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MG MGA - 1500 advance timing map?

Can anyone provide factory specs (or a source)for:

1) vacuum advance map in terms of vacuum to start advance and the vacuum when maximum advance is in and what the amount of advance is at max

2 mechanical advance in terms of rpm when it starts, and the curve between then and when it maxes out in terms of rpms and degrees?

Thank you
brucep

I cannot answer all you questions, but I have been told that once the dizzy is spun up to full mech adv the timing should be btwn 30 and 34 deg.

j

John B

What is the 5-digit code on your distributor? Every Lucas distributor has a 5-digit number stamped in a pad on its side that corresponds to a particular vacuum advance and mechanical advance rate.

The website below has the information for most Lucas distributors. You can look up your info there, or if its not listed, let us know what unit you have!

http://www.telusplanet.net/%7Echichm/tech/lucas.pdf
Jeff Schlemmer

If you have a 40510, I believe its a 7-14-10 vacuum advance and a 12 degree mechanical advance with 800-3000 engine rpm curve with a 3 degree knee around 2200 rpms.
Jeff Schlemmer

Bruce. My MGA distributor is the Lucas DM2 to specification number 40510H. It is from a 61 MGA 1600 and both of the 61 As I have owned used this model and specification distributor, so I believe it is both original and correct.

The vacuum advance can shows the following:

7-14-10 54411301

This indicates that the vacuum advance begins at a vacuum of 7" Hg and maximum advance is at 14" Hg. The maximum advance is ten degrees of distributor advance (20 deg of crankshaft advance).

The factory does not list the mechanical advance curve in any of my books. The one source I have found on the web lists the following:

300 rpm 1.5 deg
650 rpm 7 deg
1,500 rpm 12.25 deg

Since these are distributor degrees, as read from a distributor test machine, the figures need to be doubled to tell you engine rpm and crankshaft degrees:

600 rpm 3 deg
1,300 rpm 14 deg
3,000 rpm 24.5 deg

The static timing for most MGAs was 7 deg BTDC. When this is added to the mechanical advance at 3,000 rpm you have 31.5 deg BTDC, or the 32 degrees of advance that most recommend at that engine speed.

Hope this explains it sufficiently.

Les
Les Bengtson

Thanks for all the help. I think I'm making progress.

The engine is a 1958 1500. I couldn't find a number on the Vacuum Advance unit. Where should it be?

The number on the dist is 40780. Looking at the table Jeff Schlemmer provided a pointer to I find the following info

rpm1: 1500
advance1: 12
rpm2: 650
advance2: 7
rpm3: 300
advance3: 1.5
vacuum code: 150

I interpret this to mean the following:

Mechanical Advance
12 degrees dist advance at 1500rpm (24 crank degrees)
7 degrees dist advance at 650rpm (14 crank degrees)
1.5 degrees dist advance at 300rpm (3 crank degrees)

Vacuum Advance
What does vacuum code 150 mean? where do I find the number on the unit?

Thanks
brucep

I recurved my MGA distributor last year. Here is the link:

http://home.comcast.net/~eastmanb1/MG/MGA_Distributor_work1/Dizzytest_simple1.htm

It shows the early and late advance mechanisms, the setups I used to test, and a graph of several advance curves. There are links to a movie of the test and to an Excel spreadsheet containing advance curves from several sources, a spring rate calculator, and a free length/prestretch calculator.

For an early MGA, you probably have the "multilever" advance mechanism. This uses two identical springs, not a heavy/light pair.

Hope this helps. Have fun
Bill
Bill Eastman

Bruce. Please re-read my previous posting. The site that Jeff notes gives the timing in distributor degrees at distributor rotation speed. The distributor, on a four cycle engine, turns at one half the engine speed. Thus, the speeds indicated from a distributor test machine (which your values represent) are one half of the values measured at the crankshaft. Or the rpms indicated on the dash tachometer. The information I have provided to you is accurate, within one half of one degree, with the information you have determined.

Thus, I suggest you use the crankshaft figures I have provided when setting up your overall mechanical advance, then, check the figures at the lower rpm ranges to determine the degree of wear of your distributor.

Les
Les Bengtson

I second Les's last post. Bill's graph misrepresents the MGA curve as half the values it should show. Its quite easy to confuse the distributor/crank rpms as most websites don't specify which one they are referring to.
Jeff
Jeff Schlemmer

OK. Caught my mistake.

Bill: Wow! How did you do the "recurving?" change springs, regrind the weights?

Les: My 1500 has been bored out to 1588 and the carbs have been rejetted accordingly. I suppose I should use the specs for your vaccum advance can as well. Moss catalog only shows 1 can for all 150-1600s except the Mark 11. Does that mean all the cans are the same anyway, or has Moss simplified their parts inventory, like I know other suppliers for other old cars have done?

Thanks everyone for the help.
brucep

Moss has discontinued most of the old vacuum advance cans, but I'm working on reproducing them. I should be able to rebuild stock advance cans and provide new identical replacements in the next couple of months. Its my understanding that the original style advance cans are NLA for virtually all MGAs. I also offer a recurve service, performed on a snap-On distributor tester.
Jeff Schlemmer

Jeff,
I think I accurately described the graph- it is distributor degrees and consistent with most published data. It also separates the numbers from crank advance which usually includes the static setting.

Including the low compression MGB curve may be confusing. These cars ran late ignition to keep exhaust temps high so the AIR would work. Above test rpm, they added advance to compensate.

The graph shows a "target" with a 17 degree stop. I have been collecting distributors and have a 15 degree degree unit. In the end, I think I went back to the standard 12 degree unit.

On the 25d style MGA, the soft spring does not effect running. It's job is to hold the timing back during cranking. If your car kicks back against the starter, shortening the soft spring may help. On the HC MGB, the soft spring controls advance up to 1200 rpm.

For springs, I went to the hardware store and bought a bunch of stock springs about the right size. I also picked up a Chevy recurve kit at the local autoparts store. I can't remember which springs I used.

I did not change the weights. I selectively assembled parts to get a nice, tight assembly. I think the advance weights are fitted to the base plate. At a minimum, they are paired and you have to keep them together.

The bottom line- wear steals advance and makes points operation less consistent. A pertronix will fix the spark strength issue but won't fix the advance. Depending on your setup, playing with the springs may get most of it back.

Hope this helps,
Bill

Bill Eastman

This thread was discussed between 12/04/2006 and 14/04/2006

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