Subject: Vacuum advance
Date: Thursday,January 11,1996 10:05AM
I sent the following message out a few days ago, but haven't seen it
show up yet. Apologies if any of you get it twice.
> I've learned a _lot_ of things from you people, and I know lots of neat
> little things about the way my MGB works. But I'm still unsure of the
> exact function of the vacuum advance. Folks say that it helps
> driveability and emissions at idle, but can somebody explain the
> physics? Is it simply a more complete burn while at idle?
Mechanical advance takes care of making sure that the spark happens at the
same time before the piston reaches top dead center. As rpm increases, there
is less time to burn the fuel so the spark must be advanced. Since the
distributor works off of degrees beffore top dead center not time, the spark
is adjusted by a cetrifugal unit which advances the spark relative to rpm.
In reality it would be impossible to advance it enough to adjust for high
rpms. The info missing is that as rpm rises, there is more turbulance which
mixes the fuel and air and makes it burn faster. This relationship becomes
linear above about 2500 rpm meaning that after 2500 rpm, you do not need to
advance the spark due to increased rpm because turbulance makes the fuel
burn faster at the same rate that the rpm is increasing. So your mechanical
advance unit is usually curved to bring in advance in the lower rpm range up
to about 2500 rpm and then it stops adding advance. Later model emmision
cars are a little different with regards to how much and when advance is
Vacuum advance is used to compensate for changes in charge density.
Basically, when the charge density is low ( at idle ), the fuel and are
molecules are far apart and the flame front travels slower. Therefore you
need to advance the spark under these conditions to make sure that the fuel
has enough time to start burning before top dead center. When you have the
pedal to the metal, the charge density is high. The air and fuel molecules
are close togather and the flame front travels fast. Under these conditions
you need less advance.
Fortunately there is an easy way to adjust for these conditions. When you
are at idle or part throttle vacuum is high because the carb trottle plates
are preventing the engine from sucking in are that it wants. A vacuum port
between the throttle plates and the intake port will show this vacuum. When
the pedal is to the metal there is almost no restriction of the air to go
through the carb and therefore the vacuum port will show no vacuum. The
Vacuum line is attached to a diaphram which actuates a lever rotating the
distributer baseplate to change the amount of advance. The amount and rate
depends on the year of your car (cam, carb, combustion chamber, etc.)
> Anyway, I pulled the dizzy yesterday and confirmed what I suspected: My
> vacuum advance unit was frozen. Lots of WD40 and tugging later, it
> moves, but I had expected it to move a lot easier. Actually, I had
> expected to be able to suck on the vacuum hose and watch the little guy
> go back and forth. As it is, it takes a very firm push to move it.
> So what's normal? Is my unit shot? Do new units move easily? How much
> vacuum does the 18V make at idle? Will a functioning vacuum advance
> cure my backfiring on overrun (my thought is no; it's probably a frozen
> gulp valve...)?
If you have a little hand pumped vacuum pump you can test it. All advance
should be in at about 20 lbs vac. If you only have a vacuum guage you could
buy a T and add the guage to the line to see how much of a vacuum your lungs
were creating. The unit does have a spring in it. It's not an on/off unit,
it is meant to vary depending on vacuum but it sounds like yours is a little
frozen. If it is the early push on kind you can buy a new vacuum unit and
push it on. Just make sure that the problem is not with the baseplate
> Once I get it working, how many degrees of vacuum advance should I
> expect at idle?
~1967 expect around 10-12 deg
~1975 20 deg
1980 30 deg
Reply if you need more info
> Todd "so many questions..." Mullins
> Todd.Mullins@nrlssc.navy.mil On the lovely Mississippi (USA) Coast
> '74 MGB Tourer in pieces this week
> "A life lived in fear is a life half lived."