SU carbs running dry
Discussion
I've been banging my head against a wall with this one for some time now. The float chambers on the twin HS4 SU carb setup on my midget keep running dry, leading to stalling and sore forehead. I've checked all fuel lines from tank to carb for blockages and the new fuel pump I've fitted operates well. I have also fitted new valves in the float chambers and the floats are not adjustable. Can anyone suggest what might be wrong?
I'm assuming you have twin carbs (I.E. Standard car) and both are "running dry".
First off the reservoirs are actually running dry and your not suffering fuel vaporisation are you? In this weather I doubt it.
Secondly is your fuel pump new or at least know to be good? You don't mention which Midge you have but you will have either an SU electric pump or a mechanical pump mounted on the side of the engine (1500 or a Marina block used in 1275).
When is it running dry? Just ticking over on the drive or full throttle for long periods? If it's happening on the drive then either your pump is packing up (common on su electrics) and working intermittently. If it is happening on full throttle then it isn't supplying enough fuel.
Generally Midget fuel problems stem from a knacked pump (electric can be intermittent, mechanical usually either works or not) or a crack/air leak in the tank to solid fuel line flexi link.
First off the reservoirs are actually running dry and your not suffering fuel vaporisation are you? In this weather I doubt it.
Secondly is your fuel pump new or at least know to be good? You don't mention which Midge you have but you will have either an SU electric pump or a mechanical pump mounted on the side of the engine (1500 or a Marina block used in 1275).
When is it running dry? Just ticking over on the drive or full throttle for long periods? If it's happening on the drive then either your pump is packing up (common on su electrics) and working intermittently. If it is happening on full throttle then it isn't supplying enough fuel.
Generally Midget fuel problems stem from a knacked pump (electric can be intermittent, mechanical usually either works or not) or a crack/air leak in the tank to solid fuel line flexi link.
Its a 1500 with mech fuel pump which is new and pumping well as I checked it today (to compare its 'arm' with old one for size) and as I removed it from the block it was pumping fuel quite vigourously.
Could a small air leak in the fuel line cause a significant problem? It appears that the reservoirs are filling nicely when the engine is cranked and are then emptying on tickover.
Could a small air leak in the fuel line cause a significant problem? It appears that the reservoirs are filling nicely when the engine is cranked and are then emptying on tickover.
There's nothing at all wrong with the manual pumps, one fitted to my competition engine which will be using far more fuel than a standard engine has been spot on for the best part of 15+ years service. Changing to an electric pump is a retrograde step unless you suddenly need vastly more supply and it is the only option. You are not in this situation.
If the pump is pumping turning the engine over then it is a bit confusing, if I'm being honest I doubt it is, I imagine your seeing a bit of fuel spluttering out but it isn't actually pumping, take the pipe off the carb and then feed it in to a bottle or jam jar, then turn the engine over for 10 seconds solid, you want to see a decent bit of fuel in there, if there is just a dribble then you have a problem.
If you have plenty then god alone knows!!!
Other options could well be that the pump is faulty, the cam lobe is worn or the wrong pump is fitted (2 types were used, 1 needs a spacer block 1 doesn't).
Go and check it is pumping properly on turning over first.
If the pump is pumping turning the engine over then it is a bit confusing, if I'm being honest I doubt it is, I imagine your seeing a bit of fuel spluttering out but it isn't actually pumping, take the pipe off the carb and then feed it in to a bottle or jam jar, then turn the engine over for 10 seconds solid, you want to see a decent bit of fuel in there, if there is just a dribble then you have a problem.
If you have plenty then god alone knows!!!
Other options could well be that the pump is faulty, the cam lobe is worn or the wrong pump is fitted (2 types were used, 1 needs a spacer block 1 doesn't).
Go and check it is pumping properly on turning over first.
the only way they can run dry is down to lack of fuel supply.
whether thats a pump issue, or tank/line problem, you need to trace that fault.
But that really shouldnt be hard to do, given you know its a fuel suppy issue.
fill the float bowls or gravity feed via an external fuel supply.
Route the fuel line from the pump to a container and fill this as the engine runs, and it will be very clear if there is a fuel supply issue or not.
whether thats a pump issue, or tank/line problem, you need to trace that fault.
But that really shouldnt be hard to do, given you know its a fuel suppy issue.
fill the float bowls or gravity feed via an external fuel supply.
Route the fuel line from the pump to a container and fill this as the engine runs, and it will be very clear if there is a fuel supply issue or not.
Had a similar prob with a carb'd Renault 11 Turbo.
Turns out it was a blockage in the fuel system from it being stood for a while. The tank had sediment in it and every time I replaced parts they'd simply block up again with gunk picked up from the tank.
Start at the beginning, at the tank, and work from there.
I got a clue by placing the pump intake pipe in a fuel can, it ran fine as it was an unrestricted pickup.
Turns out it was a blockage in the fuel system from it being stood for a while. The tank had sediment in it and every time I replaced parts they'd simply block up again with gunk picked up from the tank.
Start at the beginning, at the tank, and work from there.
I got a clue by placing the pump intake pipe in a fuel can, it ran fine as it was an unrestricted pickup.
Having had chance to blitz this issue recently I finally nailed the cause - my idiocy.
In my keenness I treated the petrol tank with one of these tank lining treatments. Unfortunately I failed to notice that the fuel pick-up pipe in the tank had a filter over the end, which of course is now totally blocked up with the sealer compound.
Thanks again everyone.
In my keenness I treated the petrol tank with one of these tank lining treatments. Unfortunately I failed to notice that the fuel pick-up pipe in the tank had a filter over the end, which of course is now totally blocked up with the sealer compound.
Thanks again everyone.
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