Fuel tank breather

[PatD]

Hi All, I have a question regarding fuel tank depressurisation. I have recently bought a rolling restoration project Mark 2 2.4 (Solex carbs). The engine starts well and ticks over on the drive at 750 rpm, no problem. However, when I go for a short drive (1 mile to test), at junctions when I take my foot off the gas to brake and change down the idle speed drops too low and into a stall. I then can't restart the engine. The engine will start under full choke and allow me to get off the road to safety but will fail again when cold running is re selected. The engine is fully warm before I set out.
I have striped and cleaned the carbs. Blown out the fuel pipes and replaced the original fuel pump with a new SU unit. I have also fitted a paper element fuel filter to the glass bowl. Now here's the odd thing that has me thinking about the tank breather. When I open the bonnet after a nights sleep,I notice, the fuel level in the glass bowl has dropped to the bottom of the paper filter element. There are no leaks in the fuel pipe or filter housing that I can detect. Is the fuel siphoning or being sucked back into the tank?
My question, after that long preamble is. If the fuel tank has good seals on the filler cap, fuel take up pipe and sender unit. How does the make up air replace used fuel. I can find no breather mechanism. There is a tube in the filler orifice but this is below the filler cap seal.
My theory is, as I use fuel the tank gains negative pressure and gives fuel starvation issues, hence the stall under load. I will test this later by driving around without the filler cap in place but I would still like to understand the theory behind the breather mechanism.
Any other thoughts or suggestions as to my stalling will be gratefully received.
Pat

[davidr]

Hi Pat - if the tank is sealed well enough to hold a vacuum then you will hear a rush of air into the tank when you remove the filler cap

[PatD]

Thank you David, I have not done enough mileage yet to draw a discernible vacuum. However, I have now discounted that theory for my woes by driving without the filler cap. It made no difference.
Having re-balanced the carbs for the umteenth time I have managed to get a drive of 7 miles with no issue other than the idle being quite high at 1000 rpm. It started at 750 on the drive so there is some erratic behaviour going on in the carbs still.
I will retest once the engine is cold again before fiddling with either idle screw or mixture jet to try and get some consistency.
Still would be interested to know how the make up air gets into the tank though. Maybe it is relying on poor seals?
Thanks again for your suggestion.
Pat

[piman]

Hello Pat,

I don't have any knowledge about the Soex carburettors so can't help on that front. However, fuel cannot syphon back to the tank as the fuel pump effectively blocks fuel flow in that direction. ( Both discharge and suction ports only allow fuel to flow from the tank)
This means there must be a leak somewhere between the bowl and the pump discharge. I would have thought that you would smell such a leak?

Alec

[Doherto]

Pat, from my memory of stripping my 240 apart a few years ago i seam to remember a pipe running along the tank front centre lip ( with the mounting holes in it) and terminating with a plastic cone type attachment about an inch, sorry 2cm, long. It seamed possible for this to pickup debris from under the car so when i refitted the tank i brought a new pipe, connected to a fixed tube into the tank next to the fuel outlet to the pump, and allowed it to rest effectivly behind the near side boot cover panel. Hopefully is will avoid the possiblity of any potential blockages in the future. It is not possible to see the pipe as it enters the tank unless you remove it as it is behind the rear n/s wing below the fuel filler pipe access compartment. Hope this helps.

[PPV]

Do not know much about Solex carbs but have you checked for air leaks with pipes from the inlet manifold like to the brake reservoir or distributor?
Also I thought the fuel feed line filter did not have a paper element but has a flat filter gauze as per page C.7 and C.15 of the service manual. In addition there are the fuel filters in the carb banjo fitment I thought.
The problem with the Solex carbs is that they are susceptable to jet blocking with small amonts of debris but you new paper element may help. I inserted a seperate filter in my fuel line before the glass bowl one and the amount of much it collected was amazing!

[PatD]

Thanks to everyone for your comments and suggestions. The good news is that she is running well now with no sign of fuel starvation (touch wood). I think the Solex carbs are, indeed susceptible to blocked jets with small parts of debris. The paper element replaced the missing gauze in the fuel bowl. The filters, that are supposed to be in the fuel banjos are also missing and I can't find replacements having tried SNG etc. Now the carbs have a little protection things seem better.
The disappearing fuel still mystifies me as I can find no trace of leak. Thank you Pieman for confirming that fuel can not pass back via the pump.
Again, many thanks to everyone who took the trouble to reply.