What you've described above are the classic symptoms of a leaking fuel pump. You will probably also notice that you are getting worse fuel economy and that the oil in the engine is getting very thin. Your engine bay may also have a strong smell of petrol.
The fuel pump basically consists of a rubber diaphragm that is pulled down by a lever that runs off the cam shaft. A spring pushes the diaphragm upwards and thereby pumps fuel at a constant pressure. Now this diaphragm goes up and down about 2400 times for every kilometre travelled in top gear, so it's no wonder it wears out eventually.
When the diaphragm wears out it will developed a tiny split. As petrol is a highly viscous liquid and it is under pressure in the pump, it only requires the smallest of holes to leak. Most fuel pumps have a drain hole in their bottom half so that leaking fuel will drain out the back of the pump and onto the road. However, with the amount of gunk that covers the back of an average Mini engine, this hole is usually blocked. The only other place for the leaking fuel to go now is down the hole where the lever enters the engine and into the sump.
Petrol in the sump spells trouble as it dilutes the oil and we're not talking small amounts of petrol here. I car I saw recently had pumped about four litres of fuel into its sump in a couple of days. Now four litres of petrol mixed into the four litres of oil in the sump makes a pretty watery mixture and provides very reduced engine and gearbox protection.
Fuel pumps fall into two categories - those that can be dismantled and those that can't. The pumps that can be dismantled have a dome shaped top and the inlet and outlet pipes are on the side. If your pump has two cylinders poking out the top with the pipes attached then it can't be dismantled.
The pumps that can't be dismantled are easy to repair - you throw them away and buy a new one. For the other pumps a service kit is available which includes a new diaphragm and valves. You probably won't want to replace the valves unless they aren't sealing. To check this, stick your finger over the inlet and pump the lever a couple of times. The pump should hold a decent vacuum for about 15 seconds. Now repeat the test for the outlet. The pump should be able to hold pressure for about 15 seconds as well.
Replacing the fuel pump is one of those tasks that is easier if your remove the engine from the car first but as this isn't always practical, we'll persevere the difficult way. First disconnect the battery - leaking fuel and electrical sparks don't mix! Drain the fuel tank into a suitable container, the fuel pump is about level with the bottom of the tank and its difficult enough without petrol spraying everywhere. Now remove the carburettor to give you better access to the pump. Remove the hoses and undo the two nuts holding the pump on. You may have to lever a screwdriver in behind the pump to break the gasket seal but other than that it should come away easily.
If your replacing the pump, check the new pump carefully. Some pumps that are imported from England have the inlet and outlet pipes reversed to the originals. These pumps require a bit of plumbing work with the fuel lines to fit but nothing difficult - just make sure the fuel lines are a safe distance from the exhaust. Another problem with new pumps concerns those that have a metal ring at the top held down by two screws. This ring clamps the valve housings (the two cylinder things) to the body of the pump. The spacers between the ring and the pump are a fraction too big and don't clamp the valve housings tight enough. If the valve housings can be moved around then they aren't tight enough and will leak like a sieve. The solution to this problem is to file down the spacers enough so that the valve housings can be clamped tightly in position.
Refitting the pump is the reverse of the removal steps. Remember to add some gasket sealant on the new gaskets. The camshaft throws a lot of oil into the base of the pump and this tends to be a major leakage spot. Also be careful that the pump goes on straight. The cam that the lever rides on is narrow and the lever can easily slip off it and get caught down the side. Tightening the pump up when the lever has slipped off the cam will bend the lever and cause the pump not to work.
Drain the contaminated oil and replace with new oil. It is also worthwhile to drain the oil filter as well. It will take a while to get oil pressure back into the engine, so disconnect the ignition from the coil (the wire not connected to the distributor) and crank the engine over for a couple of minutes. Once you've got everything back together check the pump for leaks both before and after you start the engine.
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