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MG MGA - My dizzy

Looks like my dizzy is a 25D4, service No 40822A. TDC engineering give a table of dizzy curves, which has this as an Austin A60, which is the motor I'm working on, converting to a "1622 MGA" with camshaft change and early B head, dished pistons. The table gives the following comparison from this dizzy to the original:
12.5deg @2000 / 6 deg@ 700 / .75 deg@400, 0 @325 Original DM2P4 40510D has 21 deg@3100 / 7 deg@ 1250 /.5 deg@750 / 0 @600

The vac advance has a bad leak, so need to replace, but has no I/D on it

So what is best to do? Can I replace the vac unit (with what) and redo the weights?
Art Pearse

Art,if you have a pinhole leak in the metal housing, you "may" be able to solder the hole shut. Alternatively, if you don't want heat near, you can epoxy it, and it should be fine.
Mike Parker

The leak appears to be in the diaphragm, also the output link is seized.
Are all Lucas vacuum advance units the same? There is no ID on it.
Can I alter (increase)the mechanical advance from 13 to 21 deg by grinding the stop?
Art Pearse

All vacuum advance units are not the same. Near the vacuum connection you should see 3 stamped numbers. Mine read 7-14-10. I THINK they mean that the vac advance starts at 7psi, is fully in at 14 psi and has a max advance of 10 degrees. I have 2 distys, a std DM2-P4 in my car and a 25D4 in my parts. Both vacuum units are marked 7-14-10. I think both vacuum advance unit will swap with each other.
C Schaefer

Art. First, all vacuum advance cans are not the same. As Chuck (am I remembering your name correctly, Mr. Schaefer) points out, there should be some markings on the vacuum advance which notes at what vacuum the advance begins, at which point there will be no further advance, and the maximum advance provided at the latter point.

I am not sure of the mechanical advance figures you have provided us. When I read the first group, it looks like an advance curve taken off a distributor test machine (which measures actual distributor rpms and shows advances at the distributor which are one half of the advance seen at the crankshaft). The second group looks like it could be listed at engine rpms (twice the distributor rpms and advance seen at the crankshaft pulley).

There is a fellow, Jeff S??? who provides distributor rebuilding and recurving services. I think it might be in your interest to contact him and ask your questions of him. He has posted here before on the subject of distributors and you should be able to find a contact address in the archives if no one here can provide it. All reports are that the quality of his work is first rate.

Les
Les Bengtson

I think Les is referring to Jeff Schlemmer, Advanced Distributors.
Andy Bounsall

Yes Les. you got it right. I am Chuck. Gotta get the "C" thing changed.

A correction on my earlier post. The 1st two numbers are not PSI but "inches of Mercury" (vacuum). Sorry for the confusion.

Chuck
C Schaefer

Les, I got the data from TDC Engineering (Marcel Chichak). He does not specify dizzy rpm or crank rpm or degrees, but it does look inconsistent between mine and the 40510. He does have a very good paper on adjusting the advance curves.

Perhaps someone can read off the number stamped on the top cam of their MGA original dizzy?

Another oddity is my dizzy clamp. If I close it up on the dizzy, the bolt and bolt tunnels assume a profound bend. If I straighten it out to be in line, the mounting holes are too far apart to match the block.
Art Pearse

Art. The data supplied on that site is from a Lucas company manual giving the various distributor specifications. As such, it is all as would be seen on a distributor test machine. Multiply speed and advance by two to get the figures which would be observed at the crankshaft.

The tech article on tuning and modifying the distributor is a good one. What it does not mention is that to do the work efficiently, you need a distributor test machine. (Yes, the distributor can be tested on the engine, but this becomes a significant amount of work, sometimes requiring that you remove the distributor, make a single stroke of the file against the cam stop, and test again. The distributor test machine makes this easier in that you only use the low tension portions of the distributor on the test machine while you need to use the entire distributor, low and high tension circuits, on the engine.) It is also of note that while the referenced tech article is useful for setting the overall (maximum) mechanical advance the distributor is capable of achieving, it is far less useful in setting up the intermediate portions of the curve, something which is fully as important, on a street car, as the maximum advance.

The advance figures seen on the engine, using a dynamic timing light, are a mixture of the static and the mechanical advance. If you require 30 degrees of overall advance at higher engine speeds and the distributor's maximum mechanical advance is 20 degrees (a cam marked 10 for ten degrees of distributor advance and what would be the read out on the dizzy test machine), you have to program in ten degrees of static advance to obtain this figure. After that, the static, or pre-programed, ignition advance remains a constant and needs to be added to the figures seen on the distributor test machine for each rpm/degrees of advance reading.

Thus, a relatively complex modification to make and one best made having the proper test machine. (In my area, I only know of two of the Sun distributor test machines being available. One is my own and the second is not available for general use.)

Since it is possible to have the distributor competently rebuilt by people who have the proper equipment (John Twist and Jeff Schlemmer for sure and, perhaps, Joe Curto), I would be inclined to let them do the rebuild work. Even though I have the test machine setting out in my shop.

Les
Les Bengtson

Les, looks like I read the wrong line for the original MGA curve. Should be 12 deg @1500/7@650/1.5@300/0 @150, all dizzy rpm and degrees.
So, my A60 dizzy has about the same total advance but has to rev to 4000 to get there vs 3000 for the MGA. Not so bad for starters.
Now all I need is a vacuum unit.
Art Pearse

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

Bill that is fantastic! Thanks.
Art Pearse

This thread was discussed between 14/05/2009 and 18/05/2009

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This thread is from the archive. The Live MG MGA BBS is active now.