is properly
>calibrated for this car.
>
>Any comments would be appreciated.
I agree with the comment about making sure it is connected to the proper
(ported vs manifold) vaccum ... and make sure it's supposed to be timed
with the hose plugged.
"Goodbye" said the fox to the Little Prince. "And now here is my secret,
a very simple secret: It is only with the heart that one can rightly see,
what is essential is invisible to the eye"
from Antoine de Saint-Exupéry's "The Little Prince"
jim bilderback -- please remove the * if you'd care to email.
Halterb
>I agree with the comment about making sure it is connected to the proper
>(ported vs manifold) vaccum ... and make sure it's supposed to be timed
>with the hose plugged.<
I have the Cadillac shop manual and its instructions for setting initial
advance (6 degrees) are that it be done with the vacuum hose disconnected and
plugged shut. The vacuum source is as it was when new and seems to correspond
with the illustrations in the manual.
Brendan Spaeth
advance line at idle. Also, you spec the total advance at about 15
degrees, but are getting almost 25, so the advance unit is giving too
much advance. I know some advance units are adjustable, but not all, so
you may need to get the correct one. Do you still have the old one? I
suspect there may have been nothing wrong with it.
The advance unit's purpose is to enhance part throttle response, in that
it gives the ignition a very quick advance right off the line, and makes
the car more responsive. Manifold vacuum on the other hand will
actually drop upon pressing the accelerator, and reduce the advance,
giving diminished response and worsened fuel economy, because the driver
will push the gas pedal even harder to get moving.
Best regards.
the fly wrote:
>
> My thoughts, too.
> The vacuum advance should not be fully advanced at idle. The diaphragm
> is supposed to be connected to ported vacuum at the carb throttle body,
> not straight manifold vacuum.
>
> DDDservice wrote:
> >
> > >> It would seem that if the spring in the vacuum advance unit was weak, the
> > >> timing would advance too much--I'm assuming that is the problem. The
> > >original
> > >> GM part is no longer available, so I had to use another brand. If I replace
> > >it
> > >> again, I'm wondering how to be certain that the substitute unit is properly
> > >> calibrated for this car.
> > >>
> > >> Any comments would be appreciated.
> > >
Halterb
>I have to agree that you shouldn't have any significant vacuum in the
>advance line at idle. Also, you spec the total advance at about 15
>degrees, but are getting almost 25, so the advance unit is giving too
>much advance. I know some advance units are adjustable, but not all, so
>you may need to get the correct one. Do you still have the old one? I
>suspect there may have been nothing wrong with it.<
No, I don't have the old vacuum advance unit anymore.
>The advance unit's purpose is to enhance part throttle response, in that
>it gives the ignition a very quick advance right off the line, and makes
>the car more responsive. Manifold vacuum on the other hand will
>actually drop upon pressing the accelerator, and reduce the advance,
>giving diminished response and worsened fuel economy, because the driver
>will push the gas pedal even harder to get moving.<
The plot thickens! The Cadillac shop manual says to remove and plug the vacuum
hose to set initial timing because extra air drawn into the engine can cause an
increase in engine speed. If the manufacturer intended there to be no vacuum to
the vacuum advance at idle, then it wouldn't seem necessary to disconnect the
hose, or plug it, since it would require the presence of vacuum to draw air in.
Also, my impression of the purpose for vacuum advance is different. I thought
the idea was to retard the timing on heavier loads to prevent knocking, not to
advance it even farther. Manifold vacuum dropping when the acellerator is
pressed would provide the desired slowing of timing and would seem to be what
was intended.
The EGR valve runs off ported vacuum, but it appears the vacuum advance is set
up to run off manifold vacuum.
AJDalton7
There are also cetrifical weights that are spring controlled. Look under the
advance plate.
Brendan Spaeth
>
You wrote:
> The plot thickens! The Cadillac shop manual says to remove and plug the vacuum
> hose to set initial timing because extra air drawn into the engine can cause an
> increase in engine speed. If the manufacturer intended there to be no vacuum to
> the vacuum advance at idle, then it wouldn't seem necessary to disconnect the
> hose, or plug it, since it would require the presence of vacuum to draw air in.
There is generally very little vacuum at that port at idle speed, but it
is true that when setting timing, the procedure calls for the advance
line to be plugged. Even a little vacuum will create advance that will
make the initial idle setup incorrect. With the hose disconnected, you
will get advance with increasing engine rpm due to the mechanical
advance inside the distributor.
>
> Also, my impression of the purpose for vacuum advance is different. I thought
> the idea was to retard the timing on heavier loads to prevent knocking, not to
> advance it even farther. Manifold vacuum dropping when the acellerator is
> pressed would provide the desired slowing of timing and would seem to be what
> was intended.
> The EGR valve runs off ported vacuum, but it appears the vacuum advance is set
> up to run off manifold vacuum.
You thought the purpose of the vacuum advance unit was to retard the
ignition, you say? Funny name for the device then isn't it! I'm sorry
to point that out, but you are coming to invalid conclusions based on
limited info. Bear in mind, that ported vacuum can never be greater
than manifold vacuum. As you put the car under heavy load, all vacuum
decreases, and thus so does the vacuum advance. As I stated before, the
vac advance is meant to enhance part-throttle response. Some Ford
products did come with a dual solenoid vacuum advance/vacuum retard
setup on the distributor, and I don't have the Cadillac manual to go by,
but if the dist. has a single vac line, then it is an advance only unit,
and it is supposed to be connected to ported vacuum. Check the vacuum
line diagram under the hood.
Regards,
Brendan
litely
amazing how many people do not understand the meaning of "advance" and
"retard" as it pertains to auto engines. "advance" means/refers to
settings before/prior to the flywheel mark reaching "zero" on the
reference gauge. This means a sparkplug fires prior to the piston
reaching top dead center (TDC). Knocking occurs when the plug fires
late or after (TDC)-retarded. Changing the dwell angle of your
distributor points AFTER setting the timing, will alter the timing.
Changing the type of sparkplugs can alter the timing, also. With the
engine off try applying vacuum to the vacuum advance. If it won`t hold
a vacuum, replace it. The replies relating to where you get your vacuum
can be very important--get it it at all possible (as it should be) from
your intake manifold. This vacuum increases under acceleration and
decreases (relatively) when cruising or decelerating. Good Luck Oh, by
the way--a frozen exhaust manifold heat control can also cause some
knocking.
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Steve Bigelow
litely wrote in message <000b8d9b...@usw-ex0102-015.remarq.com>...
>Knocking occurs when the plug fires
>late or after (TDC)-retarded.
Really.
>Changing the type of sparkplugs can alter the timing, also.
Really.
How does the distributor know which plugs are installed?
Halterb
The Cadillac manual does, in fact, have a diagram of the vacuum system--a
Cadillac size diagram, measuring 18 by 32 inches.
The vacuum advance unit seems to be connected to manifold vacuum, contrary to
some of the previous posts.
I'm surprised to see it stated that spark knock comes from timing that is too
slow. That's the opposite of my impression. I assume the reason my engine
knocks is because the timing is 30 degrees advanced, when the total of
centrifugal and vacuum advance should only be 20 degrees. (Yes, I am aware that
there are two types of advance.)
Then, there's the comment that the vacuum advance is supposed to advance the
timing, not retard it. Well, I suppose--but my impression is that it advances
the timing under light loads, while providing less advance under heavier loads.
In other words, it advances more some times than other times.
At this point, it seems about the only thing for me to do is to find an
adjustable vacuum advance and set it to give no more than 20 degrees total
(including initial) advance at idle and see what happens.
litely
talking from experience. If you`re willing to put in the time trying it
out. The firing gap determines how rapidly input from the distributor
will result in ignition. You might think that .010-.020 variation
should not make a difference. You would be surprised. I found 2 to 3
degrees variation not uncommon. I have a mea culpa on the knocking
thing. If past memory serves, I had a 1973 bonne (400-Quad) which was
used only locally for several years. It began knocking. Only after
taking it out on the hiway and blowing the carbon out did it resume
normal operation. I think they refer to it as preignition knocking from
carbon deposit buildup. Think that could be it?????
N. Nelson
<obiedya...@socket.net.invalid> wrote:
> You`re correct, the distributor doesn`t know what plugs you have. Just
> talking from experience. If you`re willing to put in the time trying it
> out. The firing gap determines how rapidly input from the distributor
> will result in ignition. You might think that .010-.020 variation
> should not make a difference. You would be surprised. I found 2 to 3
> degrees variation not uncommon. I have a mea culpa on the knocking
> thing. If past memory serves, I had a 1973 bonne (400-Quad) which was
> used only locally for several years. It began knocking. Only after
> taking it out on the hiway and blowing the carbon out did it resume
> normal operation. I think they refer to it as preignition knocking from
> carbon deposit buildup. Think that could be it?????
>
Any effect that sparkplug gap (or any other form of secondary resistance)
-might- have on ignition timing would be measured in values of picoseconds
or less.
Not the sort of thing you'd see with a timing light or feel in added or
reduced performance.
It can be measured with an oscilliscope, if the time base can be adjusted
low enough.
Now breaker point gap, is a whole different animal.
--
Neil Nelson
***********************************
Send Lawyers Guns and Money,
The shit has hit the fan. WZ
***********************************
Halterb
>If past memory serves, I had a 1973 bonne (400-Quad) which was
>used only locally for several years. It began knocking. Only after
>taking it out on the hiway and blowing the carbon out did it resume
>normal operation. I think they refer to it as preignition knocking from
>carbon deposit buildup. Think that could be it?????<
Have tried it, even used a commercial top cylinder cleaning product. No help.
AJDalton7
And, indeed, the weak link in all point ignition systems.
Fortunately, there are pick-up conversions on the market now for just about
any point sytem, from your old lawn-
mower to your car. The best $$$$ value
improvement you can make to an ignition
system. With the new hi-tech switching
modules, points are a thing of the past.
Arthur
forevergone
distributor BUT....I believe that inside the cap there are these funny
shaped 'things' that actually do the 'advancing' (the 'centrifugual
[sp?] advance') and I believe that they sometimes rust up and stick.
Is it possible that this is your problem? I have no idea why this
would give you too much advance, but hot rod mags always are talking
about how important this is. You can tell simply by trying to move
them (I guess - never had this problem on GM cars) - if they bind free
them up. With the exception of the little 'module' the HEI system is
pretty reliable. In total desperation you might go to a junkyard, find
a Caddy engine, and double check where the vacumn [sp?] lines are
supposed to go - could even pull the whole distributor.
Good luck!
Halterb
>I believe that inside the cap there are these funny
>shaped 'things' that actually do the 'advancing' (the 'centrifugual
>[sp?] advance') and I believe that they sometimes rust up and stick.<
In my particular case, the centrifugal advance system is working properly, so
that's been ruled out. It's the vacuum advance that's gotten carried away with
itself. According to the Cadillac manual, the vacuum lines are connected
correctly.
DDDservice
>that's been ruled out. It's the vacuum advance that's gotten carried away
>with
>itself. According to the Cadillac manual, the vacuum lines are connected
>correctly.
If I recall correctly, you had previously said you were at 14-16in Hg. at the
vacuum advance port. That is by most engine manufacturers considered in the
upper region of port vacuum. Therefore it is natural to assume that the
distributor will go to full vacuum advance. Either the Cadillac manual is
incorrect in where to tie the line in, someone has removed a vacuum valve, or
something internal to the carb is causing port vacuum to be so high. I believe
the distributor is acting as it should. I would test the pinging issue by
removing the vac advance line, plugging it and driving the car. It should be
"doggy" without the advance, but shouldn't ping. Then you need to find out why
you have so much vacuum at that line. BTW, what is the manifold vacuum at idle
vs. port vacuum at idle?
Glenn
litely
kind of distributor you have. Either late 77 or early 78 began seeing
the HEI distributor. Some combined electronic module under the rotor
with standard vacuum advance unit. Some did away with the vacuum
advance. You indicated you have vacuum advance. If it is the standard
mechanical points job, check resistance of your external coil. If my
poor memory serves, it should not exceed 17k ohms. If you have the HEI
with the coil in the distributor cap, replace the electronic unit under
the rotor, its only and few bucks. If this don`t cut I`ll have to
admit being out of gas and bow out. Had a good run at it anyway--lotsa
luck!!!
Halterb
>If I recall correctly, you had previously said you were at 14-16in Hg. at the
>vacuum advance port. That is by most engine manufacturers considered in the
>upper region of port vacuum. Therefore it is natural to assume that the
>distributor will go to full vacuum advance. Either the Cadillac manual is
>incorrect in where to tie the line in, someone has removed a vacuum valve, or
>something internal to the carb is causing port vacuum to be so high. I
>believe
>the distributor is acting as it should. I would test the pinging issue by
>removing the vac advance line, plugging it and driving the car. It should be
>"doggy" without the advance, but shouldn't ping. Then you need to find out
>why
>you have so much vacuum at that line. BTW, what is the manifold vacuum at
>idle
>vs. port vacuum at idle?
>Glenn<
Actually the vacuum at the distributor advance unit is 21, measured with a T
fitting in the line. Sixteen is the spec for maximum advance of 13 1/2 to 14
1/2 degrees. I added 14 to the initial 6 to come up with 20 as the maximum I
would have thought I would have seen at 600 RPM, when I actually saw 30. The
other vacuum fittings off the 4GC carb service the vapor canister, the EGR
valve, PCV valve, power brake and transmission modulator. The fitting to the
vacuum advance branches off to the carburetor primary vacuum break. Our weather
is too cold to do a lot of checking of other vacuum readings outside.
This knocking problem comes up after driving around 10 miles at highway speed,
coming to a stop or near stop, then mildly accelerating from the stop. After
driving at city speeds for a while, the knock stops. I have new plugs, used GM
top engine cleaner to blow out carbon, use 93 octane gas. My suspicion was that
with centrifugal advance at highway speed adding another 7 degrees to bring the
total advance to near 40 that that was causing overheating. The pros have been
unable to solve this, that's why I'm tackling it.
Thanks for your interest and help.
Brendan Spaeth
>
> "Certainly dont`t want to insult anyones intelligence however it is
> amazing how many people do not understand the meaning of "advance" and "retard" as it pertains to auto engines. "advance" means/refers to
> settings before/prior to the flywheel mark reaching "zero" on the
> reference gauge. This means a sparkplug fires prior to the piston
> reaching top dead center (TDC)."
Correct.
"Knocking occurs when the plug fires late or after (TDC)-retarded."
Absolutely false. Knocking occurs when the fuel air mixture ignites due
to heat and pressure prior to TDC and prior to the spark plug firing.
It is more likely to occur with an overly advanced spark due to the
increased combustion chamber temperatures. The walls of the combustion
chamber get hotter and tend to set off the fuel/air charge prematurely.
"Changing the dwell angle of your distributor points AFTER setting the
timing, will alter the timing."
Correct, but who uses points anymore? The poster has an HEI, which is
electronic.
"Changing the type of sparkplugs can alter the timing, also."
False.
"With the engine off try applying vacuum to the vacuum advance. If it
won`t hold a vacuum, replace it."
True.
"The replies relating to where you get your vacuum can be very
important--get it it at all possible (as it should be) from your intake
manifold."
Wrong. The distributor advance vacuum is ported, meaning above the
throttle plates, not manifold source, which is below the throttle
plates.
"This vacuum increases under acceleration and decreases (relatively)
when cruising or decelerating."
Manifold vacuum decreases under acceleration, and increases when
decelerating. What you are describing is ported vacuum.
"Good Luck Oh, by the way--a frozen exhaust manifold heat control can
also cause some knocking."
Probably true, but this guys problem (besides being thick) is he has way
too much advance.
> You are 4 for 9, maybe 5 for 9 if you meant the right thing on the last statement. Thanks for playing. You were right that it is amazing how little people understand...
Halterb
>Suggesttions todate not withstanding--try one more. You didn`t say what
>kind of distributor you have. Either late 77 or early 78 began seeing
>the HEI distributor. Some combined electronic module under the rotor
>with standard vacuum advance unit. Some did away with the vacuum
>advance. You indicated you have vacuum advance. If it is the standard
>mechanical points job, check resistance of your external coil.<
It's an HEI distributor, no points.
> If you have the HEI
>with the coil in the distributor cap, replace the electronic unit under
>the rotor, its only and few bucks.<
If your talking about the HEI module, that has been replaced (the things don't
really come all that cheap, though).
I think I'll run this by the manufacturer of the current vacuum advance and see
what they might say.
CadillacKen
Halterb <hal...@aol.com> wrote in message
news:19991130211937...@ng-bj1.aol.com...
On the front driver's side of the carb is ported vacuum, while manifold
vacuum comes from the vacuum break. Both are used depending on the
temperature of the engine.
Check the "Thermal Vacuum Switch" (it's on the front of the engine drivers
side, behind the distributor). Make sure the vacuum connections are correct
to/from this unit are correct.
C - goes to ported vac; M - goes to manifold vac; D - to distributor.
This valve switches:
The crossover heat valve on when engine is cold.
Ported vacuum to distributor when normal temperature.
Manifold vacuum to distributor when temperature over 220 degrees.
More advance is used when hot to speed up the idle and make the engine run
cooler. (At idle manifold vacuum is stronger than ported.)
Halterb
>On the front driver's side of the carb is ported vacuum, while manifold
>vacuum comes from the vacuum break. Both are used depending on the
>temperature of the engine.>
>
>Check the "Thermal Vacuum Switch" (it's on the front of the engine drivers
>side, behind the distributor). Make sure the vacuum connections are correct
>to/from this unit are correct.<
This car doesn't seem set up quite that way. The lines, and the diagram in the
shop manual, show the same carburetor connection (apparently manifold vacuum)
feeding a tee of which one leg goes to the primary vacuum break, and the other
goes to the vacuum advance with another tee branching off to the Thermo Vacuum
Switch (position 3); then position 2 of the Thermo Switch goes to the EFE
valve; position 3 is described as a vent and does not have anything connected
to it.
Jeff Buckles
If I understand correctly, it does not ping when it
first starts, then pings after about 10 miles, then
stops pinging after you've driven a little bit longer.
I'm guessing that first, the shop manual is wrong...
Is that the official GM shop manual that you are
referencing? Have you checked for any Technical
Service Bulletins about the vaccuum host routing?
Does your local library have the Mitchell (Mitchell's ?)
manual sets? If they do, you can lookup the vaccuum
diagram there for your engine to see if it agrees with
the shop manual.
Did GM have a cold-start spark advance system in '76?
If they did, then this would use manifold vaccum
to advance the spark during initial startup and
cold running, then switch over to ported vaccum
after warm up.
So, at startup, you would have lots of advance,
but cold engine and rich mixture, so no ping.
Then as engine heats up and choke opens (mixture
leans), you get the pinging. Then when CSSA
releases, you go back to ported vac, timing advances
less and pinging stops.
Well, it looked good on paper, anyway.
You reported that the weather is too cold
to take many measurements, but the measurements
you did take showed advance vacuum at idle.
So, was the engine hot or cold when you took that
measurement? If cold (hot), have you checked
again when it was hot (cold) ?
In any case, I'm surprised it even continues to
run with 30 degree advance at 600 RPM!
HTH,
Jeff "Abhorring vaccuums since 1964" Buckles
Aloha, OR (near Portland)
'78 Country Squire
In article <19991124062935...@ng-fa1.aol.com>,
hal...@aol.com (Halterb) wrote:
> The vacuum advance on my 1976 Cadillac Sedan DeVille 500 ci goes too
far.
> Initial timing is 6 degrees at 600 RPM, with vacuum advance
specification 13
> 1/2 to 14 1/2 at 16 PSI. With the vacuum hose disconnected and plugged
the
> timing checks out at 6 degrees at idle (600 RPM).. Yet, when the
vacuum hose is
> attached, the advance jumps up to 30 degrees, about 10 degrees too
much. I
> assume this is the cause of the spark knock I get on light
accelleration.
>
> Spark knock has been a problem with this car most of its life. (I've
had it
> since new.) I replaced the vacuum advance unit a number of years ago,
but still
> had knocking to some extent. At that time, I didn't think to observe
the total
> timing advance at idle so I can't say what that figure was. I use
Shell 92
> octane or Mobile 93 octane gas.
>
> It would seem that if the spring in the vacuum advance unit was weak,
the
> timing would advance too much--I'm assuming that is the problem. The
original
> GM part is no longer available, so I had to use another brand. If I
replace it
> again, I'm wondering how to be certain that the substitute unit is
properly
> calibrated for this car.
>
> Any comments would be appreciated.
>
Sent via Deja.com http://www.deja.com/
Before you buy.
b1377
that consistently run above their thermostat's opening temperature
that suffered from ping. Then, the old, partially blocked, often
orginal, radiator craps out, and a new one is installed. Sudennly, the
car runs at or close to the thermostat opening temperature and no more
ping.
Worst comes to worst, you could install the advance kit advocated by
previous posts. It's cheaper than a replacement radiator, and,
carefully tuned, could yield MPG and performance benefits.
Whatever you do, get rid of the ping before it gets rid of your
engine.
Good Luck,
Jim
Halterb
problem. I'll respond to the specifics:
>If I understand correctly, it does not ping when it
>first starts, then pings after about 10 miles, then
>stops pinging after you've driven a little bit longer.<
Correct.
>I'm guessing that first, the shop manual is wrong...
>Is that the official GM shop manual that you are
>referencing?<
Yes, it is the GM Cadillac manual, and I agree that there is always a
possibility of errors in these publications.
>Have you checked for any Technical
>Service Bulletins about the vaccuum host routing?<
No, I haven't and don't know how to check that.
>Does your local library have the Mitchell (Mitchell's ?)
>manual sets? If they do, you can lookup the vaccuum
>diagram there for your engine to see if it agrees with
>the shop manual.<
I will check on that.>Did GM have a cold-start spark advance system in '76?
>If they did, then this would use manifold vaccum
>to advance the spark during initial startup and
>cold running, then switch over to ported vaccum
>after warm up.<
I'm not aware of any special cold start advance, and as I analyze the hose
routing and thermal valve operation, I see no way temperature would affect the
advance as things are set up.>, was the engine hot or cold when you took that
>measurement? If cold (hot), have you checked
>again when it was hot (cold) ?<
The engine was warm ( halfway between cold and hot!). I could check it at first
startup and after driving.
>In any case, I'm surprised it even continues to
>run with 30 degree advance at 600 RPM!<
Actually, it runs magnificently and gets 19.8 MPG on the highway--it's just the
confounded knocking!
PS--I'm getting to abhor a vacuum myself, and I used to live in Gladstone, also
near Portland!
Jeff Buckles
> >Have you checked for any Technical
> >Service Bulletins about the vaccuum host routing?<
>
> No, I haven't and don't know how to check that.
If you were the original owner (read: I paid you guys
good money for this...) Then a GM service dept or the
GM regional rep should be able to help with this.
(I don't know, maybe they like to help even if you're
*not* the original customer...)
Another place to look is
http://www.nhtsa.dot.gov/cars/problems/tsb/servicemmy1.cfm
> Did GM have a cold-start spark advance system in '76?
Don't know. But my '78 Ford does, and imitation is
the least expensive form of innovation.
> I'm not aware of any special cold start advance, and as I
> analyze the hose routing and thermal valve operation, I see
> no way temperature would affect the advance as things are set up.
I can't reconcile this. It goes through a *thermal* valve,
but temp change won't affect it? Can you explain again?
>, was the engine hot or cold when you took that
> >measurement? If cold (hot), have you checked
> >again when it was hot (cold) ?<
>
> The engine was warm ( halfway between cold and hot!).
> I could check it at first startup and after driving.
>
Impossible to tell what this might turn up, but
more data is better.
Good Luck,
Jeff "Thermally Expansive" Buckles
Aloha, OR (near Portland)
'78 Country Squire
Jeff Buckles
hal...@aol.com (Halterb) wrote:
> >In any case, I'm surprised it even continues to
> >run with 30 degree advance at 600 RPM!<
>
> Actually, it runs magnificently and gets 19.8 MPG
> on the highway--it's just the confounded knocking!
Suppose red-line at 6000 RPM[1]. So, power stroke at 6000 RPM
(from TDC to BDC) takes about 5ms. If I assume that the entire
charge has to burn in less than this 5ms, then I would have to
assume that ignition at can't ever occur more than 5ms before
TDC, or Bad Stuff starts to happen.
So, at 600RPM idle, 5ms equates to about 18 degrees BTDC.
The question is: Since things work anyway without launching
connecting rods through the oilpan when timing is advanced
far beyond 18 degrees BTDC, then which one or more
of my assumptions are wrong here?
Any enlighenment available?
[1] Yes, I know red-line is constrained by mechanical
limits, not burn time. But I'm just arbitrarily setting
that as an upper limit at which the engine is known to run
in order to set some upper limit for burn time.
Thanks,
Jeff "Over the top" Buckles
Halterb
exercise:
>If you were the original owner (read: I paid you guys
>good money for this...) Then a GM service dept or the
>GM regional rep should be able to help with this. (Technical service
bulletins)<
Are we living on the same planet? I am the original owner, but this sort of
help seems a bit on the scarce side--which it what brings a lot of people to
newsgroups like this.
>Another place to look is
>http://www.nhtsa.dot.gov/cars/problems/tsb/servicemmy1.cfm<
Now that's a good idea. I'll check it out as soon as I finish this post.
>>as I analyze the hose routing and thermal valve operation, I see
>> no way temperature would affect the advance as things are set up.<<
>I can't reconcile this. It goes through a *thermal* valve,
>but temp change won't affect it? Can you explain again?<
Yes. The hose runs from the carburetor connection to a T, where one leg goes to
the primary vacuum break and the other heads toward the vacuum advance. Prior
to getting to the vacuum advance there is another T, with one leg going to the
advance unit and the other to the thermal valve which opens and closes
according to temperature and admits or restricts vacuum on its way to the EFE
valve on the air cleaner.
And, commenting on your next post which at least on my computer gets tagged
into the current thread (concerning redlining and spark propagation)--I'm in
over my head on this, but doesn't fuel mixture also affect the burn? In other
words, might not a richer mixture compensate for timing that may be a bit
early?
That brings up another angle to this topic which is becoming a down right brain
teaser. I added another post noting that the engine in this car appears to be
tilted toward the passenger side. I discovered this while attempting to
determine the drive line angles. I put a protractor level on the carburetor
running from front to back to compare it with the transmission output shaft.
Then I tried it from side to side and discovered the engine was not remotely
level. A light went on! If the fuel in the carb tilts to one side, then there
would be a higher fuel level on some of the cylinders than others, possibly
meaning four of the cylinders would run hotter due to lean mixtures (and
effectively faster timing). Voila! The true source of the knock????
Halterb
> What is your engine's coolant temperature? I've seen cars
>that consistently run above their thermostat's opening temperature
>that suffered from ping. Then, the old, partially blocked, often
>orginal, radiator craps out, and a new one is installed. Sudennly, the
>car runs at or close to the thermostat opening temperature and no more
>ping.<
Can't say the exact temp, but I'm my third radiator and who knows how many
thermostats--then don't seem to have any effect on the knock.
Halterb
> What is your engine's coolant temperature? I've seen cars
>that consistently run above their thermostat's opening temperature
>that suffered from ping. Then, the old, partially blocked, often
>orginal, radiator craps out, and a new one is installed. Sudennly, the
>car runs at or close to the thermostat opening temperature and no more
>ping.<
Can't say the exact temp, but I'm on my third radiator and who knows how many
thermostats--they don't seem to have any effect on the knock.