OK, it's taken me a while to get through that tank of fuel, but, when it got down to approx 1/4, it stopped again. So the return line was not the problem...
I was determined to fix it today, and did. First test was to see just what the pump was pumping - some petrol, but also a lot of air, just as before.
I then took off the petrol tank float mechanism (the old variety, as you can see on
www.realnoteguitars.com.au/merc) and peered inside, expecting to see a horrible mess. However, the tank was pristine inside, with no corrosion at all! Even the "flower pot" and the fuel lines were perfectly clean and looked like brand new.
OK, I thought, it has to be the fuel line from the tank to the petrol pump, which might have developed cracks and once the 'gravity feed' of the pump ended at approx 1/4 tank and there would be a negative pressure (suction) on that hose it might be sucking in air at that point.
So I confidently removed the flexible line, put my finger in one end, sucked on the other and blocked it with my tongue, and...it held the vacuum perfectly. So it wasn't the fuel line.
OK, time to drain the tank and delve deeper. I made up a special tool to remove the drain plug by grinding down an old 18mm 1/2" drive socket on my bench grinder.
see pic of tool hereWhile I was making the tool (nowhere near the draining tank, I might add!) I noted that the fuel was barely trickling out, which gave a clue as to the problem.
Sure enough, on removing the drain plug and filter the problem was obvious - an almost blocked filter. Not with particles of rust or anything visible at all, but on holding the filter up to the light I could see nothing at all through it. It was only when washing the filter out in the last of the drained petrol using a fine brush that I noticed that the filter was grey, not black - a very fine substance had deposited itself over the filter and almost totally blocked it.
I can only put it down to deposits and gum from years of poorer fuels, as I swear by only using the locally available 98 octane unleaded fuel for both the SL and my '83 230TE wagon. These fuels have enhanced cleaning properties (BP Ultimate, Mobil 8000 and Shell Optimax) and my bet is that my introducing this fuel to the car after 40 years of poorer fuels did much the same as introducing a high-detergent oil to a high-mileage, high sludge engine - the deposits throughout the fuel system were gradually removed, and they ended up in the filter.
My theory is that while the tank was above 1/4 full there was enough 'head' to get the fuel through the filter, albeit at a reduced flow rate. I am sure the air being introduced was doing so at the clamped end of the flexible hose between the tank and the petrol pump, one end of which was quite loose when I removed it. Not loose enough to cause a leakage of fuel, but just enough to allow an ingress of air when the suction effort of the petrol pump became too great (combination of blocked tank filter and low petrol level).
The truth of the matter was shown when I put only a few litres back into the tank after re-fitting the cleaned filter and the fuel line. I left the float mechanism off so I could watch what was happening, and started the car. It started perfectly, and ran perfectly. Shining the torch into the tank showed the "flower pot" doing its job perfectly, with a healthy level being maintained in it by the fuel entering from the return line - quite a clever design when you see it working in real life.
To finish the job it was just a matter of fitting the float mechanism and driving stright to my local petrol station for a new tankful of 98 octane!
The problem has finally been solved!
Neil Rote, Melbourne Australia
Unrestored '64
