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MG MGB Technical - Overflow from carbs - where ?

In several of the discussions on fuel and gas mileage, it is frequently suggested to make sure one isn't getting too much fuel to the float bowls.

It is suggested that one check the overflow tubes for excess gas coming out.

I have a 72B with twin SU4 carbs (or is that HF4?) - the ones with the external float bowls.

The fuel lines come in to the bowls, and there is another line that comes out and is "T" connected to 2nd carb, and this line runs to the carbon canister near the side of the heater box.

Should I be disconnecting this line from the carbon canister to see if excess gas is entering it? If excess gas were getting to the canister wouldn't there be a stain from the gas on the flat surface below the canister?

Or is there a different line I should be checking?

As always I'm still trying to resolve issues with gas mileage, a steady 18 mpg (and that ever present hiccup). I've checked for leaks, and park the car over clean cardboard to see stains from leaks. Never have seen a gas leak. Although recent attempts at faster sharp turns, does result in the smell of gas. The tank is almost full right now so I'm not sure if this will happen at lower fuel levels in tank.

I'm going to move my brass block aftermarket fuel pump from near heater box to back by battery box. When I do this I'm thinking of replacing all the gas line with hose, not tubing and to pressure test the old gas lines too to see if I can force a leak.

I dream of the day this car gets 25+ mpg.

Thanks for any assistance.
R.W Anderson

lol Youre going through the exact same thing as I did. You have emmissions control stuff to deal with though. I know with my HIF4 carbs (you have HS4 carbs if theyre stock) the overflow line goes into a solid line and down past the exhaust manifold. I suppose in your case you could disconnect the line from your charcoal cannister and put something like a cola bottle on the end of it and drive it for a while to see if any petrol goes into it. I might be wrong there but that should work. What I would do first though is remove the dashpots with the carbs still in the car and then after disconnecting the low voltage feed to your coil turn on the ignition for a while but obviously dont try to start the engine. Take a look down into the carb at the jet. If fuel is coming up out of it your float valve isnt closing off or the float height is set wrong.
Ross Kelly

RW,

You probably have a pair of SU HS4, or SU HIF4. If the float bowl is a separate piece hanging on the side of the carb, and has a small fuel line going from the float bowl to the bottom of the carb, then you have the HS4.

Ignore Ross's comment about where your overflow hose goes, because like he said, he doesn't have an emission control system. Try what he said about looking for overflow after removing the dash pots and pistons. Don't swap those parts around.

SU carbs use low pressure fuel pumps. If yours puts out too much it may be overflowing them.

One of the first repairs I did on my GT was to replace the rubber hose from the tank to the carbs with steel line. Some lazy PO had installed rubber hose. I don't know why you would remove the steel line to check it for leaks.

As a point of reference, I get 30 mpg on the highway with overdrive. I don't know what I get around town since I don't have a working speedo/odo.

Charley
C R Huff

RW- Ross Kelly's advice on the soda can/bottle is a good one as the floats may be closing, but the jets ( especially a spat of Grose jets a few years back)may be leaking, held open by a grain of sand/dirt. If you are doing maintenance, change the jets and floats and keep them in the empty battery box when you convert to a 12v single box battery, along with spare points, fan belts, oil, brake fluid, etc. The strategy is if you have it available, then you will never have to use them. I prettymuch carry all pieces to rebuild the engine between the boot, extra batt box, and 2 splash boxes behind the fenders.
Once you fix the gas leak, check for a very rich mixture, and timing/valve adjust. You should easily be getting 25-30 MPG. Luck, Vic
vem myers

so... 25-30 mpg around town or highway? i get about 22-24 city/highway, and 27-28 highway with my gt. i do not have overdrive. is that about average? i get 35-40 highway in my midget.

i have an aftermarket square fuel pump- i think it puts out 2-4 pounds. since i'm not leaking anything, i assume i dont need to put a pressure regulator on it?

thanks-
Don
don g

This thread was discussed between 24/06/2008 and 26/06/2008

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