The MGA With An Attitude
VACUUM LINE For Spark Advance - IG-120

At 10:05 PM 9/18/2006 -0400, Bill Wilkman wrote:
"I recently had my MGA's engine rebuilt.... new Aldon/Lucas performance distributor. The vacuum fitting on the old distributor was designed for a metal vacuum tube threaded into the vacuum advance and the carburetor."
olive compression fitting
This is actually a tube with a straight end. There is a small metal ring called an olive, beveled on the outside, which fits snug on the tube. Over that is a flare nut which presses the olive into the neck of the threaded fitting on the vacuum advance unit. The tapered surfaces and the end conspire to compress the olive onto the tube for a tight fit. The compression ring is a common plumbing fitting, possibly found at a local household hardware store. It may also be found at a local auto parts store, such as Autozone.

"Mid-way in the stock metal vacuum tube is an oblong metal chamber through which the vacuum line extends."

The tube does not go all the way through, but connects on both ends of the bulb. The bulb is full of wire wool and serves as a fuel separator. When installed the bulb should stand on end with the lower end connected to the carburetor, so any liquid fuel collected in the bulb will run down to be drawn back into the carburetor on the next vacuum cycle.

"The new distributor is designed to make use of a press-on hose fitting."

This is typical of later model MGB parts. You can cut off the end of the tube near the vacuum unit to remove the olive and the nut. Then connect the tube to the vacuum unit with the right angle rubber connector used for the later model MGB.

"My question is this: Is the oblong metal chamber necessary, or can I just run a rubber vacuum line from the distributor to the carburetor without the chamber?"

Keep the fuel separator on the engine. The vacuum line on the MGA with H4 carburetors is connected to a port in the bottom side of the rear carburetor venturi, just at the bottom edge of the butterfly throttle plate. This is down stream but quite near to the main fuel jet. If (when) conditions allow liquid fuel from the jet to dribble along the bottom of the carburetor throat, the fuel can enter the vacuum port and be drawn into the vacuum tube en-route to the vacuum diaphragm on the distributor. The fuel separator is there to prevent liquid fuel from getting to the rubber diaphragm.

Later model MGB have the venturi vacuum port on top of the carb throat where liquid fuel is less likely to enter the vacuum tube. Still later models use a vacuum signal ported from the top of the intake manifold where it is extremely unlikely to ever see liquid fuel. With the bottom venturi tap on the MGA it is more important to use the fuel separator in the vacuum line to the distributor.

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