Poor Running (intermittent)

[MGB72]

Morning all, new boy here, so apologies if this has been covered (I have searched!)

An intermittent problem with my 72B roadster and only really on a short run. Power will plummet and she sounds like a tractor. Hasn't happened since the summer, but, inevitably, Xmas eve, I had to use the MG for the turkey run (about a mile). Started to act as above and I was just able to get to the village and park. Started up fine after the shopping and a sweet as can be on the way home.

In the summer I looked into this, assumed it was fuel starvation from a sticking float valve, so replaces, and also replaced the points, condenser, new plugs, balanced airflow, plugs nice biscuit brown. No problem through the summer.

The tacho was static during the rough running. I have recorded it, but the file size is too large and the Mac too peculiar to attach it!

Iain

[Ian F]

Hi Iain,

welcome to the forum. Intermittent problems are notoriously difficult to diagnose, but the tach not working during the poor running is the key here I am sure. Autoelelectrics are not my strong point, but there are folk on the forum who enjoy puzzling over electrons!
I would suspect a distributor issue - perhaps a rotor arm insulation breaking down or similar, and causing a misfire.
Poor quality distributors and distributor components have given me and others here major problems in the recent past.

Ian F

[MGB72]

By static, I mean it was at about 1500 rpm, car doing 20, probably in 3rd gear, but I can't recall for certain. Rotorarm was also replaced

[Vic Butler]

Have you replaced the distributor cap and plug leads and coil?
Check the carbon brush in the cap .These breaking down can cause the symptoms you describe. The best source is the Distributor Doctor.
It doesn't appear to be the LT circuit otherwise the tacho would be erratic.

[Paul Scott]

Welcome Iain,

Poor Condenser or maybe the little earthwire fitted to the baseplate of the Dizzy (broken or bad insulation on the wire).

I had similar issue, always got home but popped electronic points module in and issue gone. Unsure if it was a worn dizzy shaft, but my £35 powerspark module sorted it

Regards
Paul

[MGB72]

HT leads not replaced, but tested and all good, along with coil.
All points distributor etc replaced from the good doctor.
The cap wasn't replaced, but coul dbe the next on the list.

But if it is an issue with a faulty component providing a spark, why is it not more consistent?

[Ian F]

Hi Iain,

Your question- "But if it is an issue with a faulty component providing a spark, why is it not more consistent?"

My understanding (and experience) is that when some components get warm, they can fail - intermittently and probably related to insulation. My experience here was with a poor quality rotor arm which let me down when warm!

Ian F

[Vic Butler]

Iain, I forgot to ask if you replaced the rotor arm. As Ian F said, the cheap Chinese ones short out across the rivet. The red ones from the Distributor Doctor are moulded any can't short .

[MGB72]

Yes, shiny new red distrubutor arm installed.

It's reminding me though, the contacts on the cap looked a bit black, so these were cleaned. I'll check it.

Iain

[Vic Butler]

Best get a new cap. Cleaning the contacts can increase the air gap between the contact and rotor arm.