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MG MGB Technical - Point gap vs dwell angle logic???

Hello,


I have MG's for years and I know that point gap is 0.015" and dwell is 60 deg. I have to tune a friend's Spit and point gap is 0.015", it is a constant in Brit. cars. But what puzzles me is it is supposed to be 41 deg Dwell with that delco distrib. How could it be possible such a difference. May be distrib. cam profile is different. I checked or many vintage cars like Volvo, Fiat, Renault, Volks and dwell is between 50-60 deg for 4 cyl. engine. Is it and error in handbook or I missed something?

Thanks,

JGC
Jean Guy Catford

Jean-Guy. You are correct that the points gap, which is measured with the points fully open, is different than the dwell time, which is a measurement, in degrees of arc, on how long the points are closed. Thus, the profile of the points cam is of significant influence on the dwell reading. I do not know for sure, but would suggest, that the points cam on the "Spit" (Triumph Spitfire?) has a more rounded profile than the points cam used on the MGB. Thus, the points begin to open sooner, and close later, than they do on our Bs.

Many years ago, Popular Mechanics magazine ran a short question on dwell vs. setting the points using a feeler's gauge. I believe the person responding was a well known racing mechanic by the name of Smoky Yunick (sp?). He noted that the dwell reading was a very accurate indication of how the system was designed to run, but that Ferrari simply used a feeler's gauge when setting the points on their cars.

I would double check the dwell specification with some other sources. Especially if you have set the points gap at .015" and have used a dwell meter which indicates something other than the specification you have at hand. But, I would not become deeply concerned about such things. All of us know how often parts have been mixed up on cars of our vintage. It may well be that Triumph specified a more gradually opening/closing cam shape than the MG line did. Thus, giving a lower dwell reading. It is also possible that, over the years, a different distributor has been installed, or that a rebuilt distributor had a non-Triumph specification points cam when assembled from the various pieces available to the rebuilder.

Bottom line is how well the engine runs. If it runs well, note the fact, take a dwell reading and keep that for future tune ups of that engine.

Les
Les Bengtson

To add one extra item there regarding the Dwell reading on a suitable gauge. If the needle on the gauge is wavering this can indicate a bend in the distributor shaft or wear in the bushings.

Personally I have always used the feelar gauge method for normal replacement and put the Dwell meter on about every send set just as a check.

Cheers , Pete.
Peter Thomas

who is still runing mechanical points?? The issue of dwell is an anachronism. Adjusting and setting points is for old men (and women to be politically correct). Name me one valid reason not to be running an electronic points system. (Reliability is an issue in favor of electronics and you know it)

In the 1960's this was the way to go. In 2006 ?? you tell me.

Do you really need to feel competent by periodically adjusting and replacing points??

I suppose you add lead to your fuel so you can foul the plugs with lead and feel period genuine.

You ought to use the old engine oil that lost viscosity and hp performance after 1500 miles; &
leaded gas that fouls your plugs in 5,000 miles.
& alcohol anti-freeze that was lucky to last the winter.

We are all playing with beautiful antiques, but why make it harder than needs to be. I've put nearly 8,000 miles on my car in the last year, and I appreciate the performance and reliability of a modern ignition, updated electrical system and modern oils and hi performance rings, wheels tires and brake pads.

I understand if it is a show car attempting to be period exact. -- but a daily "fun" driver?? The key is the word "fun".

Barry


Barry Parkinson

Barry. As Jean Guy notes, he is performing a service for a friend. This is not his own car. Thus, he asks information in support of his friend and Pete and I have provided the information he has requested.

As to the reliability of the various electronic points replacement systems, yes, when they work properly, they are reliable. When they do not work properly, they are neither reliable, nor is the system amenable to easy troubleshooting. One of the reasons so many people, who have done the electronic points replacement system conversion, mention that they carry a spare distributor points plate, set up with a condensor and points, in their on board spare parts kit. I have, twice, had "electronic ignition systems" fail in use.

As to the statement, "I've put nearly 8,000 miles on my car in the last year, and I appreciate the performance and reliability of a modern ignition", all I can say is that, two years ago, I installed a new set of points in my daily driver. After more than 10,000 miles the points type system was capable of controlling the engine sufficiently that it easily passed our emissions test. Now, at about 18K miles, the system is in need of a tune up, including points replacement, distributor inspection and lubrication, setting the valves, etc.

Not bad for a simple, "obsolete" system which, should it ever fail, can be easily trouble shooted and repaired.

Les
Les Bengtson

Well said, Les. Personally, I'm ready to laugh at all those electronic ignition fans the next time we have a nuclear war, and EMP fries all the transistors and chips in their circuits. Then, we'll see who goes tootling down the road, and who is walking. Haaarrrr!

Seriously, there's good reasons for both systems. I just can't see trashing a tight, reliable points system if it's giving no problems. But, if your distributor bushing is seriously loose, maybe a good optical or magnetic pickup system is a viable alternative to allowing the car to just sit.

Now if I can just find a few of those old green radiator hoses, with some original wire clamps - - -

Cheers - - Alec
Alec Darnall

"Adjusting and setting points is for old men" .............. I'm near sixty - do I qualify?

When I want to have some fun on a sunny afternoon - it's my uncovered B, archaic systems and all. When I'm looking for comfort and boring reliability - it's my 740iL, packaged to the hilt with electronic gadgets and special fluids.

I enjoy tinkering with my B and I'm not overwhelmed by it. The BMW, on the other hand, scares the hell out of me and I take no real pleasure from it.
Steve Buchina

I had an elecronic ignition system on for one year. It gave trouble and came off. I do not intend bothering again when the attention needed for points is so minimal. My every day car has a coil per cylinder system, it takes longer per driven mile to change the plugs on that than it does to keep the points on the B in good form
Stan Best

Barry,

One valid reason not to run an electronic points system is, for me, that I choose not to.

Have thought about it but, frankly, experiences of others having their electronic system fail without warning have put me off. With points I can expect some deteriation in performance as a warning should I not keep up the periodic maintenance schedule.

As for the wise travelling with a back-up points system ready to install in the event of an electronics failure, well, I guess that's a second reason!

Notwithstanding, I am quite comfortable with your passionately expressed support for the electronic system. I think in a lot of these matters its a case of 'to each his own'.

Regards
Roger
Roger T

Thank you guys for your input!,

I also rely more on a feeler gauge than on a dwell meter. I also use a dwell meter as a double check.
If I remember well the latter was popular for GM products with an allen set-screw to adjust point gap in the past. I own few British cars and when originally they had points they still work fine with them. I bought 5 years ago a pertronix for my TR6 and it is somewhere in the box in my garage, as the old system is easy to fix without fancy tools in back contry roads. My friend faced that problem when we came back from USA last W.E.. A 2 min. visual re-gappig to a "precise" 0.015" and the car was running again
Concerning the irrelevant preaching habilities of Barry. It is always the same debate on classics and moderns cars. By many ways British car were old by design with lever shocks, non-OHC engines, etc. When we start in that avenue of improvement it is not a bad concept, but where shall we stop?

Anyway I will compare Spitfire's distrib. cam design with an old MGB distributor to see if it is there that is the answer for my dwell interrogations.

So long.

JGC
Jean Guy Catford

Hi
One good reason for not running electronic ignition whether young or old is what happened to me. Driving along the road about 55mph minding my own business when the engine coughed and died, electronic igntion unit dead as a doornail. Came home on the back of the big yellow truck, I was fortunate it failed in a safe place where I could coast into a layby, it started me worrying about where it could have happened.
At the time I thought it was a one off and was quite surprised at the number of people who had the same experience when I posted.
My advice to anyone with electronic ignition is, carry the points etc in the glove compartment and know how to fit them, at least with points you can usually limp home.
Ron
R. Algie

I use points to this day for two reasons:
1) They are inexpensive and easy to re-set or replace.
I ofter use my B for cross country or looong trips. There's a nice, cheap, new, set in the glove-box.
If I'm in the depths of Oklahoma with dead electronics, I'm going to have a good wait for the UPS truck.
2)Points, however ancient they seem, are far easier to time statically, in case I didn't bring my timing strobe with me.
R.S.C Caskie

One point that is often overlooked when this type of conversion is mentioned is the require for annual cleaning and lubrication of the distributor. This is a design feature of the Lucas 25D and 45D series distributors. To continue to work properly, they must be cleaned and lubricated on a regular basis.

Often, people replace the points with an electronic trigger, then, forget about the distributor for years on end. I have seen some examples of this over the years. Yes, the electronic triggers, working properly, always fired up the engine and allowed it to run. However, the various advance mechanisms, due to lack of cleaning and lubrication, did not allow the system to function at its optimum. The worst case I saw had a non-functional mechanical advance system due to a build up of rust. This prevented the weights from advancing and performance suffered.

The basis for any properly working distributor system is the basic distributor, regardless of the device used to trigger the coil, should be in good condition and well maintained. The Lucas distributor, being designed to have annual lubrication, is less desirable for conversion than a newer distributor, designed to work without annual inspection and lubrication, would be.

Thus, were I to consider a change over to an electronic points trigger I would also consider a distributor of more modern design as the basis for such a conversion.

Les
Les Bengtson

JGC,
The Delco distributor does have a much more "rounded" cam than the Lucas counterparts, just as Les surmised. That distributor is also much less likely to have issues with binding weights, as the assembly is much larger and loosely fit. In fact, it is the predecessor to the modern HEI distributor used in millions of Chevy (and other) engines. The cam design is extremely similar to the Mallory design, which coincidentally also runs 41 degrees of well.
Jeff

Jeff Schlemmer

Dunno why, but all those years of 'fixin' cars, adjusting the points with a dwell meter, seems like they run better than setting with a feeler guage.
But, I still prefer my Pertonix for starting and idle quality, plus power.

Grassroots Motorsports did a test with various igntion set-ups.
They claimed 2-3 HP at the wheels.
Late model MGB, twin carbs retro fit, stock cam/compression.

But mine has failed at least twice. but Pertonix backs them up.
SF
Dwight
DCM McCullough

Since I bought a dwell meter I have never checked the points by gap, as it is non-invasive. Before that I always found a spike on one side, which had to be filed off before I could get an accurate gap, so while I was fiddling with that I might as well fit new. Since checking dwell it has stayed in tolerance for more than 10k miles now, with only oiling the shaft and greasing the pad as maintenance. The other week I did get the distributor out as I wanted to change the vacuum capsule, and out of interest had a look at the points, and was amazed to find no spike (these are not the self-wiping points). I did find oil coating the points, and I know some big switching applications use oil-quenched contacts. Maybe the oil reduced sparking and hence spiking. I cleaned the oil off, then wondered whether I was doing the right thing, but a 460 mile trip last weekend never missed a beat. I'll just have to see if the dwell starts changing now. The V8 is just the same - at least 10k miles without changing, although I did have to tweak them slightly this year as they only have a 1% tolerance instead of the 5% of the 45D. I'm leaving both points in now just to see how long they will go, particularly the roadster set which still haven't been readjusted.

FWIW the 25D4 has a dwell of 60 degrees and the 45D 51 degrees, but the same gap.
Paul Hunt 2

I run points in all my MGs by choice and I'm hardly "old" at 34 years of age. Coments like "In the 1960's this was the way to go. In 2006 ?? you tell me" only show the difference in the reasons we own these old cars.

Some people feel that old cars should be modified to run modern parts with modern reliability and modern performance. Well, what if I want to experience the hobby at its purist? What if I want to know what it's like to own and maintain a 40, 50 or even 60-year-old car as a daily driver? Can't do that with electronic ignition.

If we really want to discuss reliability, I can only offer my personal experience. I've driven my MGs as daily drivers for 17 years and have never been left stranded because of points. In fact, I don't think a car of mine has EVER even stopped running because of points. I can't say the same for electronic ignition!

I don't own MGs to get attention, I own them for the experience of driving a vintage automobile. And I like them just the way they are, without new electronic bits shoved inside! :)

- Steve, who would buy a Miata if he wanted a modern car
Steve Simmons

Interested to read Paul's post. I do keep the dizzi oiled and have often wondered as I wield the oilcan if I am storing up trouble with an insulating film of it leaving the shaft to sieze up while it migrates to the contacts. Nice to know the current will still flow, I suppose the HT even potted down via the coil turns ratio remains high enough to blast through it.
Stan Best

Just a passing thought: I own 5 dwell meters and they all read different. I guess you just have to find the "sweet spot"?

The Pertronix is so much easier than points. Original? Better? Maybe not, but there are distinct advantages! This argument is like trying to get someone to change political parties or religions.
Jeff Schlemmer

I've been puting the Crane/Allison electronic ignition conversions in many of my family's cars for over 35 years and never had one fail. Of course, this might be due to the special magic talisman that I place in every car- A spare points plate with new points installed!
Steve S.

Couldn't disagree more Jeff, I beg to differ:

The argument is like discussing silicon vs. glycol based brake fluid.

hehehhehehhehe

warmly,
dave
Dave Braun

Paul - I have always been told that when points develop a spike on one side from metal deposition, the capacitance value is not correct for the car. I have always checked the points for pitting/spike development and if noe is present, I just leave that particular capacitor in place.

In defence of points, the only time I was ever stranded due to an ignition problem was in a car with electronic ignition (not MG). That said, I did convert our MGB to a Petronix ignition module, but like Steve S in Virginia (There are getting to be too many Steve Ss on this board), I carry that same special magic talisman that Steve carries in his car (it sure works good). Cheers - Dave
David DuBois

David, I was always curious as to the points "spike" developing myself. I had thought it was something to do with the material involved and the voltages but I have no real evidence.
Interesting thought.

Pete.
Peter Thomas

Hello,

I had some kind of compromise on a MGB for many years. It was a CDI available as a + for points systems in those days. The points were only triggering CDI. I had the same set of points on the B for at least 5 years without any pitting. I used my B all year round and it overcame starting plroblem at -30F with those 2 weak 6 volts battery. At the eab when the electonic system broke it was only to switgh the device of to had a regular ignition.


JGC
Jean Guy Catford

Is there a point to this pointless discussion?
James Johanski

Hi David - I've experienced spiking and pitting on all my points cars previously, including this roadster. It's true I did replace the capacitor in an uncharacteristic fit of generosity together with coil, leads, cap and rotor the last time I changed the points. I've always understood it was an inevitable feature of the small spark that still occurs at the points as they open, from telecoms switching experience also, and why the later 45D4 self-wiping points system was introduced. The heat of the spark melts a tiny amount of material on both surfaces, but due to ion or electron flow material only flows in the one direction to form a slike and leave a pit. It is the heat of the spark that gives you the shock when you build up static and touch a metal door knob or something. But if you touch it with a metal finger ring or something you can see and hear the spark but feel nothing, meaning it isn't the electricity that gives you the shock (don't try it with HT though!). In the 60s someone had the 'brainwave' of drilling holes through the middle of the contacts reasoning that if there was no material there a spike couldn't develop, and Lucas points came like this for a while. But of course the spike just moved to wherever the points regularly broke contact!

I still have a CDI ignition unit triggered by points that I made from a kit over 30 years ago. Never found it made any difference over correctly adjusted and maintained points, so it sits in a garage drawer.
Paul Hunt 2

James, no. ;-)
Jeff Schlemmer

I've had an electronic ignition fail out on a road trip. It ain't fun.

I've also had points failure on a road trip. Back on the road in about 10 minutes.

For my "stock" MGs....I'll take a points system, thank you.

FYI...I am a card carrying member of AARP.

:-)

rick
rick ingram

This thread was discussed between 16/08/2006 and 18/08/2006

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