Petrol draining back .............. is this common, gents?

[Linegeist]

Fitted a new Weber to the Athena a month ago - superb thing! Economy's amazing, acceleration's crisp and the hot starting's cured - but .......................

If I leave the car for a few days, it takes a LOT of churning to get it to fire. Examining the fuel system shows that the large fuel filter that came with the new carb (and fitted between the pump outlet and the carb inlet spigot) is empty, and my guess is that the petrol is draining back into the tank, the float bowl's evaporating off, and the whole circuit has to be re-primed each time after a long stand.

I've remade all the joints to the various spigots and, as far as I can tell, they're fuel and airtight ..... although the evidence would suggest they're not. Is this a known problem with the BX petrols? Logic seems to suggest that mounting the filter at the lowest point in the run would help - or is this another senior moment?

And what's the big alloy canister on the front of the engine where the three fuel lines meet?

So many questions - so short a lifespan ...............

[electrokid]

Just a guess Bob - the float chamber will have an overflow system of some kind which will either be open to air through a tubular device on the outside of the carb or it will be fed to the 'clean air' side of the air filter.

Can I sugest that - wherever air is getting in to the feed side of the carb to allow it to flow back to the tank - a one-way valve in the feed pipe should in theory sort it.

[mat_fenwick]

Oh be done with it and fit a diesel! Seems a bit of a shame now you have it running well though.

A bit of an off the wall idea - an inline fuel solenoid which is closed wit the ignition off to prevent drain back. It's kind of fixing the symptoms rather than the cause, which goes against the grain somewhat.

Although let's try and think about this logically (not easy at this time on a Friday evening!) Engine hot, turned off, float bowl evaporates, float drops, float valve opens. Therefore the fuel system is no longer sealed, so drain back can happen. Sound plausible? In which case why did it not do it before...?

[JayW]

Oh be done with it and fit a diesel!

Pure comedy... My derv-mobile was doing the same for the first couple of months!

With my experience with mine, it doesn't matter how convinced you are that it's airtight... it ain't

[Defender110]

What sort of fuel pump has it got Bob, there is usually an anti syphon / drain back valve built into the pump. If it is the lift pump type with diaphram valves the diphrams can split or just be stuck open by a piece of debris, blowing out with an airline can cure this.

[electrokid]

A bit of an off the wall idea - an inline fuel solenoid which is closed wit the ignition off to prevent drain back

Not off the wall at all - I think most carbs fitted to the BX have a solenoid 'idle cut off' valve which prevents run-on - I guess it may double as a one-way to stop fuel going back to the tank.

there is usually an anti syphon / drain back valve built into the pump.

I think that's spot-on Kevan - the diaphram valves are one-way by default.

[kiwi]

And what's the big alloy canister on the front of the engine where the three fuel lines meet?

Ah ha so it is not just the Kiwi versions of the BX that have these funny metal tin cans fitted.

When I asked this question about that can on here and aussiefrogs the only response I got was it was some sort airation chamber

Not really got a clue either to be honest as I have it fitted to both BX19s but it was not fitted to the BX16 I took apart.

Even my well versed mechanics have no idea it seems what it is for just saying it is part of the fuel system

Examining the fuel system shows that the large fuel filter that came with the new carb (and fitted between the pump outlet and the carb inlet spigot) is empty, and my guess is that the petrol is draining back into the tank, the float bowl's evaporating off, and the whole circuit has to be re-primed each time after a long stand.

Nothing unusual in that either, I just went out and checked my TZS thats been parked up the last couple of days to confirm this and indeed the fuel filter is empty.
Come to think about this on both BXs whenever they have been standing for a couple of days or more they both need repriming and dont start on the first start.
N idea the reason why either.

[JayW]

And what's the big alloy canister on the front of the engine where the three fuel lines meet?

Ah ha so it is not just the Kiwi versions of the BX that have these funny metal tin cans fitted.

That'll probably be an evaporation canister, most petrols have them.

They absorb the fual vapour from the carb/tank then release it into the engine later, otherwise you'd find the smell of petrol everytime you walked past a car (the vapour's gotta go somewhere), not to mention the explosion risk.

They usually contain activated charcoal.

[Linegeist]

That'll probably be an evaporation canister, most petrols have them.

They absorb the fual vapour from the carb/tank then release it into the engine later, otherwise you'd find the smell of petrol everytime you walked past a car (the vapour's gotta go somewhere), not to mention the explosion risk.

They usually contain activated charcoal.

Ahah! The thick plottens Jay!!!! The car's always reeked of petrol - but so did my Mk1 GT ..... that's why I like diesels. I rather think the petrol's draining back via the fuel tank return line, emptying the upstream pump chamber and the carb-float bowl line as well. As far as I can see, there's nothing much to stop it in the standard setup. I'm wondering, as a final check of this theory, if chucking a tablespoon of petrol straight into the venturi after a long stand will result in an immediate startup. This'll prove it's lack of fuel causing the fault - or otherwise.

I love the idea of a solenoid - that's elegant! As for the pump, it's brand new (I thought I'd fit a new pump to go with the new carb - sort've making sure I didn't end up playing 'Spot the Strange Symptoms" after the Weber lung transplant.) I only look dim.

I'm rather thinking along the lines of that solenoidal shutoff valve idea, ditching the 'aeration chamber' in favour of an in-line restrictor, and blanking off the large venting spigot on top of the Weber to slow evaporation.

The pain in the bottom with all this, is that with the old Solex, despite its penchant for vast amounts of bang-juice and a stubborn disinclination to start after anything more than a 10-minute run, the damned would ALWAYS start from cold, regardless of how long it'd been stood. Like a dog desperate for walkies, I swear it used to perk it's ears up when it heard the ignition key going in the steering lock.

Thanks for the input blokes. As usual, you've posted all the stuff I hadn't thought of .............. See what you've all gone an' done? I feel stupid now ................

[KevR]

This a common thing on 2CVs and H vans. Easiest 'cure' is to fit the priming bulb from a diesel in the fuel line – if it's been standing a few days or weeks, a couple of squeezes of the primer fills the carb and lines and away you go.

[electrokid]

I love the idea of a solenoid - that's elegant!

Weber DRTC and DRTM as fitted to Citroen both have the solenoid shut off valve - but they often 'add ons' in the Weber range - the idle jet (which is the only one we need to be concerned with) can be fitted with a solenoid or not as the carb builder sees fit - so it's a standard Weber part and possibly still available off the shelf from Southern Carburettors.

What is the full type number of your carb Bob ? I Have Hayneses Bumper Book of Weber Carbs (Haynes 1784) and that may well tell me if there is a standard bolt-on solenoid for your carb and other info such as the path taken by the bowl overflow which is a potential path for evaporation or syphon.

I rather think the petrol's draining back via the fuel tank return line

Normally the feed and return pipes are connected to a 'Y' on the carb with a limiting valve so that there's a small amount of flow from feed to return (so that the fuel going to the carb is reasonably cool all the time) so flow down the return pipe shouldn't matter.

If it needs cranking for say 10 seconds or less before it bursts into life I would suggest that the carb bowl has emtied - if you need to crank for longer I suggest the fuel has emptied from the feed pipe or even further back as well.

Glad to hear all its actual running probs are fixed and it's otherwise doing a good job

with the old Solex, despite its penchant for vast amounts of bang-juice and a stubborn disinclination to start after anything more than a 10-minute run, the damned would ALWAYS start from cold, regardless of how long it'd been stood. Like a dog desperate for walkies, I swear it used to perk it's ears up when it heard the ignition key going in the steering lock

Oh now please... a Solex with ears

[electrokid]

That'll probably be an evaporation canister, most petrols have them.

They absorb the fual vapour from the carb/tank then release it into the engine later, otherwise you'd find the smell of petrol everytime you walked past a car (the vapour's gotta go somewhere), not to mention the explosion risk.

They usually contain activated charcoal.

Well thanks for that Jay - I learn something new every day - not usually as useful as that though

With my experience with mine, it doesn't matter how convinced you are that it's airtight... it ain't

If I park on a slight sideways slope I sometimes need to prime - I found a potential air leak at the control spindle on the diesel pump - it's quite worn and moving it sideways sometimes produces a very faint hissing noise as the air leaks in - I've got a spindle bearing recon kit (£90 quid + ) but haven't fitted it yet.

[kiwi]

And what's the big alloy canister on the front of the engine where the three fuel lines meet?

Ah ha so it is not just the Kiwi versions of the BX that have these funny metal tin cans fitted.

That'll probably be an evaporation canister, most petrols have them.

They absorb the fual vapour from the carb/tank then release it into the engine later, otherwise you'd find the smell of petrol everytime you walked past a car (the vapour's gotta go somewhere), not to mention the explosion risk.

They usually contain activated charcoal.

Its strange that in all my years of owning BXs (22 years) I never seen them before. Although in saying that none were post 1990 petrol models these two are.

[electrokid]

Hi Bob, thinking a bit more about this and knowing that the granny suffers from fuel flowing all the way back to the tank (the Pierburg is obviously completely deaf ) I found this...

http://shop.ebay.co.uk/?_from=R40&_trks ... l+solenoid

which looks interesting - mainly solenoids for LPG conversion but hey - a solenoid is a solenoid right.

I looked at one listing that said the valve was off with 12v applied which may have something to do with the way LPG systems are set up or (more likely in my opinion) the listing is wrong.

I have put...

http://cgi.ebay.co.uk/New-Petrol-Soleno ... 19c0d704ce

on my 'watch' list - it's brass bodied and just about the cheapest as well - coming from Lithuania shouldn't be a problem. And I've sent a question re whether it's on or off with 12v applied.

I'll probably try one in the granny even though that's probably got a leaky pump diaphram - proof of principle and all that

[scarecrow]

Would it help to fit an electric fuel pump instead of the mechanical one? For a start it wouldn't be sitting on top of a very hot engine block, and they are self-priming too. I'm going to do this when mine is converted to a Weber, unless one or more people tell me I'm an idiot