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MG MGA - Lucas DM2 Cam Lobe

I am still trying to get to the bottom of the problems with my DM2 distributor that I had professionally rebuilt, reprofiled to my 1800 engine and fitted with an Ignitor electronic ignition.

I never managed to get the distributor timed properly. If I dynamically set it to 32 degrees at 3000-3500RPM I could never subsequently get the engine to start because it was way too far advanced. If I set it at 12 degrees at idle the engine ran flat above 2500RPM. The unit is now in my pending tray on the workshop bench while my 45D Metro distributor does stirling work.

However, I took the DM2 unit apart last night and noted that the rebuilder had fitted a 7 degree cam lobe, with the lobe filed away somewhat.

Now to the crux of this Thread. How do I check by bench-top measurement what is the actual advance of my 'mutilated' cam lobe? Unless I am mistaken I require about a 12 degree lobe. i.e. 8 degrees static, plus 2 x 12 = 32 degrees total advance.

Steve

Steve
Steve Gyles

In an attempt to answer my own question I measured the radius of the cam to the centre of the lobe where it makes contact with the restrictor post. I then measured the gap between the post and the lobe in its static state. Drawing these measurements on paper gave an angular movement of the lobe of 8 degrees from static to full mechanical advanced.

Is this the correct way to measure it? If so, it will account for my problems. Anyone out there understand the mysteries of these assenblies?

Steve
Steve Gyles

Steve. The stop arm on the points cam is used to limit the overall mechanical advance of the system. Total ignition advance is static plus mechanical as you have noted.

Static timing is a "better than nothing" approach which could be done without a dynamic timing light. Back in the "old days" such things were expensive and I knew professional mechanics who did not own one. In larger shops, there was a "shop dynamic timing light" available. Hence, the actual ignition advance was seldom set up for maximum performance. You might note that the mechanical advance curve for the Lucas DM2 distributor is not listed in the MGA factory workshop manual. The early MGB manuals list the advance curve of the distributor, but not as seen on the engine. As I remember it, the 68 model was the first which had an "as seen with the engine running" advance curve listed.

I have always assumed that the best factory advance curve was the one for the early MGB engines. For the 68 MGB engine this was:

10 deg at 400 to 600 rpm
24 deg at 1,500 to 1,750 rpm
32 deg, plus or minus 2 deg at 3,000 rpm

Dynamic timing was 20 deg BTDC at 1,000 rpm.

This gives us some clues/clews as to where to go. Peter Burgess seems to agree. Perhaps not perfect for any single engine, but a good basic curve for most engines.

As to how to check out your distributor, the best method is to use a distributor test machine. This will allow you to plot the exact advance at various distributor speeds. Double the noted advance and speeds to determine crankshaft advance and speed.

A less accurate method is to install the distributor on the engine, set the static timing to whatever (8-10 degrees will allow it to start), and have an assistant run up the engine, using a dwell tach to read the rpms, and use a dynamic timing light to verify what is the actual advance curve at 3,000; 2,500; 2,000; 1,500 and 1,000 rpms. Subtact the static advance from the demonstrated advance and that should give you your basic curve. Not a perfect solution, but better than none at all.

If you compare your determined curve to the curve listed for the 62-68 spec cars, it should assist you in finding out where the problem may lie.

Les
Les Bengtson

Thanks Les. That all ties in with the limited knowledge I have on the system. What I still would like to know is how the lobe is initially calibrated. They would have have left Lucas in standard lengths and marked accordingly. It is how that length was determined that I am trying to ascertain. If I am right in my technique in my second post then I can acheive the correct mechanical advance reasonably accurately with a bit more filing, then fettle the system on the engine as I don't have a test rig.

On the other hand, if someone has the correct cam for an 1800 engine that they can draw up accurately for me it could save me a bit of time and effort.

Steve
Steve Gyles

Steve. There were several "correct cams" for the 1800 engine, particularly here in the US. As I mentioned, the 62-68 spec distributor, the Lucas 25D4 to specification number 40897 is probably the best starting point in trying to develope a distributor for your engine.

As to how to modify your existing mechanical advance stop, your concept of making a degree wheel the size of the points cam, from the centerline to the middle of the advance stop, would work. Extend the lines of the protractor outwards. Align the end of the advance stop at the number of degrees specified on the stop. Use the ten degree line to mark both the inside and outside of the advance stop arm. Draw a line across the top of the advance stop and cut, then file, to that line. Carefully done, that should get you within one half a degree of the ten degrees advance you desire.

Beaware that this only governs the total mechanical advance. It does not govern when the total advance is reached, nor does it govern the advance curve at various speeds. That is done the springs attached to the distributor shaft and the points cam. Thus, it is perfectly possible to have a distributor which shows the proper overall advance, but does not show the proper advance at various engine rpms. Only by changing out springs, and testing after each change is made, can you set the proper overall mechanical advance system. Again, the mechanical advance curve I have already posted is a starting place to begin your tests. Ideally, the required advance curve is tested, for the individual engine, on a rolling road. Then, the distributor is tuned as close to the required curve as possible.

Les
Les Bengtson

Thanks Les. That is what I really wanted to know, confirming my theory about measuring the lobe mechanical advance.

Most of my learning to date is gleaned from this site: http://www.jcna.com/library/tech/tech0015.html

Steve
Steve Gyles

Just finished triple checking all my data. The lobe had been modified from 7 degrees to 9 degrees. This would have given a dynamic advance of 26 degrees at 3000RPM, consisting of 8 degres static and 18 degrees mechanical. Accounts for poor engine performance.

I have now taken 1.12mm off the lobe face to give me 12 degrees and therefore 32 degrees at 3000RPM.

I shall probably give it a test tomorrow.

Steve
Steve Gyles

This thread was discussed between 06/12/2006 and 08/12/2006

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