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MG MGB Technical - Fuel Starvation

Sorry to put this in both sections, but I initially put it in MGB General, by mistake.

Need some help with this one, as i am going nuts trying to solve it. Twin Su equipped, 1971 completely de -smogged. Carbs rebuilt in last couple of years. i have been dogged with a stalling problem which only happens after the engine has been running at low or medium revs for some time. Engine slows down, if left to idle, after say thirty minutes, and dies, and will not start immediately. An hour later, no problem. If I am running the car, it will stall at a stop sign or traffic situation and not restart. I have replaced the fuel pump for a new SU unit designated for the year model, mounted correctly,( forward of RR wheel) I believe, with the inlet /outlet at the 3 o'clock position. Finally, I replaced the fuel filter with a see through unit. All supply hoses are clear and in good order, with new Jubilee clamps. Crank case breathes into the carbs via a "Y" fitting, and overflows branch into a pipe which is set up to dump flooding fuel onto the ground. No evidence of flooding.
Turn on ignition and pump clicks away until primed. When i first check the see through, it is 3/4 or more full, and engine starts first turn, and warms up normally. After starting, the fuel level slowly goes down in the filter to almost empty, and only a tiny dribble is present, but not continuously, and never fills again. Engine idles on a little roughly, and will eventually die. If shut off before this happens, even though the see through is almost empty, the car restarts easily, but the fuel filter does not refill. If the engine is revved hard, fuel can be seen bubbling into the see through, but it never gets very much fuel in it.Less than 1/4 full. If the engine is shut off after revving, and left to idle a minute or two, and ignition turned back on, after one click, the pump shuts off, despite little fuel in the filter. Obviously, there is pressure in the system, because if a supply hose is pulled off the barb with the ignition on, a soup can fills quite rapidly. with pump clicking away happily.

I have plastic floats, which i believe cannot be adjusted, float valves appear to shut off when I blow through the supply tube and block off the exits, and as i raise the floats against the float valves. There are no leaks in the floats- and if there were, surely there would be a surfeit of fuel in the system.
tapping the pump and fuel bowls does nothing to help!
Help! That is the action word! Had to be towed home last week, not good for my almost non -existant ego. Miata becoming preferred mode of transport!
Ken R
Ken Rich

Is the car a US spec model? As a guess it sounds as if the gas tank is not venting. As a US model it would have originally had the evaporative controls and non-vented gas tank. Try taking the gas cap off and see if it helps. The fuel pump should have a vent tube as well, going up into the trunk. Be sure that this is clear as well.

HTH

Ron

Ronald Smith

Ken. See my comments on the General board. They are in line with Ron's. Les
Les Bengtson

Thanks for your prompt reply, Les/Ronald. I changed to a vented filler cap when I had the fuel tank re-done a year ago, and I removed the charcoal can and its piping, and all the evaporative equipment, and blanked off the nipple. but I think you may be closer than I, in assuming that the problem could well be at the tank end rather than at the carb end. It is "textbook" in that the fuel filter fills when the supply to the carbs is removed, suggesting an impediment at the tank end,( you got me thinking on this, and thanks! Talk about limiting paradigms) probably on the suction side. I will replace the soup can with a line into a gallon can and see if the supply weakens after the initial pressure release. and will get back to this thread in the Tech section rather than the General.
Ken R.
Ken Rich

Ken - The filler neck for the later cars with the vented tanks is different from the earlier filler necks, so the vented cap you put on may not be doing it job. Pull the cap off and run the car (I had to run about 200 miles on the freeway one time due to a bad vented cap). If the problem goes away, your problem is solved.
Good luck - Dave
David DuBois

Thanks for the help guys. Dave, the filler neck was changed before i got the car, and matches the vented cap. No difference between cap and no cap running.

Update: took off the float chamber lids once more and removed and cleaned needles and seats and re-installed. Waiting for new needles, seats and gaskets to come next week. yesterday i started up the car, filter almost full, and it idled for about two hours, getting quite hot in the process. Filter fuel level slowly went down to almost nothing, and pump supplied a dribble of fuel, unless i cracked the throttles for a short time, when fuel could be seen entering the filter en route to the carbs.( how full should these filters be? i assume, nearly full, but then i have always had metal filters, so i don't know!) Engine never faultered, and i noticed when i was under the car that a vapour emission was constant from the overflow pipe, seen against the heat wave from the exhaust.I think that had the engine been cooler, there could well have been fuel dribbling out, which leads me to wonder if i have a sticking/unsticking float or and needle.( but then why is'n fuel belting into the filter on its way to the fuel bowl and out onto the ground?) The front carb was quite warm, both "dashpot" and fuel bowl, and understandably, as the bonnet was open all this two hours time, and there is no fan shroud fitted. Additionally, some years ago, i removed the asbestos from the heat shroud, and left it bare.
Next I will replace the floats, though they are plastic and have no adjustment.
Last point. On shutting down and turning on ignition again, the pump smartly primed the fuel system, filling the fuel filter, and engine started immediately.
Ken R
Ken Rich

This thread was discussed between 30/05/2003 and 31/05/2003

MG MGB Technical index

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