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MG MGA - SU carb question, how far do I go on a rebuild?

My 1500 roadster project car coming out of winter hibernation exhibited what I think are two related symtoms.... fuel pump endlessly ticking and the car stalling after a few minutes at idle. One mystery, fuel never flowed through the overflow pipes which were not clogged.

I had planned on only replacing the needles and seat in the float chambers, but when I removed the floats I found 1/4" of rusty sludge. Last year I replaced the fuel tank and the fuel line up to the bulkhead and installed a gas filter.

I've removed the carbs, cleaned the chambers and passageways and blew the carb body passages clear. The carb needles seem OK. The carbs are now clean inside and out.

My question is do I need to do anymore to insure dunk isn't lurking elsewhere?

This is my first experience with SU carbs and I'd like to avoid pulling apart the jet and realigning the needles if I can. On the other hand I've got the carbs on a bench so this is the time to everything necessary.

Suggestions are appreciated.
Brian Denis

Check the fuel line screens where the banjo bolts connect the fuel line to the float chambers. Needles and seats are probably fine and easy to replace later. Put the carbs back on, run the car and if it dies shut it off and check the float chambers to see if you have any fuel in them. If not, you probably have a pump problem. If full, start looking at electrical.
Steven B

If fuel is not overflowing at the carbs or running on the floor anywhere underneath, then continuous running pump means it's not pumping (or pumping very little). You can disconnect the hose between the body and rear carb, and switch on the power to see how much it flows. You should get a gusher, at least one pint per minute. I'm guessing it won't flow much.

Check for a clog or air leak in the line before the fuel pump. Check/clean the inlet screen in the pump, just inside the inlet fitting. Blow a short shot of compressed air back through the line into the tank, in case there may be a clog at the pickup point. If not fixed yet, look for a bit of dirt or rust caught in one of the fuel pump check valves, which might require disassembly of the check valve for cleaning. The final straw is a possible split diaphragm in the pump.
Barney Gaylord

Steven and Barney, thanks for your input.

I did test the fuel pump by disconnecting the fuel line at the carbs and stopping the flow with my thumb over the hose... the pump stopped ticking. That's why I went to the float chamber needles as the likely suspect. Maybe it's as simple as not giving the pump enough time to overflow. I will check the flow rate as Barney suggests.

Any recommendations on the need to disassemble and clean the jets after the amound of crud I found in the fuel chambers? How difficult is it to do and then realign?

Thanks again for your assistance.

Brian Denis
Brian Denis

I did the flow test Barney suggested and certainly no gusher, a few feeble squirts as the pump activated. The pumps sounds like it's laboring. This explains why gas wasn't overflowing. So, this weekend off to clean repair the fuel pump.
Brian Denis

Brian, I had a problem with my fuel pump where it just kept running and running. No overflow from the carbs and no visible fuel spillage anywhere underneath. On my car there are several transitions from steel to rubber fuel line where the PO had installed an inline filter between the tank and the pump. It turned out that a couple of the hose clamps at the transition points had worked a bit loose, allowing air to be sucked into the pump. I snugged up all the clamps and voila...problem solved. May not be the cause of your problem, but it's a simple thing to check before you start pulling the pump apart.
Andy Bounsall

Some aftermarket fuel pumps tick continuously when they are functioning normally. Based on your thumb test, this is not the case here. I just thought I'd mention this in case some poor guy reading the archives someday doesn't realize it.
Steve Trovato


I would suggest that you might move your inline filter from before the pump to after the pump. It might seem that you are protecting the pump, but in fact you are creating a serious problem for it. A restriction on the inlet side of the pump will result in the points remaining closed and the pump burning out.
If there is sediment in the tank, that problem should be fixed.
A filter which becomes blocked, and is fitted after the pump, holds the points open and saves the pump.


Mick Anderson

Thanks Mick. Yes, I'm aware of that. You'll notice my use of the past tense.
Andy Bounsall

Brian, I would bite on the bullet and strip the carbs completely, and fit the Burlen service kits. If there was a lot of gunk present, you probably need to clean out the jet chambers and fit new seals. Tip - soak new cork seals in light machine oil for a few days before fitting - they are easier to fit and last longer. You will find that SUs are pretty simple and reliable when in good condition, and the exercise will build your confidence, so if you have problems in the future you will know how to deal with them. The Burlen kits come with very complete instructions. AB
A Bennett

This thread was discussed between 12/05/2008 and 15/05/2008

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