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MG MGB Technical - Too much fuel in my SU

Hi,

For anyone following my probs the head is now on the 1977 B GT everything went together ok.

Now I have a new problem. On starting the car for the first time there seems to be too much fuel to the carb. It will start and misfire badly then its dumps fuel out of the overflow pipes coming from the carbs. It seems to be worse on the back carb.

I have checked that the butterfly is closing and the piston is not sticking, needle is clean, oil in damper. Thats about the extent of my knowledge on carbs.

Please could anyone help?

Thanks

Graham
G Lewis

Seeing as you've just had them off the car I'd suggest the floats are stuck down. Assuming HIFs, you're going to have to take them off the car. If you are lucky a good shake may free them (check by reconnecting fuel line before putting them back on). Otherwise undo the 4 screws on the bottom move the floats by hand. You could be unlucky and have dirt in the float valve needles but I'm guessing not.
Steve Postins

Ok, I have taken the carbs off. The floats seem to move freelyand I blasted carb cleaner down every hole I could find. I removed and inspected the floats and they both seemed fine, I shock them and listened for fuel inside but could not hear any.

One thing I did notice on the top of the jets in each carb there is a metal top hat. This was at the top of the jet on the back one, but at the bottom near the pick up on the front one. I moved this to be at the top as it looked correct on the back one.

I put it all back together and is just the same. I also notice that the fuel pump keeps ticking with the ignition on, it slows but does not stop (I guess it just keeps pumping the fuel on the floor)

So what next? Did the top hat thing cause the problem? Did I miss something I should have checked?

Thanks

Graham
G Lewis

Graham,
If you are dumping fuel, you do have either a float or needle valve problem. Needle valves are the primary source because they are failing to make a proper seal within their seat (trash). Floats occasionally stick down, mostly as a result of a cracked float (fill up with fuel). If you are running HIF carbs, you'll need to pull them to correct problem. If you're running HS-4's, you need only to remove cover on side mounted fuel bowl to correct problem. Attach fuel bowl lid (with float/needle valve still in place) to fuel line, hold lid over a catch bucket). Have someone work ignition key to activate fuel pump. Fuel should flow when float is pointed down (helps to clear needle valve). Inverting the cover (float pointed up) should stop fuel flow. If it doesn't, it's time to replace your needle valve. As mentioned, trash in the fuel could be causing your problem. An in-line fuel filter helps in this regard.
Steve Buchina

Thanks Steve & Steve.

I didn't take the needle valves out. I will do that tomorrow and inspect. Could the etal part of teh float that acts on the needle valve be worn?

Can any one confirm how the top hats are supposed to be? I put them to the top of the plastic L shape, but the haynes shows them towards the bottom. A picture I have just seen on the web shows them in the middle?

Just looked at the last garage invoice for the car in July (I have only had it since Aug). It says the fron carb needle collar had fallen out and was refitted. Any ideas what this might mean? Maybe its the same problem I have now?

Cheers

Graham

G Lewis

PS: It is a HIF
G Lewis

I finaly put a pressure reducing valve in to cure my fuel dumping problems. The original system worked perfectly until about 1999 when with the car going like dream climbing up the M40 out of High Wycombe to go to the Abingdon Works Car show it suddenly stoped firing on 3 and 4.(3 guesses which carb was flooding !) I think modern petrol may be to blame. The interim solution was neoprene tipped float valves from Burlen. I've never owned anything new enough to have HIFs so cant help there. I got the pressure reducing valve from Summit in Maidenhead but I think they got it from Moss.
Stan Best

Stan, I have killed two cars on that hill!

G Lewis

Not sure about the top hat and its usual location. Any chance you can pick it out on the diagram?:

http://www.moss-europe.co.uk/MossUK/ProductList.aspx?SubSubCategoryID=MGB_DS008E

That said, it may need sorting but should only be secondary to your main problem. As has been said, the needle valves are responsible for stopping the fuel flow. If the floats are moving properly, the valves must be letting fuel past: worn or dirty or too much pressure from the pump.
Steve Postins

Hi,

Solved this one. It was dirt in the valve after all.

Thanks all.

New problem now see my new thread 'Now its not firing on 2 & 3'

Graham
G Lewis

This thread was discussed between 27/09/2006 and 28/09/2006

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