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MG Midget and Sprite Technical - Random Cutting Out

Hello everyone,

need a bit of help, basically ive got a 1973 1275 midget, its been tweaked a bit running 40 dcoe, race head, electronic fan and lumenition optronic ignition.

basically the car was running fine until a couple of weeks back it started to just cut out, it would be going along then the revs would go as if there was no spark, the engine still sounds like its running but on compression not ignition if you know what i mean the throttle does nothing and i just coast along till i can find a safe place to dump it. for a few times if i turn everything off then give it 5 mins it will fire up again but it did completely stop that too so i changed the coil andit got going again, the coil gets really hot when all this happens. I then changed dizzy cap, leads and rotor armand that cured it for 2 weeks but now its happening again. this only happens when the car is warm its currently running around 65-70 deg c. ive read on other threads about fuel vaporisation but why would the coil get much hotter than normal, is it possibly the lumenition box or optical lens failing. there is no heat shield around the fuel lines or inlet manifold but it has been running like it for 5 years with no issues and its used as a daily car in snow, rain, wind and summer so why would it suddenly start now.

any help would be greatly appreciated
Thanks very much
Dan
D Leacock

First test is it fuel or ignition?

When the engine cuts out does the rev counter go to zero or does it keep monitoring the engine speed until the engine stops turning (ie when you put in the clutch?)
Bob Turbo Midget England

When it does cut out the rev counter drops to zero but the ignition light does not come on but when i press the clutch or take the car out of gear then it cuts completely and then the ignition light comes on
D Leacock

OK so it's electrics; check the wiring to the coil, also ensure the temp gauge capillary isn't routed near the coil, it can flop around and cause a short. Next check ignition switch.
David Smith


Hello Dan,

I think I know exactly what your problem is - I had precisely the same thing a few months ago.

take a look at this thread in the archives which described what I went through:

http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=97&subjectar=97&thread=2010110719540430737

Long and short of it is that the optical sensor failed. Your symptoms sound very, very similar to the ones I describe in the thread.

To update you on what happened since - it ran fine on points for about a month and I then switched to the simonBBC magnetic system (because it's cheaper than replacing the optical sensor) and the car is running great - not a hint of a problem since.

Hope that helps?
Mark
MarkH1

Sorry, that link appears not to work...search the technical archive under 'Optronic problems Mark' and check the 'all of these words' option, and it will be the one thread it finds...

here's the key bit from one of my posts:

I've just found Lumenition's helpful ignition testing bulletin:

http://www.newtronic.co.uk/new/support/lumign/optron/TB17OptronicIgnitionCheck.pdf

Here's the bit that applies to me:

"There is a possibility that the optical switch is failing. If it is there will be a very definite pattern to the
intermittency which will be as follows:
· The ignition system works perfectly when the engine is cold but misfires or stalls as the
optical switch is heated by the rising engine temperature. If the engine is stopped and left to
cool for a few minutes (also cooling the optical switch) the engine will start and run perfectly
but fail again a few minutes later when the distributor re-heats the optical switch – and so on."

Which absolutely accurately describes what I'm experiencing.
MarkH1

Dan,

or possibly the dissy cap

a visit to a cryrton machine will probably find the cause of the problem
N Atkins

Nigel - dissy cap has already been ruled out...changed right at the start. I'd put money on it being the optronic.
MarkH1

sorry Mark, Dan I must have jumped a line or two when reading

Mark you could well be right

although never assume new parts are working correctly without fully checking them especially rotors

wires and connections including all earths may need checking for being clean, secure and protected - and the wingle test carring out on them

I assume coil lead was included in the change

twice the vauccum created by pushing on the coil end of the HT and not burping the rubber cap sleeving thing has caused my car not to start and someone else on here had a coil lead that would only seperate and cause fault when hot then would be OK again when cooled
N Atkins

Hey everyone thanks for the help, I checked all the cables and fuel getting to the carb, cleaned up the optical switch and checked all the cables but didnt find anythign took the car for a test and got 300m before it stopped took ages to get going again and did another 100m got towed back and have changed the optical switch for a spare one I was going to use on my track car when its ready but have been for another drive and have done a few miles this time with no issues so hopefully it was that causing the problems
D Leacock

well done Dan

cigar to Mark
N Atkins

This thread was discussed between 11/07/2011 and 12/07/2011

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.