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MG MGB Technical - Fuel Pump overheating??

HI, have recently had all sorts of problems lately with my 68 MGB MK1 stopping intermittently. WOuld just die and not be able to be started then about 2 hrs later it would start like nothing had happenned.
I was sure at first it was electrical so replaced ignition coil, points, condenser, dizzy cap, leads etc all to know avail. I then had the fuel tank drained, lines blown, filter fitted but still no joy. The mechanic found no water in the fuel or anything.
I thought it may have been an airlock with the fuel cap but no, was no pressure there when it stopped.

I had thought it may be the fuel pump but I was sure before when it had stopped I hadn't heard the tick, tick sound you get when the pump works to reprime the line after pump failure or when you run out of fuel.
But the other night after a breakdown, I had a better listen and sure enough. I was winding it out a bit too and it did occur to me that it does probably only do it when under a bit of load.

What would be happening ? would it be overheating or something? and then cooling down so it would start an hour or two after stopping? How do these things work, do they continuously pump or do they have an inbuilt cutout switch when they get up to pressure.
ANy advice much appreciated.
Craig WIlliams

Craig - For an explination of how the fuel pumps work, see my article on them at: ttp://www.custompistols.com/cars/articles/dd_su_fuel_pumps_101.htm
The quick answer to how the pump knows when to stop pumping when the pressure is built up in the float bowl is the volute spring in the pump. The volute spring (also called the diaphragm spring) is compressed by when the diaphragm is pulled up by the current through the pump's coil. When the points open the spring pushes the diaphragm back to it's rest position, which in turn pushes the fuel pulled into the pump back out to the carburetors. When the pressure at the float bowl reaches the pressure exerted by the volute spring, the pump stops pumping until the pressure is releived by the fuel in the float bowl being used by the engine and the needle valve is opende by the falling float. At this time the pump starts pumping again until the pressure again builds up.

The problem you are experiencing could be a bad fuel pump, but it is more likely something else in the fuel system. I am sending you a fuel delivery troubleshooting guide to help you determine where the problem is.
Good luck - Dave
David DuBois

Craig,

I'm experiencing the same behavior that you described with my '77 MGB. I'll be driving along and the engine will just quit.. So far I've been able to get to the side of the road OK. It's happened about 4 or 5 times in the last two summers (I only drive the car in the summer). It's quite frustrating. Most of the time the car will restart after sitting for a few minutes. Once it was 40 mins before it restarted. I'm not sure of the cause, but I suspect something is wrong with the fuel system. The problem seems also to be heat-related. It would run good initially, but the stalling would occur after running for an hour or two. I installed a clear filter right before the carb (I still have the ZS 175 carb. Only 26K miles on the car), and the last time it stalled (a couple of weeks ago) I immediately opened the hood and looked at the filter. It was empty, and fuel was "bubbling" at the outlet opening of the filter. I then turned on the ignition and the fuel pump tigked away and the filter filled with fuel. I'm thinking that there is something wrong with the fuel pump, but the last owner had installed one of those small "emergency" fuel pumps in series with the original pump. I'm also wondering if the problem might be vapor lock......but that wouldn't explain why the fuel filter was empty. My next step will be to test the fuel output flow before I spring for a new pump. Just wanted you to know that you're not alone with this problem.
Ray Beyler

Love MG's, Loath SU fuel pumps. Too much grief. Replace it with a low pressure Japanese one! Early 80's SUBARU or Honda civic pump.
Peter

I had a similar problem with my '68 B. It would start fine and run for about 5 minutes, then die. When it died there was always no gas at the carbs. When I bought this car it had been sitting idle for some time and the PO was going to sell it for parts. So I went through the whole fuel system: drained and flushed the tank, replaced the pump with a new Facet electronic pump, blew out the fuel lines, and added a filter between the tank and the pump and another just before the carbs. I started it up, drove about 1/4 mile and it died again. The new fuel pump was fried. Once again I replaced the pump but this time I ran a new power wire from the fuse block and a new ground wire to the frame. After that the car ran well for a couple of miles then cut out again. Now I realized that if I turned the ignition switch off and on the fuel pump would start ticking again and the car would start. I never really figured out the exact problem causing it to die, but I've driven the car a couple hundred miles since and it cut out once or twice at first but always restarted after cycling the switch. And it seems that the more I drive it, the better it runs. My guess is that one of my problems was caused by ignition switch contacts that were corroded from sitting unused and repeated use has cleaned them.
Bill Boorse

This thread was discussed between 05/06/2004 and 10/06/2004

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