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MG MGB Technical - Hot misfire then dies

Having a problem with my 72 MGB which I think is HT related. Either coil or rotor arm.
Last night was a warm night in the UK so we decided to have a run through the Blackpool Illuminations. This was basically 8 miles of stop start traffic. The temp gauge was just over half way on the clock. The car gradually got harder & harder to get moving from a standing start. Applying the throttle at low revs would cause a big misfire, almost like 2 cylinders were cutting out. However, if I gently coaxed it to over about 2k revs it would run clean and pull the car no problem. At the end of the lights we stopped for fish & chips. The car would not re start (spinning over great just no sign of it wanting to fire), until it cooled for about half an hour The misfire continued while we were in the heavy traffic but a few minutes after we got on the open road all was normal again & the 1950cc fast road I had built by oselli about 20years ago was pulling like a train as usual. I think something electrical is breakdown when hot. The Tach did not falter.

Think Ill try a new rotor arm. The one in there is quite old. After that a new coil. Unless anyone has other ideas?
G Britnell

This one will be interesting when the people that know (not me) respond. Just a guess but float sticking, or vacuum leak, or?
G Nicholas

I have had a coil behave that way, not common but has been known. Condensor can also cause a temperature-related misfire, but from what I understand of your symptoms it doesn't quite fit.
Paul Walbran

How about fuel vapourisation?
Mike Howlett

I don't have a condenser, fitted Magnetronic electronic ignition years ago. I doubt its fuel vaporisation as nothing has been changed in the fuel system & I've driven the car all over Europe in temps far higher in the past.
G Britnell

Sounds like HT breaking down to me. As you open the throttle the cylinder pressures rise, making it harder for the spark to jump the plug gap, which means it may well choose to take an easier fault path. Had that on a couple of cars. If changing the rotor then change the cap as well (keeping the old ones ...), but be prepared for coil. Check HT lead routing, and for damage.

Never understood vaporisation in SU carbs. Any vapour in the lines will be pushed out of the carb overflow as soon as the float drops to let anything in, which means the float chambers will be pretty full anyway. One exception is the HS exposed jet pipe, which could possibly cause problems if the heat shield is defective or there are other issues causing excess heat.

I had exactly this problem although with the added complication of poor starting.
Long story short, it was a so called Lucas rotor arm that eventually became a dead short after weeks of intermittent missfires. I replaced it with a Distributor Doctor component and everything has been on song since. I understand that Lucas will allow components to be sold in Lucas branded boxes if the manufacturer pays 7.5% to use their branding irrespective of source.
I stand to be corrected.

Richard Thompson

'Lucas' has been just a brand name for many years now, owned by a series of conglomerates that have little or no care for the classic car market, starting with TRW and currently with Elta Automotive. Ironically this seems to be a UK company based just a few miles down the road from me.

I remember saying when they got their 'new' logo many years ago of a green L in a white diagonal stripe it actually depicted an arrow going downhill and backwards.

DD started selling red rotors a few years ago, but it didn't take long for the unscrupulous to jump on that bandwagon and sell the same poor design as before but in red. The problem with the black ones was said to be too much carbon in the insulator, but I put it down to riveted contacts where the rivet was inside the base circle of the rotor body and therefore too close to the distributor shaft. Original rotors had no rivet, and although later ones did have them they were spaced further out from the shaft.

I had exactly the same symptoms, especially when the engine compartment was hot, with my 1970 B. I pulled the spark plugs and noticed that 3 and 4 plugs were slightly sooted. I pulled the float out of the rear carb chamber and found that it was completely full of fuel, no signs of cracks or holes, just full to the top with fuel.

Obviously the rear carb was running totally rich. A new float has mostly cured the problem but over 30 C in the desert heat, the car has a poor idle, will occasionally die and be difficult to restart. My car has A/C adapted and this puts lots of extra heat under the bonnet. Replacing the float though, was definitely a step in the right direction.
Glenn Mallory

This thread was discussed between 07/09/2016 and 16/09/2016

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