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MG MGB Technical - Fuel pump losing prime when car parked .... rrgggh

Anyone know what could cause this annoying, albeit fairly harmless, problem?: ... my fuel pump loses its prime when the car's parked for 3-4 days. Today was about the third time this has happened this year. The minuet goes thus:

1. key in ignition, coupla pumps of gas, whiiiiiiiiiiiiiiiiiiir starter sure sounds great but she ain't firin'

2. is there gas? ... yep, plenty

3. is the fuel pump working? ... yep, whirring away nicely

4. hmmm ... the DPO put in a fuel pressure gauge in the engine bay and it says 0 ... conclusion: no gas getting to motor

5. disconnect fuel line from carb; attach Mityvac, suck air bubbles then eventually fuel through system; reconnect fuel hose to carb

6. starts just fine now


Why is this happening, and how can I prevent it? I was wearing my Sunday best today when this happened and I didn't appreciate the indignity. Any ideas?
Ted

Did the DPO install an aftermarket fuel pump?

I ask because original equipment SU pumps don't "whirrrrr".

SU pumps "click-click-click..."

Ordinarily, if the SU pump and carb float valves are OK, the pump will click a few times until fuel
pressure is built up in the system, then stop.

Three things come to mind when an SU pump clicks away endlessly:

1) something is stuck in one or both of the IN or OUT valves inside the SU pump. Debris, junk,
a hunk of rust (from the fuel tank) caught in the valve(s) can hold a valve open and prevent the
pump from pressurizing and delivering fuel.

2) A valve(s) is cracked and needs replacing. The IN and OUT valves on the SU pump is a thin, clear,
plastic "reed" design (it's shaped like a "+"). When these wear out, they develope a hairline crack
and fuel delivery first becomes weak, then irractic, then worsens more and more until it quits.
Victoria British sells replacement valves for this (their part #: 3-2002) and their catalog shows
a fairly good exploded view of all this.

3) The pump diaphragm is cracked and leaking. You'll know this when you disassemble the pump
and find that everything under the plastic cap (points, springs, arms and wires) are coated in black, moist soot. You'll need to measure the length of the diaphragm spindle whe you order a new one
because it comes in two sizes. Of the three possibilities, this is the least likely fault to happen,
however.

If yours is actually an SU pump... my bet is on #2.
Daniel Wong

Daniel, it's not an SU. It's a Mallory (gerotor-type design; there's no diaphragm). The only thing I can guess is that I've not sealed up the fuel lines or something, and air's getting in when parked. However, it doesn't seem to leak gas so I don't know. It has run for years without doing this and I'm wondering what the heck has changed.
Ted

Ted, suit yourself, but i wouldn't mess with this for a minute, I'd toss it out and buy a "pusher" type standard SU pump for your year model- I believe there are only two. From your description, you must have the fuel pump under the bonnet, pulling from the tank. One tiny leak causes bleed back and no fuel. The rear mounted SU diaphragm pump is a simple system, and when it does go wrong, a wack with a large screwdriver handle or a lump of wood seems to cure most problems.
Ken R
Ken Rich

Ted,
If you have a fuel filter between the tank and pump, I read some where recently that you should create a loop in the line so the filter is vertical rather than horizontal, with the outlet pointed up. This maintains a solid column of fuel up to the filter. I don't know if it works, but it's worth a try.

Regards,
Jerry
Jerry Causey

If the pump is indeed in the engine bay as Ken hinted, that could be a part of your problem.

The stock location (down by the right side battery box) is almost level with the fuel tank and this helps
to keep the pump primed. Whenever I fiddle with my fuel pump I have to plug-up the fuel lines
otherwise gravity will slowly siphon the fuel out of the tank.

Locating the pump next to the battery box also saves it from engine heat and vibration and helps to
prevent vapor lock (air bubbles) in the lines.
Daniel Wong

Jerry, by Jove, I think you've got it ... it's in pretty much the normal SU location on the RHS of the RH battery box, but because Mallory strongly advises doing so, there's indeed a pre-pump filter.

(An SU doesn't push enough gas, at high enough pressure, for a V8.)

It's amazing what you guys can divine here!
Ted

This thread was discussed between 16/04/2003 and 17/04/2003

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