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MG MGB Technical - Fuel pump keeps ticking
|The fuel pump on my 67 MGB has just started to keep ticking all the time. It doesn't shut off at idle like it used to. It's an original type SU pump that has been on the car for 5 years and has around 10,000 miles on it. Even though the pump ticks all the time at idle there is no fuel overflowing at the carbs or at the pump. The car drives great and there is no hestiation at all. Is this a problem or should I just ignore it.|
Any suggestions much appreciated.
|It is one of two things:|
The pump is sucking air on the inlet side from a loose or leaking connection, especially common if there is a rubber hose involved. Connections can leak air in but no visible fuel out. This will get worse until too little fuel is delivered. Common.
The pump has a leaking internal valve - the inlet one. This may be due to dirt, and could stay the same or even cure itself if the dirt washes through. Not common, unless the tank is rusty.
|The pump should deliver way more than the carbs need - 2 Imperial pints per minute or more - so can get pretty ropey before it starts affecting running. It's your car talking to you and telling you attention is needed, don't ignore it for long. You can get an idea whether it a suction leak or a stuck valve by doing a delivery test. Remove a pipe from a carb and direct it into a container and switch on the ignition. As I say it should deliver a minimum of 1 Imperial pint per minute and in practice double that or more, in a continuous series of pulses with minimal bubbles. If you get a lot of bubbles it's sucking in air, if not but the delivery rate is low then it's a valve. If it keeps clicking with the ignition on but the engine stopped, and still doesn't overflow after a couple of minutes (longer on cars with the charcoal canister, disconnect the coil to stop it overheating) then it's the inlet valve. If it stops clicking then, and there are no bubbles, then it could well be the outlet valve (when the diaphragm pulls back it pulls some fuel back from the pipe to the carbs as well as some from the tank).|
|Thanks FRM and Peter. I checked the fuel delivery and I have just over 2 imperial pints per minute with no air bubbles visible. The pipe from the tank to the pump is a solid pipe and is not leaking. None of the pump connections are leaking. The fuel bowls are not overflowing even with the pump left on with continuous ticking for over 2 minutes (coil disconnected). You both suggest that it is probably the inlet valve. I assume that the pump has to be removed and disassembled to correct this problem. Do you need to drain the fuel tank to remove the pump?|
Valve trouble is usually a hair or piece of grass etc. (stringy stuff) in my experience. This is pretty unlikely to cause total failure, and may clear itself - but then the hair will get stuck in a float valve and make it flood! If you have an inline filter after the pump the filter will catch it.
You don't have to drain the tank, but the fuel level must be below the outlet pipe connection, and you have to loosen that a tad to break the vacuum or the fuel will siphon out.
Two different pumps were fitted.
HP has the connections about 30 degrees apart, outlet straight up, and a filter under a plug straight down. The valves come out when you unscrew the outlet adaptor.
AUF300 pump has both connections parallel & horizontal with outlet on top, and a 4 screw cover at the top. Has to come apart, as the valves are inside the diaphragm chamber.
Pics in WSM. Good idea to contact Dave DuBois, and/or look up his website. Surprised he hasn't jumped in already. SUfuelpumps@donobi.net
|"Good idea to contact Dave DuBois, and/or look up his website. Surprised he hasn't jumped in already."|
Really no need to Fletcher, you and Paul covered things well enough.
I would add that on a 67 MGB, the pump should be the AUF 300. Once the coil housing and diaphragm is removed from the pump body and the body is placed on a work bench with the inlet/outlet fittings pointing to the left (cover that looks like a top hat pointing up (12 o'clock), there is a square cover held in place by two screws. Remove the screws and cover, the inlet valve it the top one. You will have to pry the cover off the valve and the valve out of its seat. Once the valve is out you can remove the reed (plastic disk like item with four lobes) by carefully lifting each lobe with a very small screwdriver to pop the lobe out from under the flange. Once the reed is out, you can flush everything with fuel or mineral spirits, then flip the reed over (so the unused side is against the valve seat) and put it back in place, then reassemble the pump. Any problems, contact me at the e-mail address that Fletcher provided. My web site is at: http://homepages.donobi.net/sufuelpumps/ Cheers - Dave
|The open end of the pipe from the tank must be above the level of fuel in the tank to prevent siphoning, and that is difficult to judge of course. Where the outlet pipe is bolted to the tank isn't really relevant, as the other side of that pipe i.e. inside the tank goes right to the bottom of the tank. On a chrome bumper the pump is mounted relatively low, so there is a significant chance of fuel siphoning when you are removing and attaching the pipes (BT, DT). On a rubber bumper the pump is above the tank, so no chance of siphoning while you keep the end of the pipe at that height. On a CB if you have a correctly functioning fuel filler cap you may well find driving for a few miles before you tackle the pump will prevent fuel siphoning, even with a mostly full tank, BT DT also. But in any event once the pipe is off, raising it above the top of the tank should allow fuel to drain out of the pipe, and so prevent further siphoning even if the open end is left to dangle lower.|
|Paul - just loosening the fuel line from the fitting on the tank is sufficient to break the siphon, even on a full tank and particularly with the right side of the car jacked up to get at the pump. Cheers - Dave|
|That's true, but I'd rather not open up parts of the fuel system if it specifically needed, and possibly cause more problems later on. Things get pretty rusty round there, I'd hate to split the tank wall.|
|Guys, many thanks for your advice. I'll tackle the project next week and see what I find. I've got a good method of pumping gas out of the tank so we run the fuel down low and then pump the rest out into a gas can for my lawn mower.|
Thanks again, and apologies Paul for calling you Peter in my first response. I don't know if you remember me but I visited you around 8 years ago when I was visiting my Mum in Birmingham.
|Stand by Paul, you have been called Peter and Paul, can Mary be far behind? ;-) Cheers - Dave|
|I did go out with a Mary once, but there was no Peter involved ...|
I do remember Andy, have assumed it was you in your postings over the years.
This thread was discussed between 18/08/2011 and 21/08/2011
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