MGB Register Forum

I'm hoping someone can help shed light (in an non-technical way for a novice please) on an issue I had at the weekend.

Setting off on Sunday for the North Wales Walled Town Run in pouring rain, by roadster started first time as usual, but within half a mile I noticed the rev counter bouncing up and down.

It was running ok, so I continued, only for it to start backfiring badly once warmed up and then stall and not re-start - a premature end to my Run.

I towed it home, pushed it in to the garage and it's been sat there since, I've not had a chance to go back to it.

Any ideas?

Good Evening,

What year is your car?

Points or Electronic inside the dizzy?

Regards
Paul

Hi Paul,

It's a December 1970 build, K-reg - not registered until August 71 for some reason.

Electronic ignition was fitted by a previous owner in 2011.

Did you notice any other stuff stop working?
Indicators wipers?

Do you know which electronic module is fitted to the car?

I had a similar issue which turned out to be the condenser which would stall the engine and nearly always start instantly showing no issue again for several miles.

I have a tell til light fitted across my coil to assist in finding an issue. If the light is on a spark should occur. Which meant if the light went off it was a low tension issue. The tacho gets a pulse from the lt side in my case.

It could be a faulty electronic module.

Regards
Paul

No everything else seemed to be working fine.

It seemed to be blowing out a bit more than usual when first started, though it was a cold morning, so perhaps more noticeable.

When trying to restart it, the engine seemed to be turning over ok, so I'm hoping it is an electrical issue.

I tried starting it a couple of times within a couple of minutes of the breakdown, to no avail. I've not tried it since.

The backfire seemed to be coming from the exhaust rather than further forward.

Connect a 12v test light accros the coil. If it flashes its ht side if not its LT.

If it does flash whip a plug out or connect a spare plug and touch to the engine. If you have spark then it is fuel issue.

Good luck

Regards
Pau

Thanks Paul,

Sounds like it's not as serious as I'd initially anticipated.

I'm no expert, but thats how I found my issue eventually!!

Points system I think you can fiddle and carry on. Electronic no major warning just stops - so they say :0)

I whacked in a cheap module and all my probs vanished. I know what you mean by bacfire I followed my wife driving our B. As she accelerated off every roundabout. Pop & flaming tailpipe!!

I knew I should not allow her to drive it ;0)

Paul

As the rev counter needle bounces then the fault is definitely in the LT circuit. My BGT recently had a misfire but the rev counter was steady. It was in the HT circuit, dodgy plug.
What make of electronic ignition is fitted?
It's worth reading the thread Vacuum Advance Backfire as your problem sounds similar.

Erratic/unusual behaviour of the Tach does suggest the LT circuit as Vic previously stated. The exhaust backfire is commonly unburned fuel - unburned due to ignition fault causing weak or no spark in many cases.

As the car will not start, it may be easier to locate than an intermittent fault so time to pull the plugs, look for a spark for a kick-off.

But the first question should be - what was the last change you made to the car? checked anything before it happened?

G