The Rogue Distributor

This story relates to a 1935 20/25 but Tom thought it could be useful for all pre-war Rolls Royce. The car had been off the road for over 50 years so needed a lot of restoration and recommissioning. Whilst thinking about the way forward with the body I attacked various mechanical components and searched for missing parts. I removed the distributor because the spindle holding the rotor arm waved about and indicated there was a major fault. Once disassembled it was obvious that the bearing had seized at some stage and the spindle was very, very worn. The spindle carries the bob weights for the automatic advance/retard mechanism. The spring on the bob weights was also broken.

I thought I was lucky when I managed to get a replacement spindle with working bob weights to rebuild it. Some months later I felt I had enough parts to try and start the engine. The car had been completely de-wired so I thought I could use my Ebay found magneto to have a go. After a bit of fiddling, I was greatly heartened to see the engine burst into life and run. Not for long though because there was no water in the engine and the fuel tank had yet to be installed. My only concern was a tinkling sound which I could not identify, but nonetheless progress.

Jump ahead several years and the restoration was complete and I re-registered the car and put it on the road. Performance was disappointing – lack of power and barely able to obtain 40mph. Failing to resolve the matter I resorted to the experts and the car spent several expensive months until they pronounced the car was ready to go. I drove 50 miles home but was not completely happy and on subsequent trips locally performance fell away to where I started. As a result, I fiddled and messed up all the adjustments done by the experts and got nowhere.

One day I was in my garage and I rediscovered all my tuning equipment which I had used in the past for tuning everyday cars and classic cars. Using my dwell angle meter I set the gap to give 36 degrees as that is pretty standard for 6-cylinder engines. However, I could barely obtain 2000rpm which should equate to 40 mph. I then set up the engine and marked the crank pulley wheel to see how much advance I had using a strobe. The engine reacted as expected using the advance/retard lever on the steering column but when I pushed the throttle down there was no further advance. This led me to suspect the distributor so it was removed and inspected. Inside the dome of the bob weight carrier there was a deep score mark where the bob weights were coming into collision with the casing. This looks to be the cause of the problem. I succeeded to obtain a replacement 20/25 distributor. Dismantled it, cleaned it and re-installed it in the car. Engine started okay and would now rev to 3000rpm for the first time. Road tested and the difference was surprising. Now the engine pulled more strongly and the car went faster. After a few adjustments to timing etc the car is now running quite well.

Visually both sets of innards looked the same so why did the bob weight carrier foul the distributor housing. Careful measuring of the components shows no significant differences, except!! – the distance from the boss which abuts the top bearing to the top of the bobweight carrier is 2mm shorter on the rogue rebuild. In fact, the overall length of the spindle is also 2mm shorter than the original. Because the base of the distributor is domed this means that the bob weight carrier sits higher and therefore fouls the casing preventing the operation of the bob weights. The other benefit of this operation is that the tinkling sound from the engine has completely disappeared.
The lesson is that even when using genuine Rolls Royce parts, check and check again to be sure that they are exactly the same dimensions in all respects. I lost a year trying to fathom this particular problem.

 

2 Spindles, left 2mm shorter than one on right – wear clearly visible:

 

 

Gouge in dome clearly visible: