Mazda Eunos Roadster
Steep nose-down parking results in a petrol lock for John’s MGB engine
1973 MGB GT
My MGB GT spent three days on a very steep drive, parked nose down so it did not leak out of the gearbox tailshaft seal.
I thought the battery was flat when the starter wouldn’t turn the engine over.
Then I noticed the petrol smell and the gauge, which should have read near full, was showing less than a quarter. A quick look and a dipstick sniff proved my hunch that the engine wouldn’t start because it was full of petrol.
Standing back, I could see the petrol tank was a good 12 inches above the carburettor floats and gravity had taken its course. I’ve consulted MG Owners’ Club guru, Roger Parker, and wondered if my car had a fault. His opinion is probably yes, but not a serious one: ‘Gravity will give the effect of the fuel pump running so the fuel system is operating as though the engine is turned on. Fuel to the float chambers should shut off once the fuel level reaches the ‘full’ point and the float pressure on the float valves should cut fuel flow. ‘However, wear between a float valve and its seat may not result in significant leakage while the engine is running, because the engine’s fuel demand will have masked any negative effect. Gravity-induced fuel pressure will result in a tiny leakage over a long period with fuel continually entering the float chamber with the car parked nose down. This raises the fuel height and causes fuel to trickle out of the main jets into the inlet tract of one or both carburettors. That will have, over time, trickled past both the closed (although they are never fully closed) throttle and onto the inlet valves where only one inlet valve will be closed fully, if that one is not worn. That results in a constant dribble of fuel into one or more cylinders and subsequently into the sump.’
With 50 quid or so gone, I wasn’t best pleased but I knew that I mustn’t start the car until it had proper oil in it. I managed to get the sump open and drain a fair bit of the now thin oil and replace with cheap 20/50, so I could start the engine.
I then freewheeled to a flatish bit of road, started up and drove at low revs to an area where I could work. I did an oil and filter change using Millers 20/50 Classic Sport Semi-Synth I always run the engine on, and filled the oil filter first to avoid any damage on start up as the petrol had washed out the engine.
All seems well, but I’ve learnt a valuable lesson; don’t park old cars on steep slopes facing downwards with a full petrol tank.
Filling oil filter before fitting is a good idea and helps avoid damage on start up.
Oil change forced by petrol flooding engine while MG was parked.