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Previous Technical Articles: Fuel Consumption Spark Advance TEFBA Radiator Filters



AHOC Website last updated on 6th April 2008

NEW SOUTH WALES

Technical

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All Technical Articles

Vacuum Advance

Here’s some information of vacuum advance which explains in detail how the unit operates. If you look at the original advance specifications for the BJ8, the mechanical advance of the distributor is 18º distributor. Add to that 8º vacuum advance equals 26º. Because the distributor turns at half engine speed you double this for crankshaft degrees, 52º and add the static advance of 10º and the total advance at 6400 rpm is a massive 62º. This is 2º more than one cylinder! Much too high and my only guess is that in 1966 it was important to improve the MPG.

The rev limit is 5200 rpm but even then its 58º total. The six cylinder engines are much better at 34 – 36º maximum plus 16º vacuum. You’ll also notice on the graph that the vacuum advance is constantly moving in response to speed, manifold depression and the amount of throttle.

… yes folks now you CAN service Lucas vacuum units so don’t throw them away.

It’s not too difficult to repair them and many of you will be able to do this with a few simple tools and a bit of time.

There is the coiled spring type connection to the points-plate and the lever type connection. With the level type, drill out the rivet and remove the lever, noting its original position. You’ll have to replace this rivet with a 3/32” one.

Roll back the folded edge of the housing. I use a pair of 5” long pincers, similar to nail removing pincers. One of the blades has been ground to a curved sharp edge and the other to a concave flat edge that suits the curve of the housing. By working around the edge 5 to 6 times the edge will be opened up enough to separate the housing and reveal the old diaphragm, hardened and split by petrol that finds its way there from the carby and down the vacuum pipe.

Remove the old diaphragm by drilling out the centre, save the retaining washer. This material is fabric reinforced neoprene and available from pump repair places of gasket makers. It is about .016” thick; (.4mm) cut two circles 2.46” dia. and with a leather punch a small hole in the centre for a retaining screw 1/8” or 4 BA. Drill and tap a hole in the control rod to take the screw and reassemble using a small amount of RTV silicone. Don’t forget the retaining washer.

Using a small amount of RTV silicone on the edges of the diaphragm, reassemble making sure the control rod is in its original position, and start bending the folded edge back into place with some pliers. Finally tap the edge down with a hammer and allow the silicone to set. – VOILA – a vacuum unit that now works.

On vacuum units most of you know what the numbers mean but I’ll repeat them for you. A 5.12.8 unit means the advance begins at 5” of mercury vacuum is complete at 12” of mercury vacuum and provides 8 degrees of distributor advance (that’s 16 degrees at the crankshaft).

Vacuum units for the BN1 is 7.18.12 for the BN2 5.17.10 and from the BN4 to the BJ8 5.12.8. Modified engines should be recalibrated using a vacuum gauge and test runs using light throttle at specific RPMs to optimize fuel economy and avoid early pinging. Using these figures you can try to find a vacuum unit that matches the modified engine or use one of the Hexagon head units and alter the spring rate and degrees of advance. A final coat of silver paint will improve the overall appearance.

If all this seems too hard then give you vacuum to Don Hardie or me and we’ll do it for a modest fee of $40. More on vacuums next month (will it ever end?)

Have fun,
Barry Campbell