Lotus Elan

Gas Leak - Weber Carbs

PostPost by: lotusbob » Fri Feb 25, 2005 3:16 pm

Have a 71 Elan with twin Webers (40DCOE18). After turning the car off, several drips of gas leak where the carb is connected to the manifold (for the 1st and 4th cylinder, to be specific). Not a lot, but no leak is good.

Anybody have any suggestions as to the source of the leak and what I should do about it? I am a novice with respect to carburetors - but maybe now is time to change that.

Thanks. Bob
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PostPost by: type26owner » Fri Feb 25, 2005 3:40 pm

First remove the inspection cover from over the jets and remove those two mainjet assemblies. With a flashlight look down into the 'well' and you can judge whether the fuel level is too high by how close it is to the brim of the passageway leading off to the auxiliary venturi. You can safely do this while it is idling. Of course if the engine shakes around violently while idling it could spill out fuel and catch fire so use your best judgement then. To be okay it should come up within 1 to 2 mm of the brim and not spill over. Now shut the engine off and watch the level. If it goes up and dumps fuel into the passage the float needle valve is leaking and needs to be replaced. Webers float valves are overwhelmed with excess fuel pressure. Best not to have it go above 2 psi.

For extremely accurate reading of the fuel level make yourself a pipette with a tubing diameter of about 1mm. The passage is ~ 23mm down into the well and is drilled in there diagonally. It's ~6mm in diameter.

The o-rings on the compliant mounts should seal well enough so that it is also a liquid tight connection. You need to fix the leak there too. Buy Viton o-rings only because they don't get attacked by the fuel and swell up. McMaster.com has them if you're in the USA.
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PostPost by: lotusbob » Fri Feb 25, 2005 4:38 pm

Thanks for the quick and detailed response. A couple of practical questions:

1) is removing the mainjet assembly straightforward (does it screw out, lift out...)
2) if it is the float needle valve, is that replaced reasonably easy or will I need to take the carb substantially apart to do that
3) if it is not float needle valve, what would be the next likely cause of the leak

Just trying to assess the risk that I take something apart and either cause damage taking it apart or cannot get it back together. I do have a security blanket, er, local experienced car enthusiasts, who can help, although they have little carb experience.

Well, since I am going to take off the carbs to fix the mounts, I might as well replace the mechanical fuel pump with an electric, turning one task into something bigger! If you have any thoughts on that, let me know (previous group opinion was to replace, although a few said leave the mechanical pump). (Fuel pump, inline filter, blanking plate, inertia switch, cutoff switch...now about reading those wiring diagrams...)

Bob
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PostPost by: type26owner » Fri Feb 25, 2005 5:37 pm

Since you're new here let me advise you to use the advanced search engine on this forum and go back and read all the posts I've done on the Webers. There is a lot of them. Some of the info you will not find anywhere else. It'll take a couple of hours to plow through them. After that if you're unsure about things fire specific questions at me and maybe I can help and maybe not. I'm still on the learning cuve about what actually happens when doing the main jetting but it's on my list to pursue it soon.

I'll answer your three questions later.
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PostPost by: saarhus » Fri Feb 25, 2005 6:36 pm

Due to valve timing overlap and other airflow dynamics, there is a tendancy for reverse pulsation in the induction path. This is especially noticable where each cylinder has a complete carburator, as with the Webers. It is common to notice, while running, a fine mist of fuel "standing off" from the intake trumpet. This reverse pulsation could be responsible for small quantities of fuel, that have dropped out of the air stream to collect and dribble out at the manifold junction. Also, that may indicate that the rubber "o" rings are not quite tight enougn and allowing this dribble. This is all purely speculative, but plausable?. What does the group think?

Stan
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PostPost by: type26owner » Fri Feb 25, 2005 8:12 pm

One other symptom that can lead to the fuel overflowing when the engine is switched off is if the carbies are hot to the touch when the engine is running for at least twenty minutes then the mixture is wrong. If they are hot to begin they get even hotter by heatsoaking when the engine is switched off and the gasoline expands. Properly tuned carbies are cool to the touch when the mixture is about right because of the latent heat cooling effect.

1) is removing the mainjet assembly straightforward (does it screw out, lift out...)

Just need a blade type screwdriver. Don't overtighten it putting it back in though. A couple of inch-pounds is plenty.
2) if it is the float needle valve, is that replaced reasonably easy or will I need to take the carb substantially apart to do that

Just need to remove the top cover which is held on with I think just 5 screws. Go here for the info to do this.
<a href='http://www.lotuselan.net/uploads/weber.pdf' target='_blank'>http://www.lotuselan.net/uploads/weber.pdf</a>

3) if it is not float needle valve, what would be the next likely cause of the leak

You got me there. A hole in the housing? Maybe a loose, leaking lead plug. A plug would only dribble out fuel if the induction vacuum is removed like the engine switched is off.
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PostPost by: lotusbob » Sat Feb 26, 2005 2:46 am

Thanks. Will let you know what I find. The service manual link was very helpful.

Bob
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PostPost by: lotusbob » Mon May 02, 2005 2:42 am

Float levels are correct. Needle valves and needles look fine. Took the airbox off and looked into the throats after driving, no leaks detected.

Was suggested I look at the choke mechanism. Well, there is clearly a leak that runs along the sides of the inspection cover below the choke mechanism, fanning out in a triangle shape from the top corner of the choke mechanism. Reaches the bottom of the inspection cover and then goes over to the mount, making it appear as if the leak is from the mount.

Cannot tell if the leak is from the choke mechanism or the inspection cover until I clean it up and see, even then may not be able to tell.

Anybody have any idea which is more likely to leak?

Bob
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PostPost by: type26owner » Mon May 02, 2005 2:10 pm

Bob,
The inspection cover can only leak air into the cavity between the bores. One should remove the dried out paper gasket and glue the cover into place so it's airtight with a adhesive which is easily dissolved with a solvent. Having an airleak there will degrade the tuning performance of the first progressive hole by a lot depending on how big the leak. The bigger the airleak the more the throttle plate have to be closed to maintain the same idle speed and a lean hole will open up when coming off idle onto the progressive hole.

If you do this look for the casting flaw on the recessed surface that the cover rests against. Both my carbies have in the same place on the bottom a section missing which is about 6mm long. The paper gasket doesn't seal worth a hoot if it's not touching anything. Wondering if all the 18s have this casting flaw or not.

The cold start choke is the likely source of your fuel leak. Mine leaks in the same way. One alternative is to buy the blanking plates and remove the cold start choke mechanism altogether. Since I've recently tuned my chokes to work correctly for the first and use them everyday now I'm planning on stopping them from leaking if it's at all possible. Just haven't gotten around to devoting a full weekend of Weber tweaking yet to proceed with that plan. Suspect the sealing surfaces are marred. The back of Weber manual illustrates all the special tools for repairing those surfaces. I'll make my own tools but prefer to hone rather than dent and move metal.
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