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MG MGB Technical - positive earth electronic ignition

Anyone have experience to share about pos earth elctronic ignition? Seems the choice is more limited.
What about losing the points drag on the cam? I'd heard this messed up the advance; the lack of drag causes the centifugal advance to operate too soon, anyone found this with any contact-less setup?
Thanks, Rich.
Rich

Rich, typically an earlier advance curve will provide a little more power. The points drag only changes the vacuum advance curve by a truly marginal amount since spring tension holds the points plate in position. The points have no influence on centrifigal advance.

Try Crane or pertronix for the electronics. Both are good.
Jeff Schlemmer

ive had problems with electronic ignition, my problem was the car would not run with the vacume advance connected. my mg as the later dizzy ,throw it away and get a aldon 101by2 and then eather use points or pentrax which aldon sell.there is no amp needed and easy to install,dizzy 110pounds and pentrax around 60pounds cheers daren
daz

A tuned B I had 25 years ago persistently pinked after fitting an optical Lumenition - no other changes - and that was pre-unleaded.
Now, in these web days I came across this which may explain it, so obvious it hadn't occurred to me.
Fast forward to "The Solution" to see what the problem is about,
http://www.mgcars.org.uk/mgcc/sf/980904.htm
Springs are tricky to get now and I wondered if it was a problem in general which other users had noticed or not much of an issue with a standard engine.
Rich
Rich

Rich: I installed a Crane pointless system in my 74 USA car about 6 years ago. Other than setting the timing I didn't do any before and after advance checks. The car did run better with the electronic system. At the time my engine was a stock LC engine tuned for US emissions with a 41491 distributor. The 41491 has a lazy and long advance curve, Full advance is 39* + - 2* @ 4800 rpm. Maybe your link explains why I felt the engine ran much better after adding the electronic ignition, it probably did cause the centrifugal advance to advance a little quicker. No pinking problems with the LC engine after adding the Crane system.

Clifton
Clifton Gordon

Rich
The article on setting the springs for advance curve is very good. His story of cut and try is very true. It is a time consuming frustrating but challenging job. A distributor advance machine which allows you to chart your advance is a great help. But even with that machine, you still need to test it out on your engine to compare performance. The author is intent on duplicating factory settings. With a modified cam the compression pressure at low rpms is much less than stock and more advance is necessary at lower rpms.

With a modified cam you set the static timing with more advance. The result with lighter centrifugal advance springs, is an advance curve the advance being all in around 1800 rpm, depending on the cam grind, compression ratio etc. You probably should have less total centrifugal advance also.

As indicated in the article, you can reduce the total centrifugal advance by changing the weights (in width) or by putting a small tube around the stops which reduces the travel of the centrifugal weights.

You can also have two different size weights and/or springs and design an advance curve with an initial early advance and more advance coming in later at a higher rpm.

Ironically, many people with a hot cam go to a distributor with no vacuum advance. It always seems to me that a vacuum advance is even more necessary with a modified cam. At light throttle the compression pressure will be less and more advance will be useful. At full throttle the compression pressure will be greater than stock and much less advance is required.

Barry

Barry Parkinson

Thanks All. Previous exp and that article made me realise there might be a bit more to electronic ignition than "fit and forget", seems it could work for or against a given engine.
Daz, I was thinking standard or fully programmed electronic would be the way to go but the latter is expensive. As the article and Barry says, reprogramming the mechanical advance would be another option.
You could get the springs a while back and I did a Ford distributor once which included removing the vac and using 2 heavy springs, instead of one light and one heavy, to get a flatter advance curve. Seems Aldon can do this sort of thing so I'll give them a call. Rich.
Rich

That article started "a lively debate" in Safety Fast . I fitted a sports coil at the same time as my Electronic ignition and I now believe that it never worked properly due to being overloaded ( electrically , not mechanically ). I may try again one day when I get too old to see the points.
S Best

the simplest and safest mod for the points is a "dwell extender". It is used with your stock points and only has one wire on the ground side of the points. The points break the circuit when they open but the silicon diode, capacitor and one little resistor together immediately ( a fraction of a second determined by the combo of the resistor and the capcitor)ground the circuit and, in effect, close the points instantly.
The result is a longer dwell and hotter spark and the points last nearly forever. You adjust the points wide as it doesn't matter how quick they close as the diode does that. As the points wear on the distributor cam the gap will close over thousands of miles and the timing will change slightly. If the diode fails, clip the wire and you are on your way using just the points as an ordinary system. There are a couple web sites that give instructions on making them. I found one in a cheap tool store the other day (new) for $15.00 I used them in the '70's on a triumph, vw, mustang, etc. Radio shack used to sell a cute little kit for $9.95. As long as the diode is adequate size you can run hot coils. I had one failure on a modified vw with a 6 volt hi power coil. It just quit one day after about 45,000 miles. I disconnected the wire to the dwell extender and drove home with no problem.
I had a dual point Mallory that I wanted to use for my B, because of the easily adjustable advance curve timing system built into those distributors. My plan was to use one set of points and a dwell extender. In the end I didn't use it because I wanted to use a distributor with a vacuum advance for street use.

The modern transistor "electronic ignition" is better in that it eliminates the mechanical points and can "shape" the voltage pulse to the coil for more spark per amp, but it costs more. And if it breaks you may not be back on the road without new parts.
Barry
Barry Parkinson

Thanks Barry,

I like the idea of improving the standard points set up and looked for a Sparkrite unit which does as you say but found they are only for neg earth. If I had a circuit for a DIY ignition unit I may be able to redraw it for pos earth. I'd appreciate any links you may have.

Rich.
Rich

Rich
I found it by google searching sometime ago and didn't save the address.
You really should convert to neg earth. I did it on my '67 and put an alternator in place. Everything works except the tachometer. You have to reverse the flow of current in the induction loop on the back of the tach.
Barry
Barry Parkinson

This thread was discussed between 27/03/2005 and 31/03/2005

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