Practically Classics Forum

My 1968 MGB GT has been acting up, starts well, runs for 5- 10 minutes well, then mis-fires and rev counter starts dancing and ammeter goes to discharge, checked alternator and earths sounds like short somewhere but where? Any suggestions welcome. Thanks................ Ron

jagiron wrote: ↑

Fri Mar 17, 2017 4:09 pm

My 1968 MGB GT has been acting up, starts well, runs for 5- 10 minutes well, then mis-fires and rev counter starts dancing and ammeter goes to discharge, checked alternator and earths sounds like short somewhere but where? Any suggestions welcome. Thanks................ Ron

I had the same problem on my p6 rover V8 - turned out to be a hairline crack in the distributor cap and has the car got warm the crack expanded with the resulting arcing within the cap leading to a misfire - I think from the symptoms that you have described that the problem is ignition related - reason for the misreading's on the rev counter / ammeter is because it is misfiring in the first place

Actually, given the dancing rev counter, I'd suspect the low tension side rather than the high tension - the rev counter takes pulses from the LT side, so if the fault was on the HT side the rev counter would still work fine I'd have thought? I'd be suspecting dodgy points and/or condenser - if they are new then this definitely doesn't mean that they are good, there are so many shoddy quality ignition parts out there. I know JPB will disagree with me here, but I'd be getting all new quality parts from someone like Distributor Doctor, all my classics are kitted out with his distributor parts and they all appear to be well made and reliable - works for me, anyhow.

Check other items like the ground strap between the rotating points mounting plate and the dizzy body, and the quality of the LT wiring to the dizzy and coil. This latter is important as you say you also have the ammeter going to discharge, so there may be a 12V feed issue, as it's intermittent then all you can do is to thoroughly check all the wiring - especially check the fuse box here too, they're not the greatest design in the world.

It could be a faulty coil on the LT side too, hard to prove though unless you've got a solid hard fault, apart from just swapping it for a known good one.

Lots to do, but all pretty straightforward.

Definitely the low tension side. I think the clue is in the ammeter reading, either the low tension feed to the coil is being shorted or the low tension winding of the coil is breaking down when it gets warm. The only to test the coil, as Luxo says, is by substitution. If the coil is a ballasted type I would check the wiring round there as well. Do you have anything extra added to the LT side, such as an immobiliser?

suffolkpete wrote: ↑

Mon Mar 20, 2017 10:21 am

If the coil is a ballasted type I would check the wiring round there as well.

That's a good shout, I think they are indeed ballasted on MGBs. - well worth a check as you say.

Luxobarge wrote: ↑

Mon Mar 20, 2017 11:08 am

suffolkpete wrote: ↑

Mon Mar 20, 2017 10:21 am

If the coil is a ballasted type I would check the wiring round there as well.

That's a good shout, I think they are indeed ballasted on MGBs. - well worth a check as you say.

Along similar lines, if the car is an earlier one, built before ballasted coils were fitted, then someone replaced the coil without specifying the year, a ballasted coil that continues to be fed with the full potential once the engine is running - instead of the correct 6-7 Volts from the dropper - will fail in the way Pete described in his post, so check a) whether the car has a ballast resistor, which is usually a ceramic dropper either round or cubiform that carries a winding and if it doesn't then b) make sure that the coil is not a 6 Volt one but a 12V one.
The symptoms certainly do match what will happen, sooner or later, when a coil designed to run on 12V only when starting the engine is supplied at 12-14V permanently.
Either way; be careful if you do buy a replacement for the coil, there are many counterfeit ones around these days and worse than this, Intermotor ones are still being sold.

Good advice about the fake coils JB, I bought 2 fake ones off Eblag, innate end, I went to my local Aircooled VW garage who are also a Bosch stockist and paid a bit more for a genuine one.

Kev

73 on the ballast resistor is actually part of a wire in the loom, so the OP may not even be aware that it's even there.

P45.

Thanks guys for all the info. lot to think about.I have changed the coil(non ballasted type)made no difference.Will check all ignition comp. and see what happens.
thanks again.

Hi Guys, tried all suggestions + and still a problem, not sure where to go next
thanks for all suggestions.