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A Bosch distributor mod

Author: John H

A recent conversation with John Fort set me thinking it was about time that I sorted out the basic problem with the Bosch distributor centrifugal advance in 'Pearl' and as I have time on my hands at the moment, I set to and came up with the following solution.

The problem with the Bosch distributor, that many of us use now, is that the advance curve is nothing like the original Lucas item, it also has far too much advance, which meant that I had to set the static timing back to 6 deg. retarded in order to get the engine to rev. without knocking in the intermediate gears.

The effect of this was to cause the engine to go 'flat' when I changed-up from 3rd to 4th at 30 mph. and when I plotted the advance curves it was easy to see why, the advance dropped back from 18 to 8 deg. Something needed to be done! Chatting to Steve Jones, from the West Yorkshire Group, he told me what he had done to cure the same problem in his trials car. So, here we go:

A Bosch distributor modRemoving the two screws that fix the cap retaining springs and also the contact-breaker base-plate, enables this to be removed with a bit of 'jiggling' - see Pic. 1. The centrifugal advance mechanism is now revealed. There are two bob-weights but only one has a spring. The one without a spring is the one that gives the initial advance from 900 to 1200 rpm. before coming up against its stop-post at 8 deg. The other one with the spring starts to advance from 1700 rpm. up to 2600 rpm before coming up against its stop-post at 24 deg. The Lucas DK4A reaches 16 deg. at 3000 rpm.

It was clear that the easy solution to restrict the maximum advance to a more reasonable 18 deg. is to alter the stop-post by sliding a sleeve over it to limit the travel of the bob-weight. I will not bore you with the details of the measurements and calculations I made to arrive at the correct thickness of sleeve to limit the advance to my target of 18 deg. except to say that pi came into it quite a lot!

A Bosch distributor mod0.75 mm. is the answer and a section of the plastic sleeve off a spade connector is the correct thickness and is the right shape. (See this in Pic. 3 on the right of the graph sheet). You may just be able to make this out on the stop-post at 7 o'clock in Pic. 2.

After re-assembly and setting the contact-breaker gap to 0.45 mm (18 thou.) as recommended by Bosch, I set the static timing to TDC and a quick road-test showed an immediate improvement to the mid-range torque, 'Pearl' sailed up our local Holborn Hill in top with a bit in hand, whereas before it was a case of staggering up in top at 30 mph or revving its nuts off in 3rd to keep out of the way of other traffic.

A Bosch distributor modA nice simple solution and one I can recommend with confidence. My graphs in Pic.3 show the advance curves as supplied by Bosch and the Lucas curve came from the Austin 7 Club of Australia. I am sure there are other ways of modifying the Bosch distributor, by using different springs perhaps, so if anyone out there knows more I would be glad if you would share your information with us all. I had hoped to further test my mod during the Yorkshire Experience on all those lovely hills, but 'twas not to be!

Striving for Perfection – Cheers

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