Lucas Distributor Service and Vacuum Diaphragm Failures
By Jim Leach
Joseph Lucas usually gets blamed, but you would be surprised at the damage to Lucas distributors caused by careless and inept mechanics. I have rebuilt or replaced over a dozen units that have failed because of ham-handed tune-up-artists making the same mistake: they all have dropped one or more of the small screws that attach the ignition points and condenser, into the space below the breaker plate...and then sent the car back down the road without removing them. The screws interfere with the mechanical advance counterweights and jam between the weights and the inside of the case.
This usually causes at least one of three things to happen:
It was a very cold winter morning and I was in a hurry, so I used a can of aerosol carburetor cleaner or brake cleaner to blast away some of the grease and dirt off of the breaker plate. I oiled the cam as usual, dwelled the points, changed the spark plugs: everything looked normal and the car ran well. The following morning, after 20 minutes running time, the engine barely ran and then stalled. A complete distributor tear-down and inspection revealed the (previously shiny) cam was rusty brown, the case was full of muck and the drain holes were blocked. The solvent had pooled in the bottom of the casting and it vaporized when I ran the engine and it heated. The vapor rinsed the lubrication off of the cam and overnight it rusted; the roughened cam "attacked" the rubbing block on the new ignition points and the gap closed.
On Series Rootes cars there were two versions of Lucas DM 2 distributors (early and late) and they were equipped with two different model vacuum advance units. The early version has a 3/4" hexagonal tube and nut projecting from the body, and a threaded portion at the outlet to accommodate the fitting on the steel vacuum tube running to the carburetor. Beginning about 1960, the hexagonal tube and nut were eliminated, and the threaded portion remained. They were available under a variety of part numbers (superseded many times) and had slightly different operating characteristics from each other. (The original version used a different breaker plate top-half, and was connected to the vacuum unit from below the plate by a Mechanno-set style link. The design was short-lived and was replaced by the spring style, about 1957. This model distributor may be updated by replacing the breaker plate top half and the vacuum unit.)
There may be many British cars in service that have failing vacuum advance units. I was recently surprised to discover seven of the DM 2 type vacuum advance units in my storage (both types) have failed while sitting on the shelf. I last tested them about two years ago and they were functional. I suspect they suffer from old age and the natural rubber (?) diaphragms have dried out and lost their flexibility and their seal. The later model vacuum advance units as fitted on 25 D series Lucas distributors introduced in 1963 will interchange with the earlier types; however, the threaded portion was eliminated and a nipple is provided to accommodate a rubber fitting or hose.
To run efficiently, an engine requires a certain amount of ignition advance; often in excess of 30 degrees. The mechanical advance unit located under the breaker plate and the vacuum advance unit described above work together to advance the ignition timing depending on an engine's speed and its load. At speed, a defective vacuum advance unit will reduce the (necessary) total ignition advance and may be a cause of overheating: retarded ignition advance will also reduce engine performance and mileage. A leaking vacuum diaphragm provides extra air to an engine and may contribute to poor idling. At every tune-up, be sure to check the vacuum advance mechanism and ensure it is working correctly.
Joseph Lucas usually gets blamed, but you would be surprised at the damage to Lucas distributors caused by careless and inept mechanics. I have rebuilt or replaced over a dozen units that have failed because of ham-handed tune-up-artists making the same mistake: they all have dropped one or more of the small screws that attach the ignition points and condenser, into the space below the breaker plate...and then sent the car back down the road without removing them. The screws interfere with the mechanical advance counterweights and jam between the weights and the inside of the case.
This usually causes at least one of three things to happen:
- the driving dog on the bottom of the shaft shears off
- the mate to the dog on the oil pump shears off
- the distributor's case explodes.
It was a very cold winter morning and I was in a hurry, so I used a can of aerosol carburetor cleaner or brake cleaner to blast away some of the grease and dirt off of the breaker plate. I oiled the cam as usual, dwelled the points, changed the spark plugs: everything looked normal and the car ran well. The following morning, after 20 minutes running time, the engine barely ran and then stalled. A complete distributor tear-down and inspection revealed the (previously shiny) cam was rusty brown, the case was full of muck and the drain holes were blocked. The solvent had pooled in the bottom of the casting and it vaporized when I ran the engine and it heated. The vapor rinsed the lubrication off of the cam and overnight it rusted; the roughened cam "attacked" the rubbing block on the new ignition points and the gap closed.
On Series Rootes cars there were two versions of Lucas DM 2 distributors (early and late) and they were equipped with two different model vacuum advance units. The early version has a 3/4" hexagonal tube and nut projecting from the body, and a threaded portion at the outlet to accommodate the fitting on the steel vacuum tube running to the carburetor. Beginning about 1960, the hexagonal tube and nut were eliminated, and the threaded portion remained. They were available under a variety of part numbers (superseded many times) and had slightly different operating characteristics from each other. (The original version used a different breaker plate top-half, and was connected to the vacuum unit from below the plate by a Mechanno-set style link. The design was short-lived and was replaced by the spring style, about 1957. This model distributor may be updated by replacing the breaker plate top half and the vacuum unit.)
There may be many British cars in service that have failing vacuum advance units. I was recently surprised to discover seven of the DM 2 type vacuum advance units in my storage (both types) have failed while sitting on the shelf. I last tested them about two years ago and they were functional. I suspect they suffer from old age and the natural rubber (?) diaphragms have dried out and lost their flexibility and their seal. The later model vacuum advance units as fitted on 25 D series Lucas distributors introduced in 1963 will interchange with the earlier types; however, the threaded portion was eliminated and a nipple is provided to accommodate a rubber fitting or hose.
To run efficiently, an engine requires a certain amount of ignition advance; often in excess of 30 degrees. The mechanical advance unit located under the breaker plate and the vacuum advance unit described above work together to advance the ignition timing depending on an engine's speed and its load. At speed, a defective vacuum advance unit will reduce the (necessary) total ignition advance and may be a cause of overheating: retarded ignition advance will also reduce engine performance and mileage. A leaking vacuum diaphragm provides extra air to an engine and may contribute to poor idling. At every tune-up, be sure to check the vacuum advance mechanism and ensure it is working correctly.