cobraboy
Well-Known Member
This is not a how to guide, more of a wot I did to mine story.
I have moved away from the SU carbs to a four barrel, in doing so I went from the stock vacuum distributor take off and installed a manifold vac take off and now use manifold vacuum to the distributor.
I have tried various ways to get my timing right, but the car has not been totally happy.
On my other Rover V8 I modified an Opus distributor and have been happy with that, so I thought I would have a crack at the 35 DLM8.
Two problems at the outset. One the mechanical advance, two the vacuum advance. In order to get the 38 degrees all in mechanical advance I wanted, the mechanical advance at idle became 16 deg, which was fine, until you connected up the vacuum, 22 degrees, now on full vac at idle the advance was a crazy number and the car did not like it at low throttle openings, mind you it went well at WOT.
So the plan was to get a static advance of around 10 deg, but still obtain 38 deg at 3500 rpm and above mechanically. This meant getting more mechanical advance from the bob weights in the base of the distributor.
The second part of the plan was to limit the vacuum advance.
The bob weights in any distributor are limited by a stop, this in the case of the DLM8 is a pin. If you allow the weights to throw out more you get more advance.
The question was how much to allow ?
The diameter of the distributor body is 95mm, a circumference of 298.4mm, so every degree = 1.2mm around the outer edge. ( SEE AMENDMENT )
Using a cocktail stick from a recent G&T fixed to the rotor arm with Plasticine and some scribe marks I first plotted the original mechanical advance then added the required amount of extra advance I wanted, this gave me an indication of how much to grind off the bobweight stop. BTW the degrees at the distributor are half the degrees that you see at the crank damper with a strobe light.
Pointer
Working clockwise, Datum mark under pointer, original limit of mechanical advance, new limit of mechanical advance.
The area ground on the bobweight to increase advance ( yellow marker )
That is the mechanical sorted. 10 degrees at 800 rpm with all in at 3500 and 38 degrees.
Now for the vacuum.
I wanted to limit the vacuum to 15 degrees from the stock 22 degrees, I got 16 degrees. I did this by fitting a length of hard plastic pipe over the rod coming from the vacuum capsule to the base plate, this limited the stroke of the rod and so limited the advance, it was purely trial and error as to how long the plastic pipe needed to be. I do not have a pic of the pipe , but here is a location pic. I intend to turn up a spacer tube from aluminium as a permanent solution.
So the result is.... At idle I have 10 degrees mechanical advance with 16 degrees vacuum added = 26 degrees total, and the car is very happy at cruise / light throttle.
At WOT the vacuum advance drops out and at 3500 and above all of the 38 degrees mechanical advance is in.
At cruise I will have the given amount of both mechanical and vacuum advance in play, I expect this to be in the region of 45 degrees.
Spark plug colour is an even mid grey and the oily plug threads are drying up.
The car goes rather well
All of the figures given are subject to manufacturing tolerances, age, wear and tear, and a pinch of salt.
If all of this seems a little too much hassle and effort, remember. Burt Munro never gave up making his bike go faster !
I have moved away from the SU carbs to a four barrel, in doing so I went from the stock vacuum distributor take off and installed a manifold vac take off and now use manifold vacuum to the distributor.
I have tried various ways to get my timing right, but the car has not been totally happy.
On my other Rover V8 I modified an Opus distributor and have been happy with that, so I thought I would have a crack at the 35 DLM8.
Two problems at the outset. One the mechanical advance, two the vacuum advance. In order to get the 38 degrees all in mechanical advance I wanted, the mechanical advance at idle became 16 deg, which was fine, until you connected up the vacuum, 22 degrees, now on full vac at idle the advance was a crazy number and the car did not like it at low throttle openings, mind you it went well at WOT.
So the plan was to get a static advance of around 10 deg, but still obtain 38 deg at 3500 rpm and above mechanically. This meant getting more mechanical advance from the bob weights in the base of the distributor.
The second part of the plan was to limit the vacuum advance.
The bob weights in any distributor are limited by a stop, this in the case of the DLM8 is a pin. If you allow the weights to throw out more you get more advance.
The question was how much to allow ?
The diameter of the distributor body is 95mm, a circumference of 298.4mm, so every degree = 1.2mm around the outer edge. ( SEE AMENDMENT )
Using a cocktail stick from a recent G&T fixed to the rotor arm with Plasticine and some scribe marks I first plotted the original mechanical advance then added the required amount of extra advance I wanted, this gave me an indication of how much to grind off the bobweight stop. BTW the degrees at the distributor are half the degrees that you see at the crank damper with a strobe light.
Pointer
Working clockwise, Datum mark under pointer, original limit of mechanical advance, new limit of mechanical advance.
The area ground on the bobweight to increase advance ( yellow marker )
That is the mechanical sorted. 10 degrees at 800 rpm with all in at 3500 and 38 degrees.
Now for the vacuum.
I wanted to limit the vacuum to 15 degrees from the stock 22 degrees, I got 16 degrees. I did this by fitting a length of hard plastic pipe over the rod coming from the vacuum capsule to the base plate, this limited the stroke of the rod and so limited the advance, it was purely trial and error as to how long the plastic pipe needed to be. I do not have a pic of the pipe , but here is a location pic. I intend to turn up a spacer tube from aluminium as a permanent solution.
So the result is.... At idle I have 10 degrees mechanical advance with 16 degrees vacuum added = 26 degrees total, and the car is very happy at cruise / light throttle.
At WOT the vacuum advance drops out and at 3500 and above all of the 38 degrees mechanical advance is in.
At cruise I will have the given amount of both mechanical and vacuum advance in play, I expect this to be in the region of 45 degrees.
Spark plug colour is an even mid grey and the oily plug threads are drying up.
The car goes rather well
All of the figures given are subject to manufacturing tolerances, age, wear and tear, and a pinch of salt.
If all of this seems a little too much hassle and effort, remember. Burt Munro never gave up making his bike go faster !
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