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

[Defender110]

I would have thought an electric pump would be the perfect cure as it will fill the carb when you turn the ignition on then 2 seconds later turn the key to start.

[electrokid]

I've had confirmation from the seller of the solenoid in the last link I sent, that it is 'normal' IE: normally closed, open when powered, so it should do the job in the feed pipe as flowback prevention.

Would it help to fit an electric fuel pump instead of the mechanical one?

Yes. Webers are normally happy with around 4psi. Injection pumps are way to high pressure of course - they'll just flood the engine compartment with petrol. Diaphram pumps are usually a little on the low side and cylinder pumps are often a little on the high side. The ideal is a cylinder pump + regulator.

Although it's ended now I've been watching...

http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... K:MEWAX:IT

for the pump and...

http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... :MEWAX:VRI

which is a filter and regulator combined.

[B-Hive]

I decided a while back the the TRS has fuel drainage issues resulting in hard starting after even just an over night rest. I decided this because I began to notice that if i parked on a right slope angle, starting the next day seemed easier. Also I noticed that if i lowered the car after shutting off, starting the next day was easier too..

It makes perfect sense as the fuel feeds/return are on the left side of the carb, so if its on a right incline fuel has to travel up to escape.... and with the BX's propensity to stay up at the front for quite extended periods after shut down, this is also providing an opportunity for fuel to run back to the tank.

There is no other logical reason for the cars difficultly starting cold..So until I have sorted the problem all I do is pump the be-jeebers on the accelerator then keep doing so as i hit the ignition. Still not first kick usually but better.

[Linegeist]

Right! That's a new Facet electric pump ordered Ebay rules!!!

Thanks for all the advice and anecdotes blokes.

This pump's rated at 4.5 -7psi ....... and it'll be feeding a brand new Weber carb. Do I really need a fuel pressure regulator?

[electrokid]

Do I really need a fuel pressure regulator?

Probably not - but if in doubt check with the people who supplied your carb and of course make sure all the pipe clamps are nice and tight

I might be able to check if you let me know the model No. of the carb. Looks a decent pump for the money.

[Linegeist]

Hi Brian, thanks for the kind offer ....... but I'm first trying to get my geriatric head around the engineering involved here. I also think I'm not making myself clear (as usual)

The petrol delivery system on the BX is a constant circulation type - as I'm sure you already know, the pump's delivering petrol to the float chamber feed line all the time the engine's running. What the carb doesn't need (when the float valve snaps shut) simply vents back to the tank.

My reasoning here is that there's never going to be anything more than residual pressure in the delivery line(s) at any one moment (leaving float valve shutoff transients aside). So, provided that the restrictor on the fuel return line isn't too small, the carb'll never feel any real back pressure, and certainly never enough to overcome the float valve.

Or am I annunciating from my exhaust ............... again.

[electrokid]

The petrol delivery system on the BX is a constant circulation type

Not on all of them - which is why I asked which carb you've been supplied with as a 'BX' carb. The TLP for example is normally fitted to the 14 but could just as easily be set up for anything else and called a BX carb because it will do the job - just not as well as the DRTC for example. Most Weber / BX installations include an Idle Vacuum Unit - a couple do not - if you have a DRTC without an IVU (where there should be one) then that might impact on your emissions unless the carb builder has changed the emulsion tube or made another adjustment to fix. The DRTC (which is the right carb for the job) can be supplied with or without the idle jet cutoff solenoid - the DGAV which could be set up for the BX with a bit of math and a degree of good luck can also be supplied with or without the solenoid. There are quite a few variables depending on which model carb you have.

I'm sure you've got 99% of the job done because the carb is performing well and the new pump should indeed fill the float chamber prior to starting so that will help but there are half a dozen different implementations of float chamber venting and probably another couple of things that could affect the way fuel may or may not drain back down the feed - without knowing which model carb you have I have no way of knowing where that problem may be.

My reasoning here is that there's never going to be anything more than residual pressure in the delivery line(s)

With your new pump the fuel line pressure is likely to be around 7psi on idle - the max quoted for the pump, the spec for which suggests that it could drop no lower than 4.5psi on full bore. In reality the drop is likely to be less than than considering the quality of the pump.

[Linegeist]

Wow!! A dissertation on carbs!! <impressed> Thanks Brian - only several things that hadn't occurred to me there then .....

The only thing I can't get my head around is how you can get pressure in what is effectively an open pipe (the tank return).

In a closed circuit, the closing of the float valve forces the pump diaphragm to 'float' on pressurised fuel in the line - and therefore the pump to 'idle'. On an open return system, the pump never idles - it's always shifting liquid, n'est pas?

[electrokid]

Well I'm a big fan of Webers Bob I have both books on Weber carbs by John Passini plus the aforementioned Haynes regurgitation of the manufacturer's stuff but my only claim to fame is the recon of the Weber on my 2 litre Granny - but keeping it to 65 on a run it would return 50mpg so I figure I've learnt enough to get something right

I'm talking about the pressure in the feed pipe - not the return. As you say the impeller pump never idles - it produces fuel pressure - ideally that should be enough pressure to provide enough flow both to dribble through the restriction and into the return pipe, and to supply the engine with enough to keep the fuel in the float chamber at a constant level because the carb calibration depends on that.

[scarecrow]

but my only claim to fame is the recon of the Weber on my 2 litre Granny - but keeping it to 65 on a run it would return 50mpg

Have I woken up in an alternative reality? My experiences with 2L and above petrol engines have taught me that they never do more than 35mpg. I'm not doubting what you say Brian, more what I thought I understood

[electrokid]

It was closer to 35 around the town and the reason for checking it on a run at 65mph (boring !) and not higher was it built in 1982 - just before they started fitting the 5 speed gearbox - the engine was the excellent Pinto and had some 150K on it - overall a very good motor and 35 / 40 overall is still very good - and yes - it is a kind of alternative reality when you can just jump in the car and go wherever you want without being concerned about fuel costs - even better when it's a big white handsome lump

The work on the carb (and on the cooling system to get the auto choke and inlet manifold heating system working properly) took around 3 days to complete but obviously well worth it.

The Peugeot engines are excellent too so I've no doubt that high mpg is achievable with a Weber and a lot of fiddling about and that's the reason for me getting up to speed again with Webers for when I eventually get around to fettling the 4x4 I've got - at least I think I've still got - it's at a one-man-band garage on an industrial site where there might be problems - we shall see.

I'd recommend John Passinis 2 books though they are quite old now - they are very detailed but they don't pretend to know everything and let you know when it's time to stop reading the book and start experimenting for yourself

[Linegeist]

Could you possible post up the book details please Brian - ISBN number - stuff like that. I'll then have a trawl and see if I can find copies for myself.

Having been out of the motor trade for nearly 20 years, I really need to get up to speed on Weber carbs again and your books sound like the canine's accoutrements.

Excellent thread - top info.

[Defender110]

sound like the canine's accoutrements.

love it

[electrokid]

You probably won't need ISBN Nos. - search eBay and Amazon - 'weber passini' will work well in Amazon but ebay listers may not put the author's name in the title. Just like junior school - book 1 is red and book 2 is green - can't miss 'em Bob

[Linegeist]

Just like junior school - book 1 is red and book 2 is green - can't miss 'em Bob

Ohhh trust me guv', I can!

Never heard the term, "Idiot Savant", Brian ............?