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.

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