Fuel problems in a Midge.
(Apart from the actual cost of petrol)
I’ve been having a conversation with Peter, his Midge won’t start without a lot of cranking if it has been standing for a while. I suspect he and I are not alone in this.
I’ll ignore things like damp spark plugs and stale fuel for now, and concentrate on specific faults that you might find in a Midge.
Properly set up, the Triumph 13/60 twin SU and Stromberg carbs produce few problems unless seriously worn, the 12/50 downdraft has a charming tendency to leak, mainly age related incontinence and over complication. Ford Cross-flows usually have a fairly reliable twin choke Webber, and a return pipe which can complicate matters somewhat.
There are a number of weak points in the fuel system of both engine types, but in the end the faults are the same.
•Fuel run back
•Weak fuel pump
•Older pieces of plastic being dissolved in modern petrol which has ethanol in it.
By the roadside.
For a quick fix you can take the spark plugs out, loosen the fuel cap, and spin the engine more easily while you air dry any flooded plugs. Check for sparks and bubbles. If that doesn’t sort it then disconnect the fuel line at the carb and check the pump action (mind any loose petrol). Delicately clonk the float chamber to free off a stuck float valve.
What do you mean you haven't got a tool kit? What kind of Midge driver doesn't have a basic tool kit? Call the AA. No mobile phone? Achieve Darwin Award, write a letter, and die of exposure. Nitwit.
Back in the work-shop.
Essentially the problem, irrespective of the cause, is usually a low fuel level in the float chamber, If the car has been standing for a while, some of the petrol will have evaporated or leaked and the delay is the refilling, usually compounded by flooding, swearing and flat batteries. The usual and simplest answer is to prime the chambers by manually operating the fuel pump, sadly these, with a manual override lever are thought extinct in the wild now. If you have one, do that then remember that the pump is on a cam, and if the cam happens to be at peak then your pumping it will not help because the spring cannot de-compress. Half a turn of the engine should clear that. If there’s no manual lever on the pump then you can spin the engine with the spark plugs out, but in the longer term I recommend an additional electrical priming pump, which can be fitted in line, ideally near the tank and at fuel level. The sooner it is pumping fuel rather than air the better it will work. The best place for any fuel pump is next to the tank, but that cannot happen with a mechanical unless you put the tank in the engine compartment, which I advise against.
If, as on the Ford carburettor, there is a return pipe to the tank, then you may want to fit a switch to the electric pump so that it doesn’t run the whole time, leave the mechanical in place, and just prime the carb with the electric at start-up.
Electric pumps, (can be skipped if that’s not what you want). I liked the SU type mounted on the Morris1000 and Minis bulkhead, but they were a bit prone to dirty contacts, and occasionally I had to get out and bang it to get it going. On the upside they were noisy, you could tell when they hadn't started up when you turned on the ignition, and, in those impoverished days, when the tank was getting dry you'd hear the rattle of drawn air and you could wiggle the back end of the car to throw the fuel around a bit, thereby getting to the garage. That kind is rare now, though you can get new, if expensive, ones. More modern styled ones don’t have the dirty points problem (or the small spark they are prone to). Mine is relay operated which takes the loading off the switch, old style switches were heavy enough not to care.
Diaphragm pumps To the best of my knowledge, electric or mechanical pumps all use sprung diaphragms to actually pump the fuel, and so the thump you hear is the diaphragm and spring being compressed, drawing fuel up. Pumping fuel to the carb is done by the spring, and it will not pump again until the spring has decompressed. That is why the diaphragm and a sound spring are the most important bits. Proper pumps have a 2½" minimum diameter, 3" is better. Smaller means less petrol pumped per stroke and more noise.
If the pump is working then the next area of interest is air being sucked into the pipe between the pump and the tank, that point might also leak fuel out when standing. Check your joints and clips, especially where the pipes change diameter. The internal diameter of the pipe is relevant in that a large pipe has greater capacity and will take longer to fill if it has ‘run back’.
If there is a leak between pump and float chamber, being under positive pressure, it will leak fuel out (instead of sucking air in) and so be more obvious but more dangerous.
Additional non-return valves, vibration absorbing flexi-tube and filters can all help (unless they leak) but all add joints to the system, and resistance to fuel flow. A healthy pump should be able to push past them but remember that there are three valves and a filter to push past before you add anything.
I was rather surprised to find that my Triumph Midge didn’t have a tank vent, which explained why it wouldn’t start if I did a long run and then parked for more than a few minutes. The vacuum in the tank sucked the fuel back, so that the pipe was full of air. I hadn’t helped by tightening up the filler neck joint. The cure was to put a vented cap on the tank. A very small hole will do, but remember not to smoke when standing next to an upside down car, especially if it has crashed, as that loosens the joints.
A tank vacuum is not going to siphon the fuel out of the float chamber as there is an air vent in it, above the float level, but a leak will empty it. Look for dark stains around the base, a clean carburettor is easier to check. The 12/50 Solex is the worst for that as they are the oldest, most worn, and overly complicated. New ones are available at firstname.lastname@example.org
The float valve can stick of course, blocking the input, or over-filling, and the manuals always suggest perforated floats as a likely cause. All are possible, but rare. Assuming you would have noticed the smell of a petrol leak, the problem is usually worn non return valves allowing back flow, weak pump action, or air being drawn in by the vacuum. (check, if your filter is transparent, for bubbles).
Diaphragm pump. Bigger diameter = bigger volume = faster flow.
Fuel pipe. Bigger diameter = bigger volume = slower fill rate.
Non-return valves. Two in any pump, become inefficient when contaminated or worn.
Fuel cap. Vented or non-vented. Does it hiss if you release it after a long run?
Filter. Near the tank protects more components, especially if you swing the tail as described above, but is harder to check or change
Pump. Nearest the tank is best, but more likely to leak petrol than draw air.
Electric pump. Can be mounted near the tank
Mechanical pump. Quieter and designed in. Non return valves may fail with age or dirt. Only works when the engine is turning.
Return pipe. Reduces the effect of stale petrol and flushes bubbles out faster, but complicates electric fuel pumps.
Down-draft carbs. Fit inside the bonnet easier, more complex.
Best route. Tank to filter to electric pump to non return valve to long thin pipe then Mechanical pump if fitted and eventually Carburettor. Some prefer to put the filter after the electric pump.
Petrol. Modern petrol has chemicals that will dissolve some plastics
Siphoning. If the fuel line is below the fuel level then it will draw fuel more easily, but it can drain your tank if it leaks, and will still siphon it if the leak is below the fuel level irrespective of what height the pipe reaches in between.
I prefer plastic, if it is tolerant of modern petrol, all the way from the tank to the filter just before the electric fuel pump, high on the bulkhead. But the pump does have to be quite powerful, and it is an old fashioned way of doing it. Don't start the engine until the pump has settled down unless you want to pre-load the oil pressure.
Best route. Tank to filter to electric pump to non return valve to long thin pipe then Mechanical pump if fitted and eventually pipe to Carburettor. Some prefer to put the filter after the electric pump.
Simplified version. Plastic pipe, about 4mm (3/16) internal, all the way from the tank to the new SU electric fuel pump, high on the bulkhead. Same pipe to the carburettor avoiding the exhaust and other hot bits. Metal braided rubberised pipe with stainless high quality jubilee clips for connections. The old fashioned and simple way of doing it, but with no extra filters, in spite of the vulnerability to dirty fuel.
Does mine look like that? No chance. The above is how to do it properly…..One day…..