Weber woes - pressure and leaking into primary chamber

[Brenjacques]

Hi all

I posted on the Facebook group about this initially, I noticed at the weekend when I was going to replace the cam cover gasket that my weber DMTL was leaking fuel from the end of the primary throttle shaft. I presumed that the seal and or bushings had worn despite the carb only being three years old.

Yesterday I had a chat with Emmanuel at Classic Carbs who advised me that excess pressure in the fuel line or a fault with the float/needle could also cause this by dripping fuel onto the closed throttle valve and causing it to leak down at the end of the shaft. Not really ideal as it drips straight onto the manifold.

Today I drove the car into work to allow everything to get up to temperature. I found thaat the fault begins about 5 minutes after the engine has been switched off when I can begin to hear a sort of spluttery breathing/venting sound coming from the carb and the top of the closed throttle valve slowly becoming wet.

The fuel line to the carb holds no remaining pressure after switch off so it looks like the fault is within the carb itself and possibly building up pressure within the float chamber after switch off, perhaps caused by the residual heat of the engine and manifold and the lack of fuel flow. I dont know if the float chamber has a breather which has became blocked, having never stripped one down before I'm not very familiar with them.

Has anybody experienced this fault with their weber before? I'm hoping to get it sorted soon as I need the car for a trip this weekend, typical it would start playing up a week before!

[WreckTangle]

I did not get on with the Weber fitted by the previous owner and restored mine to using a Pierburg. One of the established "tricks" to making a Weber function is to restrict the fuel return line. If this has been done in such a way that you no longer have a functioning return line then I suppose the system could be over-fuelling and maybe even retaining pressure after you turn it off, depending on exactly how it has been plumbed in. A non-return valve and absence of a fuel return would definitely ring alarm bells.

[Brenjacques]

I did have problems with my Pierburg which is why I now have the weber fitted, I do still have it so I could refurb and go back to it if it was really necessary. It was okay when it worked well but it started giving me trouble so I opted for the weber, which is also now giving me trouble. My car still has the swirl pot and unobstructed return line so it should have no pressure problems in the fuel supply.

I think first off I need to get any excess heat away from the carburettor, I'm sure it is the float chamber percolating after shut off. The carb currently has the little button heater element fitted so I'll start by removing this, not that I think it is really required here in the UK. I don't know if there's much else I can do but I'll look into what sort of heat shielding is available and if any can be fitted to the scirocco and maybe go from there.

I'll also give the carb a strip down and clean just in case anything else is causing it but it definitely looks to be heat related.

[WreckTangle]

If your fuel line still runs over the top of the engine that can be a source of heat. Some people fit a longer fuel line from the vapour separator to the carburetor so it can be routed away from the engine.

If you're planning on stripping down the carb anyway you can obviously check the height of the float valve and maybe reduce it by about 1/8th inch if you've got enough leeway.

Other possibilities which I considered, but never actually did, when I was running a Weber are to fit a fuel pressure regulator and/or the float vent and idle cut-off solenoids which the Weber supports but which are not normally provided on the version supplied for Volkswagens. These options are only really worth exploring when you're confident the fundamentals are correctly adjusted.

[Brenjacques]

Cheers, I'll see how I get on with trying to reduce the heat first. Interesting to hear of the float vent solenoid, maybe it's worth going down that route as it'll be easy enough to feed a 12v supply to it.

I think the car has suffered this issue for a long time as starting has always been a bit rough and smelling of fresh petrol and hot starting takes its time too. Perhaps now things are wearing in on the weber it has begun leaking at the end of the throttle shaft whereas before it may have only been leaking past the small gaps around the throttle valve and into the manifold.

[Brenjacques]

Well further to all of the above, I got home this evening and had another look, carb actually appeared to feel only very slightly warm to the touch so I waited a few minutes again until I began to hear the breathing sound coming from the carb again. This time I pulled the fuel line from the carb and found there was quite a bit of pressure built up in it so I think actually this is the cause, it can't be percolating as there isn't enough warmth there.

The set up is still the same as when the pierburg was on with mechanical pump, swirl pot and then a return hose and hose to the carb. Is it possible that the swirl pot is faulty or blocked? The return hose is clear as I can easily breathe down it and hear it in the tank but I'd expect any excess pressure left in the system to be drained back into the tank via the swirl pot.

If it's worth changing I have another spare which I could fit in its place.

[GT_II]

The tank itself will be pressurised after a journey, due to the temperature rise caused by warm fuel returning to the tank, so I'm guessing the pressure released from the fuel line when you disconnected it would have been due to that. I wonder if the float valve in the carb may be faulty, allowing the carb to overfill, but I don't have any practical experience of the carb to back up that theory.

[Brenjacques]

Yeah I only have experience of older cars and SU carbs in the past, not much at all with the webers. That said I've never had as much pressure in a line as I did here on other carb cars, maybe a small spurt when pulling the line off the carb but this was really flowing with pressure behind it. Even with the cap off the tank it continued to be under pressure as I was holding the hose end shut whilst somebody else took the cap off, it was still under pressure after that too. It looks like the pressure build up is between the pump and carb hence why I thought possibly a blockage in the swirl pot return.

I'm really no expert when it comes to swirl pots and return lines as my experience is virtually nil but to me something just doesn't seem right with how much I had there.

[Brenjacques]

Further to yesterday's saga I had another look when arriving at work today, again engine up to temp and a good run. I found that there is still a fair bit of pressurised fuel in the line from the swirl pot up to the carb after switch off and a short 20 minute rest. Not just a spurt on removal but the hose will continually flow when removed.

I disconnected the return hose on the swirl pot and found virtually no petrol at all, in fact just a single drop of fuel but I could see vapour rising from the outlet.

I'd expect with the amount of pressure down the carb hose I'd have had a fair bit of fuel come out of the return outlet as it would have de-pressurised through that but it seems not, the carb hose was still under pressure when connecting everything back up.

So I'm guessing that all of this points to a blocked swirl pot or at least the return part of it and virtually the equivalent of having a hose direct from fuel pump to carb. Also, swirl pot is very likely the original so it's now 38 years old chances of it being full of rust and crud are probably close to 100%.

Anyone think the above is a fair judgement or have I been dumb and gone down a rabbit hole

[WreckTangle]

Those metal fuel separators are fairly cheap. No harm trying a replacement if yours is behaving oddly. It should remain full of fuel with excess going back down the return line whilst providing a constant supply via a pick-up to the carburettor.

However, I refer you to my first post in this thread where I said that when people install a Weber they often restrict or block-off the return line. This may have been done in the return outlet from the separator or, more likely, in the black rubber pipe that connects to it. A common trick is to insert a welding tip. When I used this method (briefly) I put a thread in the output from the separator to hold the welding tip. Most people will just stick it in the hose where it may well migrate further down towards the see-through part. Maybe try disconnecting both ends of the first section of return line and check that it is clear.

Be warned that if/when you find your return line has been nobbled and restore its operation you then run the risk of the Weber suffering from fuel starvation particularly under heavy acceleration or when ascending hills.

[Brenjacques]

Thanks for the tips. I have a new spare so I've spent some time this evening replacing it. Once I removed the original I gave it a test by blocking off the input and blowing down the outlet to carb. Some fuel did come out of the return but I found the pressure to be all over the place, sometimes it was difficult to get air to pass through and then suddenly it would ease up and allow me to breathe through easily before giving more resistance again. The new one has virtually no resistance when doing the same test.

All seems okay, I haven't fitted a weld tip yet though I do have some, there isn't one currently in the line as I replaced this a few years back and previous to my ownership it had the pierburg on. Gave the car a good drive to test and there's currently no starvation issues but I'll keep some tips in the car just in case I need to give it a quick roadside repair if it catches me out.

After parking up there was no excess pressure in the line to the carb and no breathing coming from the carb either. Not 100% that it's certainly fixed yet but it seems promising so far, I'll run it for a few days first before I cast any judgement.

[Brenjacques]

Just a couple of more questions more so out of curiosity. How much pressure should I have down the return hose? If I blow down it there's quite a lot of resistance but it does flow as I can hear it back at the fuel intake, is this normal or should it be easy to force air down the return hose say to allow free flowing of fuel

Secondly, I seein the Haynes manual that there's a sort of ball valve at the top of the fuel tank that opens and closes at various angles, is it worth checking this for correct operation?

Perhaps really I should drop the tank at some point and give it a proper inspection having never done so whilst I've owned it. It does look to be a replacement tank anyway hence why I haven't bothered but it wouldn't harm taking it off and giving everything a check and replace necessary parts. Especially the rubber hoses connecting the tank to the copper lines as they're still the originals.

[WreckTangle]

Recently I had to restart my Scirocco after about a year of not using it. The fuel lines had completely dried out and no amount of cranking would get the flow started. I resorted to using an oil suction gun (like a large syringe) on the fuel line before the mechanical fuel pump. I was surprised how much force it took to get fuel flowing. However, I have had no problems since so assume everything must still be broadly in spec.

There are a number of factors at play here. The fundamental one is that pressure drops proportionate to the length of a tube. This is due to friction between the fluid and the walls of the pipe. If you try blowing air through a pipe normally used for liquid it will behave differently anyway as air is compressible. Water/oil/petrol are not. Parts of the pipework are quite restrictive. There is a long see-through pipe in the fuel return which has a smaller internal diameter than most of the rest of the fuel pipework. This will increase resistance. It is also possible that some of the rubber tubing is now getting sloppy with age leading to greater resistance. And, of course, rust from the fuel tank could be causing partial blockages. In a nutshell, I doubt it's too much of a problem provided the tube isn't actually completely blocked.

Moving on to your second point, that is not a ball valve. It is the float for the fuel gauge. It is probably best not to mess with the fuel sender unit if everything is working. They become brittle with age. If you really must, then you can access it via a hatch under the rear seat cushions which unscrew at the front. Be aware that if you break it finding a new one will be hard and expensive. You can see from the attached picture that the fuel feed is taken from near the bottom of the tank and the return goes in near the top.

It is only really necessary to replace the hoses if they are worn or damaged. As alluded to above, if the rubber hoses are tending to collapse under suction from the pump then it would be a good idea to change them. If they are still holding up well and don't look perished then they will probably do a bit longer. You don't want to be taking chances with petrol leaks though.

The system should primarily operate at something under 5psi. 2.5-3.5psi is, I think, what is recommended for the Weber.