Fuel Leak

[Mazzini]

The 40DCOE18's on my S3 were recently rebuilt by Gower and Lee. The seem to work well, but the front carb leaks fuel. I've checked the float heights and they are spot on. The leak isn't bad, but any leak from a carb is not good. Any ideas?

[mbell]

Leaks into the throat of the carb or outside?

If outside, where do you see fuel? Options are probably bottom cover, fuel inlet/filter housing, carb to airbox or engine.

Have you checked your fuel pressure?

How did you check the float heights? Via actual fuel level in jet holder or by float to lid distance method?

[Mazzini]

Outside, under the body. It seems to be where the carb body meets the o-ring carrier, but that's where it would collect I guess.

It might be the bottom cover.

I haven't checked fuel pressure. I'm running the standard glass bowl mechanical pump. How does one go about checking that?

I checked the float heights using the float to cover method.

Thanks

[nmauduit]

to check fuel pressure statically I've modified a Weber banjo bolt (drilled and tapped) to adapt a small pressure gauge on top... cheap and convenient to rule out an issue, then I keep it in my Weber box.

Weber fuel pressure gauge

[mbell]

Fuel pressure is tested by inserting a fuel pressure gushes between pump and carbs.

I think you need to directly check the fuel level in the carb. It's possible they are flooding due to needle valve not sealing. In that case the fuel level can rise and run down directly into the throat where it could leak out around the o ring.

You should be able to check the level by removing the main jet assembly from the carb and running the engine. The fuel level should be a few mm below the hole that goes down to the throat.

[SENC]

Rob, is it leaking just sitting there or only when running? The former would have me looking at the bottom covers and gaskets, but that seems unlikely. New fiber washers on both sides of the banjos? Any sign they are wet? I wouldn't think fuel pressure if you're running the mechanical pump but will be interested to learn.

Check your fuel level with the tops on by removing a main jet and using a straw with your finger on top. Around 25 mm from the top of the well (around 28 from the top of the lid ring) is about right. I've also learned recently you can remove the idle jet and look down that orifice and that fuel right at the bottom where the jet sits is essentially the same level. These levels put fuel 1-2mm below the channel to the venturi.

[Mazzini]

Morning gentlemen,

Thank you all for your replies.

@nmauduit - great idea, I shall try and make something similar.

@mbell - I will check as per your suggestion. Something tells me that you're on the Sidedraft forum.

@Henry - good to hear from you and I hope all is well. The fuel only leaks when the engine is running. The banjos are copper and fuel tight. Yesterday I did notice a small pool of fuel in the alloy casing, there is a well by the mixture control screw and the vacuum take off cover (not sure if that's the right name). I have no idea how the fuel got there. I will buy some straws. I know you're on the Sidedraft forum.

Just out of interest are the float levels stated in the workshop manual still good?

Many thanks to all of your for your advice.

[ericbushby]

Hi Rob,
Try measuring the fuel level from the top with a dipstick such as a cocktail stick.
If you remove a main jet stack and lower the stick in, watching with a small torch, as the stick touches the fuel the surface will shiver and marks you have put on the stick will give you the height.
The engine has to have just been running of course.
Kieth Frank insists that the correct level is 25mm below the top of the float chamber, plus or minus 1mm

As the rim of the jet cover is 16mm above the float chamber, then I measure 41mm from there.
I use the depth gauge on the end of a vernier caliper for this because I have one, and a small torch.
Recently I have been experimenting with 27mm to try to get a weakure mixture,(also with weaker jets and hotter plugs)
I am watching Kieth's research with interest, but have not bought his jets yet.

Hi Henry,
I did try using a clear tube and a finger for this, but was surprised how much the fuel ran up the tube by capillary action and lntroduced an error. The smaller the tube the worse it gets. I suggest trying it on the bench first with some petrol in a jar.
Regards, Eric
S3SE DHC

Edit, I just realised that I am assuming Weber carburettors. For other makes please ignore the above.

[Mazzini]

Eric,

Thanks for that. Yes, my S3 is on Webers.

I have been talking to Keith Franck, I want to buy some of his tubes, but first I want to get my carbs working properly.

I'll work on the carbs this afternoon.

Thanks again,

Rob

[Craven]

Must be the most talked and debated problems with old Webers. Consensus being worn inner shaft bearing bore, shaft runs directly in body, and when worn allows the depression in the main venturi to create a vacuum in the closed chamber that houses the pump jet control rod. This pulls fuel from the pump jet piston cavity via the control rod into the closed chamber. It’s from the closing plate the fuel leak reaches the outside then runs along and accumulates and drips at the ‘ O ‘ ring flange.

[Hawksfield]

Hi Mazzini

I would go with Craven "leak from the cover plate" for that area or "O" ring leaking and running to the body
What work was done during rebuild!, did the carbs leak like this in the past?

[alan.barker]

Is this a leak when you stop the Engine then open bonnet and see fuel at back of Carb.
Alan

[HJF]

Must be the most talked and debated problems with old Webers. Consensus being worn inner shaft bearing bore, shaft runs directly in body, and when worn allows the depression in the main venturi to create a vacuum in the closed chamber that houses the pump jet control rod. This pulls fuel from the pump jet piston cavity via the control rod into the closed chamber. It’s from the closing plate the fuel leak reaches the outside then runs along and accumulates and drips at the ‘ O ‘ ring flange.

I have had exactly this In the past, and not just with old Weber’s. Solution is to ensure the small closing plate to that chamber is the one Weber offered with the hole in it preventing vacuum build up in the chamber and the consequential suck-through of fuel.

[alan.barker]

+1 spot on
Alan

[Mazzini]

Craven, John, HJF and Alan,

Thank you for your comments, I haven't had a chance to look at the car today, but I will tomorrow.

Thanks again,

Rob