That Machine Gun Sound

[billlobban]

I now spend more time watering the garden - than I do on Forums!

Yes Roy and pigs do fly

[alex_holden]

The answer is to relocate the fuel pump lower down

I saw a car at the National that had this modification - along with a two-speed electric fan, transistor-assisted points, thermostatic air intake control, radiator expansion tank, and a float chamber heat shield.

[HarryMango]

My fuel pump was going mad today coming down the M1 from the National Rally - struggled to restart after petrol station stop - made it home ok though.

Rog

[Kevin]

so if it gets flooded suction from the tank = no more problems.

Can you clarify that bit as flooding is not the problem.

[alex_holden]

I have a theory that the reason why some cars have this problem and some don't (mine never have, even in very hot conditions) is down to how much resistance there is in the pipe and strainer between the tank and the pump. The more vacuum the pump has to draw in order to lift the fuel up from the tank, the more likely it is to vaporise in the feed pipe. Perhaps blowing through the feed pipe with compressed air would help a bit, though not if it's really bunged up with that horrible gummy brown 'varnish' you get in petrol tanks that have stood for a long time.

[MarkyB]

By "flooded suction" he means that the suck side of the pump has little work to do lifting petrol from the tank.
It suggests that the standard pump is a bit marginal on the suck side, which fits in with alex_holdens theory too.

It would help if we actually knew what the causes of this problem were, rather than theories and speculations.

[alex_holden]

By "flooded suction" he means that the suck side of the pump has little work to do lifting petrol from the tank.
It suggests that the standard pump is a bit marginal on the suck side, which fits in with alex_holdens theory too.

I'm not saying it's a weakness in the pump itself that is causing the petrol to vaporise, I'm guessing it's the strength of vacuum in the feed pipe caused by the pump being located high up on the bulkhead, possibly exacerbated by having to suck the fuel through a partially blocked strainer/pipe.

The fact that the pump runs away ("that machine gun sound") when it happens suggests the problem is that the pump is drawing vapour in from the feed pipe rather than that the fuel is vaporising after it has left the pump, so I can't see how doing anything to the pipe between the pump and the carb would help.

[bmcecosse]

It is caused by the pump's in-ability to suck-up the fuel from the low mounted tank. This gets worse in hot weather as the unleaded fuel of today tends to vapourise more easily. Some pumps may not be set up to give the best stroke either - and some folk mis-guidedly fit filters in the inlet line which makes matters worse - even more resistance to the sucking! Pump mounted low down - gets flooded by fuel from the tank due to gravity - and then has just an easy upwards pumping action to the carb. It works.

[Kevin]

Right now I see what Roy and Mark are implying, however the term Flooded Suction will still cause confusion as it can only occur when the tank is open topped and above the pump (a similar situation to domestic plumbing where a head of water is needed to make a shower perform correctly) Also gravity will have little to do with it as the fuel comes from the top of the tank not the bottom, also I have not seen a filter fitted before the pump only on the pipe to the carb from the pump. But thinking on Alex direction I wonder if the filter in the pump itself could be making the problem worse if its got dirt in it.

[MarkyB]

Sorry Alex, I did misrepresent your theory and I didn't mean too.

As a small data point the minor pump is "long pull, short push" while the mini pump, which looks identical, is "short pull, long push"
Not sure how this is achieved within the pump though, maybe BMC can enlighten us?

Assuming that gumming up of the pipes and filters between the pump and tank could be the cause, does anyone know of an effective petrol additive that will actually sort it out, NOT just say that it will.

[moray]

g'day all,

down here in OZ one of the solutions to this problem is to cover the fuel line with airconditioning pipe foam insulation.
It's cheap and easy to fit, and if done properly doesn't look to bad.

this sort of stuff.
http://stores.optimumhouse.com/Detail.bok?no=182

cheers
Moray

[RobThomas]

Not directly relevant, but on my aeroplane there is a vent pipe (equivalent to the hole in the Morris filler cap) but it is located underneath the wing so that it gets atmospheric air from an area of high pressure, thus putting a few PSI of air pressure above the fuel level in the tank. This helps to force fuel down the line to the pump and helps to prevent any 'cavitation' at the pump. The equivalent thing on the Morris would be to seal the filler cap and to run a vent pipe to the area at the base of the windscreen where it is at high pressure whilst driving along. Obviously wouldn't work whilst sitting still.

About 16 inches of vertical drop from tank to pump is equal to 1psi of pressure. My carb therefore runs 1psi whilst stationary and about 3psi at speed.

[MarkyB]

Interesting, but standing still or barely moving is when the problem tends to manifest itself.

[Kevin]

Interesting solution Moray, and it sort of reinforces what Rob Thomasson has confirmed in the latest Minor Matters in that the modern fuels are far more prone to vapourisation in the fuel pipe line and is offering some exhaust wrap for owners to try.
As my own car has a larger bore exhaust system and generates quite a bit of underbonnet heat I was thinking of giving this a try first, also twice in the last week I have been able to pinpoint it to the pipe feeding the pump as when the problem was occuring I was almost at home and on arrival it was still doing it and I could feel the pulsing in the fuel pipe which was also quite hot. The float chamber did not seem to bad heat wise but I will give the wrap a try and see what happens, although for a standard car Morays solution seems a good idea.