Modifying a Lucas 35DLM8

cobraboy

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
IMG_2631.JPG
Working clockwise, Datum mark under pointer, original limit of mechanical advance, new limit of mechanical advance.
IMG_2633.JPG
The area ground on the bobweight to increase advance ( yellow marker )
IMG_2641.JPG
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.
IMG_2636.JPG
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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Sorry guys, I did proof read it honest. But that wont help if you are looking at three lots of notes made over different days :rolleyes:
I am not going to edit the post. What I wanted was to gain an extra amount of degrees specific to my distributor, which won't be applicable to anyone else's distributor or engine. I did achieve the required gain in mechanical advance, and the post shows how I did it. To edit the post now following the highlighted discrepancy would make no sense.
The method is there, even if the numpty factor is also present.
Thanks for the scrutiny :thumb:
Edit
The setting out on the distributor rim is to try to figure out how much movement there is at the bob weight for a given amount of degrees at the crank to give you an idea of how much to grind off the stop to increase the advance. I was prepared to have to have more than one go at it, taking it gently. The first grind gave me what I needed. The important figure is the all in figure, what you get at idle can have some wiggle room. I wanted to get the 38 degrees all in and as long as I had near 10 at idle I was going to be happy.
 
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When I put in a 4.6 engine my distributor was marked 13, I found one marked 9 swapped the bits over, put in a spring off a Ford 1600 GT motorcraft dissy. this was to get the best total advance, I didnt know what I was doing, ! a friend told me to do it this way. I dont have the vacuum connected as I have a unusual carb setup.
 
What kind of unusual carb set-up, if I might be nosey, and divert this thread off topic a little bit?
 
Its a 1961 Buick 215 Edelbrock aftermarket manifold with two DGAS Weber downdraft twin barrel carbs adapted to fit.
 
That's nice! Is that intake manifold polished, or powder coated? I gotta see a pic of the whole car now, Dave!
 
That's a wicked street rod! Does it have functional trafficators? Was that photo taken at Crystal Palace?
 
Those big-base Rochesters are indeed very rare: what you have there is 2/3 of a Pontiac Tri-Power setup; I have a NOS Offenhauser manifold sitting in a box that uses the same carbs; tho’ I have seen these modified for a pair of downdraft Weber’s...
 

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TokyoP6B said:
Those big-base Rochesters are indeed very rare: what you have there is 2/3 of a Pontiac Tri-Power setup; I have a NOS Offenhauser manifold sitting in a box that uses the same carbs; tho’ I have seen these modified for a pair of downdraft Weber’s...
Click to expand...
When I said rare I meant the Edelbrock manifold, the manifold is made for 1 inch venturi Rochesters, that is double what a Buick 215 came with, I had two sets of twin Rochesters but sold them, one set were almost new, even had philips screw heads ! I have Webers on mine DGAS from a Ford V6.
 
Dave lowe said:
When I said rare I meant the Edelbrock manifold, the manifold is made for 1 inch venturi Rochesters, that is double what a Buick 215 came with, I had two sets of twin Rochesters but sold them, one set were almost new, even had philips screw heads ! I have Webers on mine DGAS from a Ford V6.
Click to expand...


Sure! Enjoy, nice to have something different, Hot Rod magazine ran an article in ‘61 or ‘62 where they used that Edelbrock manifold on a Buick 215 with 2GC Rochesters and got 233HP on a dyno., Pierce Manifolds in the States were selling a similar DGAS setup and your manifold is the only other one of these I’ve ever seen in use, or for sale; Rare indeed!

G
 
Yes I have that article, it was included in a hard back Hot Rod book of the period that I have, they do come up for sale in the USA once in a while, they were popular with the Drag boat people.215 Buick raceboat.jpg
 
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