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MG MGB Technical - Fuel overflow

I have problems with my 1964 MGB (and I am a newbie ;)).

I first start the ignition and heard the fuel pump "ticking". Then I start my engine which goes smoothly. However after around 15 seconds the float bowls on the carburator start overflowing. Then after a while it stops overflowing but my engine stops after around 30 sec as well and I can not start anymore (only temporary but that makes bad noise and then it stops again).

When I look at the transparant fuel filter right before the carburator it is empty! I then wait for a few secs and initiat the ignition, and it started filling. But then it also starts overflowing again.

So it seems to be pressure related. How does this regulation work exactly? At what point will the fuel pump "know" it has to start pumping. Is this something electronically? Or does it always pump "blind"? I can hardly believe that since the flow of fuel should be controlled quite well?

Any help is again highly appreciated!
A.J. van Donselaar

Sounds like a classic case of worn float hinges.

The floats should swing freely - but should have little
sideways movement or "slop". It is this "slop" that is
giving you the erractic filling, starving and flooding
of the float chamber.

Fortunately this part is very easy to replace.

Remove the fuel lines from the carbs, and undo the
three screws on top of each float bowl and carefully
lift the lid out. The float hinge pin is easily
pushed out using a solid wire or drift.

While you've got everything apart, you might as well inspect the float needle valve, too. Use a magnifying
glass to check for any wear along the face along the
tiny "cone" surface of the needle.

Since you're new to all this, I might suggest that you
somehow get a copy of the book: "The Complete MGB"
by Robert Bentley, for your year model. You can get a
new copy ...but sometimes, you'll see these being sold
on eBay.
Daniel Wong

The floats are actually new and I have opened them but (although they were filled with petrol) the mechanism seemed to be fine.

Are there any other possibilities?

I have taken the fuel pump apart and put it back in. Could it be that I haven't "closed" the vibrating part enough (you can adjust it by turning it around) so it would pump too much now?
A.J. van Donselaar

A.J. - Adjust the pump exactly as the book directs. The amount of fuel and the pressure of the pump is regulated by the volute spring behind the diaphragm and will not force the fuel past the needle valve in the carburetor if the valve is operating correctly. Your problem is the floats filled with petrol. In that condition, they will not "float" at the top of the fuel in the bowl and shut off the fuel by pressing against the needle valve. You need to get all the fuel out of the floats and seal them against any more fuel getting into them, or get new floats. Good luck - Dave
PS. For an explination of how the SU fuel pump works, see my article at: http://www.custompistols.com/cars/articles/dd_su_fuel_pumps_101.htm
David DuBois

I am sorry. I meant the valve is filling up. There is no fuel in the floats (having problems remembering all English terms for each part).

I'll try to get the float valves out and see if they're working fine again. I read online the level of the floats can be adjusted. Maybe something is wrong there.

I'll also try to verify if my fuel pump is not set too fast. Although all I could change is rotate this diaphram. Is it indeed possible that this diaphram is set too far from the volute spring and therefore processing too much fuel thereby overfloating my float valves? Or can't this be the issue and must the problem be with the float valves and the float level?

At this moment I am not at my car so I won't be able to fix anything before the weekend. Hope to be able to have acquired enough knowledge about all this from you guys by then. Thanks so far for the answers.
A.J. van Donselaar

Btw http://www.custompistols.com/cars/articles/dd_su_fuel_pumps_101.htm is not working at this moment?
A.J. van Donselaar

I had to put neoprene tipped float valves in to seal my carbs float chambers , since then OK. The level adjustment is just to put the static fuel level at a point in the jet where the carb will work , you can neglect it in this problem .
S Best

It's not the float chambers themselves that are leaking. It's the overflow opening on top of them..
A.J. van Donselaar

Hi A.J,
Have a close look to the top of fload bowl and feed tube connexions, they should be tight, also there should be a seal between the bowl and the cover. If it's floading from the upside you will see it. On the other hand, when we say floading, generally that means the overflow dripping out, downside the bottom exhaust meaning, float bowl valve failure .
Good luck .
Renou

http://www.mgcars.org.uk/cgi-bin/gen5?runprog=mgoc&p=emg/needles.htm seems to give a good view on my problem. If anyone else needs it ;)

Now I am still doubting whether the pump is just going too fast (since that's the last thing I worked on) or whether for some mysterious reason the float indeed has to be fixed. I can hardly believe that since it is both (although the righ one is leaking much less) and they're completely new.
A.J. van Donselaar

Here's a flash of thought:

Be sure that the hoses are going to the correct fuel feed, and vent, nipples that are on the float lids.

On HS4 carbs, it is very easy to mis-orient the proper
position of the lids of the float chambers. If this is so, then it is very easy to wrongly install the
fuel feed hose into the vent nipple, and install the
vent hose into the fuel nipple. Doing this, will certainly result in a flood of fuel leaking on the
floor.

Examine underneath the float chamber lids in order to
trace the route of the fuel and vent barbs and to verify the proper orientation of the float lids.

====

Some folks have had problems with Grose Jets (2 ball)
leaking - even when new - and have had to revert to
conventional type float valves.

====

To set the fuel pump diaphragm:

1) Be sure that the points are gapped to specs and
that the points mechanism is working properly.

2) Unscrew the diaphragm by a few turns.
There is no need to unscrew it completely.

3) Very slowly screw the diaphragm back in - until
the points just begin to tick over - then stop.

4) Unscrew the diaphragm by counting 4 of the
diaphragm screw mount holes (or about 1/3
of a turn).

Daniel Wong

A.J. - The link seems to be working now. If you still can't get it to work try going to http://www.custompistols.com/default.htm, then click on Classic MG.
The fuel pump cannot put out any more pressure than the volute spring force. That is set by the spring pressure regardles of how far the diaphragm is run in. The shop manual shows how to adjust the points and the diaphragm.

I believe that Daniel has the answer to your problem, the float chamber caps are rotated 180° and you have the fuel line attached to the point that the overflow tubes should be connected to. A quick way to check is to blow into each tube with the cap removed and holding the float hard against the needle valve. The correct fuel inlet should be the side that you cannot blow through until the float id released.
Good luck - Dave
David DuBois

This thread was discussed between 01/06/2004 and 02/06/2004

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