View Full Version : Vacuum Advance Question

Brian N.
11-06-2003, 12:10 PM
Not being sure if the vac advance was working propery, I detached the tube at the carburetor end and tested with a mighty vac tool. There appears to be constant leak down.

Before I shell out the iron men for a new one, I thought I would ask what might seem like an obvious question: It is not supposed to leak down at all, right?

The reason for the question is that on some (newer) cars that used a lot of vacuum operated engine controls, some devices are SUPPOSED to have a calibrated rate of air passage.

11-06-2003, 01:04 PM
It's not supposed to leak. I'd assume that the rubber diagphram inside is perished.
The vacuum unit mostly helps with low speed power. If it's not working (or working poorly), you may notice a slight loss of torque in low to mid-range RPM. With a big, torquey Healey 6, you might not even notice the loss.
At higher RPM, the vacuum unit does virtually nothing and the centrifugal advance system takes over. Many racers remove the vacuum advance unit.

11-06-2003, 01:05 PM
Hello Brian,
if by leak down you mean that the vacuum falls off then no it shouldn't. Does the diaphragm move when you apply a vacuum?, that is what should happen. If the unit is defective, e.g. has a holed diaphragm then there will be no movement.
It may be that the leakage is in the line or the connection to the unit. If you do see movement when applying vauum I would just connect it back up and forget it.
This device allows the manufacturer to set up the car to run a weaker mixture when cruising, giving better fuel economy.

Alec graemlins/cheers.gif

tahoe healey
11-06-2003, 02:55 PM
I may be wrong in this but you may simply have constricted plumbing. Detach the metal tube a both ends and blow through. Check down the tubeon the carb too. I'm not sure what a mighty vac is but I put a small hose on the vac advance unit with out the cap on the distributer and sucked(shouldn't take much). Watch for movement of the advance connector inside. Because a constant vacuum is created in the carbs it should be working even with a small leak in the diaphram. I foun the leak to be in the rubber conector at the carb. It had dried out.

Brian N.
11-07-2003, 04:04 AM
No movement of the advance mechanism upon application of vacuum. Also, the unit was tested at the carb end of the plumbing and also at the vacuum advance unit itself. No joy either place.

Mike, at Dr. Nock's office says there is to current replacement. Their rebuider up and quite a while ago and they have not source. The item is not available at Moss or VB.

Anyone else have a source? Or a used one?

Dave Russell
11-07-2003, 04:26 AM
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Brian N.:

Anyone else have a source? Or a used one?<hr></blockquote>

Try Tuxedo Motorsport
Tuxedo Park, NY

They will rebuild your unit & turn it around in a few days if you are in a hurry. They did mine for $70 incl. shipping.

11-07-2003, 06:10 PM
Hello Brian,
I personally would not worry about an inoperative vacuum adavance unit, but there again I drive with a lead foot so it would not be working very often in my situation.
My current car, a Triumph 2.5 PI sedan doesn't have a vacuum advance unit at all (Lucas 25D6 distributor).

Alec graemlins/cheers.gif

Dave Russell
11-07-2003, 09:43 PM
Hi Alec,
I guess that the lower the power to weight ratio, the more times you have to stuff your foot in it. Big Healeys are operated in cruise mode a lot of the time.

I have had several cars with & without vacuum advance. For street-road use the vacuum advance gives sharper part throttle response & definitely better gas mileage. True - this doesn't matter if you always drive at full throttle.