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MG MGA - Vacuum issue

Hi
I applied a new distributor and a new vacuum pipe, yet I can not feel the vacuum from the carburetor and distributor does not move. What might have happened?
Thanks for the help.
Rui
Rui Fonseca

Ported vacuum from the MGA carburetor is zero at idle speed, as the port is just in front of the throttle plate. The vacuum signal comes up as you crack the throttle open.

Check the vacuum unit on the distributor with a hand-op vac pump. It should be able to hold a static vacuum indefinitely. If it leaks it's trash.
Barney Gaylord

If you don't have a vacuum pump, you can suck on the pipe. You should then see the distributor points plate move.
May need two people - one to suck one to look into the distributor.
Peter.
P. Tilbury

Hi
Thanks for the tips. But the problem is that I do not feel the "vacuum" in the small hole that exists in the carburetor body. Therefore, the distributor can not advance.
Rui
Rui Fonseca

There will be no vacuum at that port at idle speed. You have to hit the throttle when running to trigger the vacuum signal.
Barney Gaylord

Rui. It may not be possible to "feel the vacuum" in this circumstance. One of the reasons test instruments were developed was the instruments allowed us to measure things which might otherwise be difficult to feel or to see.

All vacuum comes from the working of the pistons within the cylinders. The vacuum within the cylinders is changing, during the intake stroke, as the intake valve opens, the piston moves downwards, and the air/fuel mixture rushes into the cylinder due to the lower air pressure in the cylinder and the higher air pressure outside of the cylinder. The vacuum pressure becomes more stable within the intake manifold, and that is were we commonly measure vacuum pressure for test purposes. The vacuum inside the intake manifold draws air into the area of lower pressure/negative pressure any way it can. Thus, we have air coming in through the openings in the carburetors, called venturi, and moving a mixture of air and fuel into the intake manifold and cylinders. Any places on the carbs and intake manifold, commonly the junction areas between the carbs/intake and intake/cylinder head, that do not make tight contact with each other will allow vacuum leaks to be present. We test for these leaks, which can have an effect on the operation of the engine, by spraying a thing oil or solvent around the junctions of the carbs to intake manifold and intake manifold to cylinder head. This is your vacuum system and, at the intake manifold, a vacuum gauge can be hooked up to determine the amount of vacuum present. Also, when one places his finger over the vacuum port used with the vacuum gauge, one will feel a slight force drawing the skin to the vacuum nipple, indicating the presence of vacuum within the intake manifold.

IT IS SELDOM POSSIBLE TO FEEL THE SORT OF VACUUM PRESENT IN THE INTAKE MANIFOLD AT THE CARB'S VACUUM ADVANCE TAKE OFF PORT. The primary source of vacuum is the cylinders with the primary vacuum measuring point being the more stable vacuum reservoir, the intake manifold. The air, rushing into the vacuum source from the outside will, under some circumstances, pass over a small hole in the carb body. This hole will have a nipple attached so that a line can be attached to the carb body, then go to the vacuum advance canister on the distributor. Because this is a secondary source of vacuum (i.e. vacuum developed by the motion of the main body of air passing over a hole which is open to the outside, thus inducing a vacuum within that secondary system) the amount of air flow is minimal. Sufficient to, as part of a closed system (e.g. carb, line, distributor with the distributor and line having no opening/leaks to the atmosphere,) create useable vacuum--sufficient to operate the vacuum advance mechanism on the distributor or to indicate on a vacuum gauge. Thus, a system which produces vacuum at a level which cannot be felt with the human body is capable of moving the rubber diaphragm in the vacuum advance capsule.

Testing the vacuum advance and port systems:

The two types of vacuum take off sources for a distributor are called manifold and "ported". It would be better, since both use a port (nipple or screw attachment) to hook the line up to, to call the systems "manifold" and "carburetor"sourced. The method of testing the distributor vacuum advance assembly is the same with each--hook up a vacuum pump to the connection terminal on the vacuum advance can and apply a vacuum to the can while watching for movement within the distributor (engine turned off, distributor cap removed) or at the timing marks (take a dynamic timing reading at about 1,500 rpm with the line to the vacuum advance unit unplugged, then have an assistant attach the line to the vacuum advance can and see if the strobe light shows a timing advance.

Vacuum advance is a very crude method of controlling timing advance. The vacuum advance capsule will show three numbers (e.g. 4-14-10) which indicate the vacuum needed to begin to activate the system (4" of mercury in this example) the vacuum point where the system is fully engaged (14" of mercury) and the number of DISTRIBUTOR degrees of advance provided (10 degrees distributor which results in a 20 degree advance at the crankshaft). As you can see, as the vacuum in the intake manifold varies, it will have an effect on the vacuum advance system either directly (manifold vacuum) or indirectly (carb sourced vacuum). Not a great system, nor a particularly accurate one. But, it does work fairly well and provides somewhat better fuel economy than the straight mechanical advance only distributor.

I hope I have gone into this manner in sufficient detail and depth that you now understand how the system works and why you will not be able to "feel the vacuum", even on a properly working system.

Les
Les Bengtson

Hi Les

Thank you for all the developed explanation of the operation of the vacuum system. Indeed is a real lesson.
My gratitude and I learned scientifically as the vacuum system works.
Rui
Rui Fonseca

So, what is the difference in measured vacuum between the manifold tap and carb tap, at various throttle openings, or engine speed. Why not use the manifold tap?
Art Pearse

Art,
The manifold has high vacuum at idle and it drops as the throttle is opened.
Carb vacuum has it's lowest reading at idle and it increases as the throttle is opened. They work opposite of each other.
Ed Bell

My understanding is that manifold vacuum and ported vacuum (at the carb) are essentially the same, except at idle. Manifold will continue to see vacuum at idle whereas ported will see virtually no vacuum at idle as the vacuum port is on the opposite side of the closed throttle plate.
Andy Bounsall

More to it than that. Indeed the manifold port gives manifold vacuum, high vacuum at idle and lower vacuum with greater throttle opening. The carburetor port on the MGA is at bottom just in front of the throttle plate, therefore exposed to atmospheric pressure (no vacuum) when at idle. When the throttle plate starts to open it quickly passes the small drilled port, and the vacuum signal jumps immediately high, almost as high as manifold vacuum. This gives you a nice pop to spark advance with initial poke at the throttle.

With slightly more throttle, still relatively small throttle opening, the radially positioned port acts as a venturi port, generating vacuum due to the velocity of air passing the port at right angles. So the vacuum signal remains fairly high for moderate throttle opening and high velocity of intake air. This gives good spark advance for good fuel economy at cruising speed.

When you press the loud pedal to the floor for max power, vacuum at the throttle plate drops quite low, almost down to atmospheric pressure. Air velocity near the wall location of the vac port is immediately reduced some as the total mass air flow is distributed over the full area of the intake throat, and the (ported) vac signal drops dramatically. At low engine speed and wide open throttle air velocity in the throat is not so high, but intake air velocity increases with engine speed. So with increasing engine speed the vac signal can rise slightly due to the venturi effect. Maximum air velocity is ultimately limited by engine displacement, engine speed, and restrictions to air flow in the intake and exhaust paths.

Even though the vac signal is rising a bit with increased engine speed, it is still relatively low. There is a threshold vac signal level required to start movement of the vac advance diaphragm on the distributor. Any time the throttle is wide open this ported vac signal is too low to move the diaphragm, so the vacuum advance goes away entirely with wide open throttle.

End effect of all this is, no spark advance it idle, immediate rather dramatic increase of spark advance with small throttle opening, and the vac signal is maintained to higher engine speed as long as the throttle is only moderately opened. Combination of vacuum advance and mechanical advance at cruising speed and partial throttle (2000-4000 rpm or 35-70 mph for instance) gives high total spark advance and good fuel economy.

The kicker comes with the kick, so to speak. When you floor it for max power the vac signal drops, and the vac advance goes away. For full throttle operation the engine runs on mechanical spark advance only (and why race engines often do not use vacuum advance). You can set maximum spark timing to about 32d BTDC at high engine speed (when mechanical advance is maxed out).

At cruising speed with moderate throttle the vacuum advance piles on top of the mechanical advance, and total advance can be up around 45d BTDC. Punch it for max power and it's instantly back to 32 at speed (so you don't have pre-ignition or detonation with max power). So the advantages of ported vacuum advance are good quick throttle response and good fuel economy.

This is why you need to disconnect the vacuum line to check and set spark timing dynamically at any speed higher than dead slow idle. When you rev the engine up with no load and fairly small throttle setting, the vacuum signal is still there in a big way and does not represent the low vacuum signal you would have under load with wide open throttle. So do disconnect the vacuum line when setting timing with a strobe light.

For the "other" setup using manifold vacuum, the vacuum signal is high at idle, is reduced with increasing throttle, and goes very high on overrun (closed throttle at high engine speed). The vacuum advance function on the distributor has to be dramatically different when using manifold vacuum. The vac signal can be applied to opposite side of the vacuum module to create vacuum retard at idle.

When you hit the throttle for power manifold vacuum drops, the vacuum retard goes away, and you get spark advance for quick acceleration. If you close the throttle at high engine speed you get very high manifold vacuum and maximum vacuum retard on overrun. This setup was used to cope with emissions issues in later model MGB.
Barney Gaylord

This thread was discussed between 15/12/2011 and 20/12/2011

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