Hot starting problems after winter storage
The MGA normally starts and runs very well after it's winter hibernation, but this year there were running problems. It's not a normal MGA, and had a 1950cc stage 2 engine, lumenition optical ignition, and a ballasted coil.
Slow running and hot starting
The car took a little time to start from cold, but seemed to run OK as I took a test run down to the petrol station. It would run OK apart at speed, but misfired a lot in town. Stopped at the petrol station for some fuel, found the car already has 30 liters in, but topped it up anyway.
The car refused to restart at the petrol station even after it had cooled down enough for the temperature gauge to fall to 1/8. Eventually I decided to try bump starting it which thankfully worked.
The first obvious thing to rule out was the petrol. The petrol bought in the summer (last time the MG was filled up) is much less volatile than fuel sold in the winter. Volatile fuel needed to run a car in the winter would cause vaporisation problems in the summer, and the non-volatile stuff from the summer causes running problems in the winter. So garages sell different fuel depending on the time of year.
Added to the likelihood of non-volatile fuel, the high ends of the fuel will have vaporised over the last 6 months, so the petrol will be a little like diesel.
High compression engines, especially older ones with weak ignition systems tend to be sensitive to fuel quality, so I syphoned all of the fuel into my Renault 4 (that's got a very low compression engine and will run on anything).
First problem cured - with fresh winter fuel the car ran very well at low speed. But would still not restart when hot.
Very often when a problem is difficult to diagnose it is being caused by more than one thing. Chances of two things going wrong at once can seem very slight, but I've found it to be very common.
The traditional method of removing a spark plug from the engine, earthing the end and connecting an ignition lead is not a good test. It is much easier for a spark plug to spark in the air than in the engine. The air gets less conductive when the air is compressed (as in a cylinder) and even less conductive when you add a little fuel into the mix. That's why cars with weak sparks are prone to flooding. Once the air is heated (hot starting) it becomes even less conductive.
A better technique for diagnosing hot starting problems is to turn the engine over with a ignition timing strobe attached to check for a spark. It's a real test of whether the spark plug is sparking under real conditions.
Mine wasn't sparking when hot. It would spark if I removed the plug from the engine. Diagnosis weak spark. They are the worst to diagnose.
There are many possible causes for a weak spark. Poor connections in the system, poor coil, worn points, low battery voltage, excessive starter current drain, too much fuel in the engine, engine too hot, etc. One very common one for cars with points is a bad earth in the distributor base plate.
My MGA has a ballasted coil. The coil operates on 6V as standard, the voltage being reduced with a ballast resistor. The reason for this is the starter motor takes so much power on starting that the voltage supply drops causing a weak spark. That's the last thing you would want on starting. So with a ballasted system the starter solenoid provides a temporary 12V supply to the coil while the starter is turning and gives the spark a boost.
On the MGA the starter solenoid turned out to be faulty - it wasn't supplying the boost to the coil, so the spark was weak enough to be overcome by all the adverse factors I mentioned.
My temporary fix was to wire a 12V boost through a switch on the dashboard. Longer term I'll need a new starter solenoid.
This didn't completely fix the problem. The car was suffering from ignition problems when hot which resulted in a recovery truck ending a second trip to the Alps. The problem was traced to a rotor arm breaking down when hot, but the car still wasn't right.
Took a full year for a proper fix. Replacement Lucas rotor arms are a disaster. See a thread about rotor arm problems. The last poster Jeff sells alternative rotor arms from Advanced Distributors in the US (also available from Distributor Doctor in the UK). Both pointed out that the side exit caps with screws that pierce the leads are not suitable for the carbon leads sold by many spares places - they put a lot of strain on the rotor arm. Instead use the copper cored leads available as "original style leads" from Moss, or cheaper from the guys above.
The car is fabulous when hot now - beautiful smooth idle. Finally I'll be able to try a trip longer than 5 miles.