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MG MGB Technical - '72 MGB Roaster Won't Start

mgb roadster
J. K. Chapin

I'm pretty sure I need a new battery. The one in it is an Extreme Magna Power 12v. that appears to be primarily for snowmobiles and skidoos. It has garnered several negative reviews on the web. Can you recommend a 12v. battery that will fit in the MGB battery box intended for a 6v battery? Thanks. Jud
J. K. Chapin

The Varta B35/B36 gets good reviews.

I fitted one in my brother's B and I have one to fit into mine, when I get it back together.

The footprint is the same as a 6v, but it is slightly shorter. Plenty of power, though.
Dave O'Neill 2

Looks good but wrong side of the pond. Thanks. Jud
J. K. Chapin

If you flatten a battery then the cars charging system will only ever put 50% of the capacity back - Lucas Fault Diagnosis Manual.

One option is to charge at a higher voltage - up to about 16v if it is still fitted in the car, or you can go as high as 20v if off the car and in a well ventilated space. Another option is a conditioning charger, some of these have programmes that charge in short pulses up to about 20v and should recover the capacity.

The non-starting now may just be an unfortunate coincidence.

A wildly swinging tach when the engine is running indicates an erratic connection in the circuit through the coil and points, so that is the area I'd be looking at first. On a 72 the coil feed comes through the tach pickup, changed to direct in 73 feed in 73 with a trigger wire off the points terminal to the tach, but other than that voltage tests are the same. With the ignition on you should have 12v on the coil +ve which should be a white wire. With the points closed you should have 0v i.e. an earth on the coil -ve or white/black, and with the points open you should have 12v on both sides of the coil. If you close and open the points by hand when they open you should see a small-ish spark and get an HT spark, remove the coil lead from the distributor cap and connect it to a plug laying on the block and you should clearly see it. If you get a lot of sparking and spitting from the points and either a very weak or no spark from the coil then the condenser is probably dead. You can check this by temporarily connecting another condenser between the coil -ve i.e. points wire and earth. However new condensers can be very iffy these days, so really it needs to be one that you know works. However unless the condenser has gone short-circuit (which would mean you still have an earth on the coil -ve when the points are open) this fault doesn't affect the tach, and normally they fail open-circuit which means the DC tests are OK but there is no spark.
Paul Hunt

If Paul's suggestions don't yield any positive results, a common replacement battery is the one designed for the Mazda MX-5. RAY
rjm RAY

Thanks Ray, that's what I ended up getting. I'm going to try Paul's diagnostic tests and I'll let you know what I find. Thanks Paul. Jud
J. K. Chapin

IT RUNS!! The rest of the saga - Part one:
COINCIDENCE?? CAUSE AND EFFECT??
ONLY THE SHADOW KNOWS!
Well, the first thing I did was to post in inquiry on the MG forum describing the problem and asking for advice. The second thing I did was to ask some FBCCers (members of the Foot Hills British Car Club) for advice. Both resources came through for me.
Even though I knew I had fuel and the points, cap and rotor looked good, I did not know whether the problem was lack of fuel to the engine or lack of spark to ignite the fuel. Doug Hansen gave me a test to perform from the fuel side and one of the forum members gave me a test to perform from the ignition side.
Doug’s test couldn’t be simpler but it’s not something I would have just thought of. What he said was, “Squirt some starter fluid into the carbs and if it fires the problem is on the fuel side.” Doesn’t get any simpler than that so I gave it a try. The initial test was misleading because the engine fired up on the first try. “Aha!” I said, it’s a fuel problem. But no. After about half a minute the engine sputtered and died. Squirting more starter fluid down the carbs didn’t result in a restart. It cranked and cranked but no fire. Ergo, spark is the problem – or so it would seem.
The ignition side test was also fairly simple. Without a helper I couldn’t hold a spark plug against a head bolt while the helper cranked the engine. Turns out the test was something I could do myself. Instead of holding a spark plug and cranking the engine to open and close the points, just take off the distributor cap, hold the center hi-tension lead close to a head bolt and manually open the points with a screwdriver. The engine almost always stops with the points closed. With the ignition key on the coil immediately gets charged. If all is working properly, when you open the points the coil field will collapse and, Presto! You’ll get a spark to the block. Unfortunately, this test also proved to be misleading. Yes, I got the hoped for spark but the engine still wouldn’t keep running after initially starting and it wouldn’t restart unless I waited many hours.

So, what has this told me? When dead cold, the car starts and seems to run perfectly for 20-30 seconds. Then it dies. Then it won’t restart. I concluded that it must be something that fails after about 30 seconds and then, Lazarus-like, heals itself over night and works again for another 30 seconds. What could it be? I bet you can guess.
During all of this I discovered that the wire from the “-“ side of the coil to the distributor was badly frayed and hanging on by only one small strand (0.0085 inches) at the distributor terminal. Aha! Thought I – insufficient conductor that heats up and forms a high resistance block after just a few seconds. I replaced the wire and crossed my fingers. Varoom!! And I thought I was a genius and had fixed it. Thirty seconds later the engine died and I was back where I started. Drat!!
Continued ...
J. K. Chapin

The Saga Continues - Part Two:
So, what has this told me? When dead cold, the car starts and seems to run perfectly for 20-30 seconds. Then it dies. Then it won’t restart. I concluded that it must be something that fails after about 30 seconds and then, Lazarus-like, heals itself over night and works again for another 30 seconds. What could it be? I bet you can guess.
During all of this I discovered that the wire from the “-“ side of the coil to the distributor was badly frayed and hanging on by only one small strand (0.0085 inches) at the distributor terminal. Aha! Thought I – insufficient conductor that heats up and forms a high resistance block after just a few seconds. I replaced the wire and crossed my fingers. Varoom!! And I thought I was a genius and had fixed it. Thirty seconds later the engine died and I was back where I started. Drat!!


Could it be something inside the distributor? I’ve heard lots of complaints about the lousy quality of the current crop of condensers so that seemed a likely candidate but how to test it to find out. By now it’s the day after Thanksgiving and the Bee hive in Clemson is closed so I’m not going to just drive over and get a new condenser.
But wait. I recently ordered two complete sets of points and condensers for the MG-TD so I do have two spare condensers. I have no idea if the condensers for the TD are the same as those for the MGB but they’re the ones I have so I give it a try.
I’ve always wondered why a part that’s prone to failure such as a condenser is located inside the distributor where it’s almost impossible to get to. I looked at some wiring diagrams and decided that electrically speaking “inside the distributor” and “hanging off the ‘-‘side of the coil” are the same thing. The wire from the points to the condenser is a bit longer but electrons go really fast and I hoped they wouldn’t notice the little extra time taken to get from the points to the condenser.
I popped off the distributor cap (for about the tenth time), disconnected and removed the condenser, screwed everything back together and put the cap back on. I unscrewed the nut on the “-“ side of the coil, attached the condenser flag there, put the nut back on and grounded the condenser to the frame (verifying with an ohmmeter that there was a good path to earth). Then I crossed my fingers and turned the key …
VAROOM!!! And not only that, the engine kept running and purred like a kitten. Forty to fifty miles later and it’s still running like a champ. I don’t know how long the TD condenser will last but hopefully at least until I can buy several MGB condensers to put in the ditty bag in the boot.

So, did the dying battery cause the condenser to fail? Did the almost broken wire create a high resistance point that prevented the coil from properly functioning? Did the failing condenser cause the battery to discharge and fail? Did none of these failures cause the other failures and it was all just an unfortunate set of coincidences? ONLY THE SHADOW KNOWS!

J. K. Chapin

Jud,
No reason why the TD condenser should fail if Td's are 12v.
On your question of cause and effect or coincidence i've been messing with cars long enough never to commit to either view unless glaringly obvious!!!
Allan Reeling

As far as a condenser being 'prone' to failure (modern examples from some sources excepted) goes I personally have never had one fail in almost 50 years during which time I have rarely been without a car with points and condenser ignition.

Yes, temporarily connecting a spare condenser between the points terminal of the coil and earth is indeed a quick way to deal with a failure rather than fiddling about inside the distributor at the roadside. Unless it has gone short-circuit of course, in which case snip the wire to the condenser.

Condensers are built (or should be built!) to withstand at least a couple of hundred volts as that is the voltage you will see on an oscilloscope when the points open - generated from the coil in much the same way as the HT spark is by the collapsing magnetic field, and is why you get a spark across the opening points. The upshot being that a TD condenser in an MGB or vice-versa should not cause any problems. Any difference in capacitance shouldn't have a significant effect either, just for kicks I've viewed the HT and LT waveforms on a 'scope with two condensers in parallel and in series with only fractional changes in the waveform, and no change to HT spark amplitude.

I've also had a coil wire hanging on by a single strand with no ill effects, even if that strand does heat up it isn't going to increase the resistance and reduce the current enough to affect the spark - until it parts company altogether that is.

Likewise a flat battery won't cause a condenser to fail, otherwise it would have happened to almost everyone at some point or another.

Neither can a failing condenser cause the battery to go flat - unless you keep cranking a non-starter until it does so, that is.

Which leaves coincidence.
Paul Hunt

Paul, thanks for the good info. Yes, I think the wire hanging on by a thread was a red herring and I think you're right that it was all an interesting set of coincidences. Anyway, it was an interesting experience and All's well that ends well. Jud
J. K. Chapin

This thread was discussed between 23/11/2015 and 28/11/2015

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