Lotus Elan

Weber Carbs Twin 40dcoe

PostPost by: khamai » Sat Feb 14, 2004 5:49 pm

The plug unscrews.

See #48 in this diagram
<a href='http://www.piercemanifolds.com/40DCOEa.pdf' target='_blank'>http://www.piercemanifolds.com/40DCOEa.pdf</a>
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PostPost by: JACKJABBA » Sat Feb 14, 2004 9:09 pm

My carbs do not have screws, they have brass blanks fitted, like an upside down bucket. They are TYPE 18 Webers, I don't know if they are different to the type 31, which the +2 workshop manual says the car should have. I have noticed that they also do not have the leather dust seals and springs for the throttle spindle, but a brass washer which covers the bearing and a spacer fitted between it and the outer washer.

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PostPost by: khamai » Sun Feb 15, 2004 5:32 pm

Jack,

Are your carbs new? There's lots of ideas about what the diff the last 2 numbers in the DCOE model means. From what I've seen the diiferences are jets, chokes, emulsion tubes, and the location & number of idle bleed jets.

Given the difficulty of removing & replacing the plugs covering the bleed holes of your carbs I guess you're better off using the synchro tool. BTW, does the idle speed change when you place the tool over one of the carb throats? If the idle is unaffected I would say it's a decent tool that will give you reliable readings.

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PostPost by: JACKJABBA » Sun Feb 15, 2004 6:53 pm

Kiyoshi, the carbs are what came with the car when I got it. Looking at the condition of them, they are quite old. On stripping them down I have found that it is possible to blow some fuel through the throttle spindle supports/bearing where it passes from the throttle plate to the area of the pump contol lever. This is on one carb only so it may need to be restored by a pro. Other than that I could find no problems.

The synchrometer does not seem to make a difference in rpm when in use.

I will try and get it on the Dyno this week, so the guys there can tell me how well balanced they are when I watch them do it.

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PostPost by: type26owner » Sun Feb 15, 2004 10:32 pm

Jack,
My DCOE_18's have a brass threaded plug just like Kiyoshi describes which covers up but gives access to the three progressive holes. It's about 25mm upstream of the idle mixture screw on the carb top and offset off of the throat centerline sligthly. Straight blade slot for a screwdriver and about 10mm in diameter. If these leak air you'll never get rid of the stumble.

Since you have your carbs off at the moment let me mention an easy performance modification you might consider doing now with a burr and a die grinder. The stock throats of the induction manifold on the head are as cast and left extremely crude. If you leave the dual o-ring carrier in place and look at the mating precision of it to the head's throat you'll likely be appalled at the amount of material that should be removed to have a smooth transition for the air to flow past at high speed without encountering severe turbulance. Chamfering the 3mm wide sharp edge away on the head partially around should easily be good for another 10 hp when pulling above 5k rpm. Don't worry about any roughness further down into the throat. That won't matter diddly.
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PostPost by: JACKJABBA » Mon Feb 16, 2004 3:35 pm

Keith, I have taken a look at the carbs and can confirm that mine have plugs inserted and not a screw type plug but a push fit type. They look like anti tamper plugs.
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PostPost by: type26owner » Mon Feb 16, 2004 5:14 pm

Bummer. Having visual access is important on the intial alignment procedure reinstalling the throttle plates. The limit of doing this though is the machining precision achieved back then. The positional tolerance variation between the pattern of progressive holes and the bearing pockets for the throttle spindles most likely is on the order of 200 microns. To balance the airflow by positioning the plates via their location to the first progressive hole technique means to know what the error envelope is beforehand between adjacient throats and to compensate accordingly. Mine are misaligned by that amount or more now that I've just got the plates to close in perfect unison. Means I'll have to document this for later fault diagnosing some number of years from now.

The person that originally assembled the carb altered the shape of one the first progressive holes of one throat to compensate for the positional error oneside to the other. This gets the holes perfectly in sync with the throttle plates. Look for it. One hole will be slightly oblong but the downstream edges will be uncovered by the throttle plates in perfect unison. Few microns error here makes all the difference between idling perfection and total junk with a stumble.
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PostPost by: type26owner » Sat Feb 28, 2004 4:42 pm

Recommend glueing the coverplate into place to seal it vacuum tight. I used Permatex Gasket Eliminator. It's an anaerobic adhesive which is made for sealing large gaps.


I'm retracting this advice. Removing the coverplate is a pain since it will be glued into place with a relatively high strength adhesive which has no solvent. Now recommend appyling a non-hardening gasket cement. The old low tech stuff. The cover comes free with a wetting of laquer thinner or acetone and waiting a few seconds. Cleans up easier too. Butter it around the edges too. Particularly in the flawed area of the carb body sealing surface which is mostly missing.
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PostPost by: type26owner » Sat Mar 20, 2004 4:54 am

Hey Jack,
Check out the section labeled 'Dust-proof carburetors' on page 17 of the Weber Tuning Manual. This is the only reference I've found which might explain the hole in the coverplate. Trouble is it doesn't list the carbs this applies too. Go fish!

With more intimacy with the DCOE_18 I discovered that the cavity under the coverplate actually is indirectly connected to the air space at top of the float bowl. I was right the first time I posted afterall but by only by dumb luck. The passageway is provided via the location clearance fit between the plunger and the housing which is lifted by the throttle plate spindle lever. The plunger then is connected to the accelerator piston assembly which is situated just beside it in the housing. The housing's bore for the piston has an air passage hole to the air space above the piston's range of travel. I suspect this was done intentionally to connect those areas but provide some dampening of pressure waves too. It also allows any fuel which gets past the piston to be returned to the float bowl. More Voodoo stuff.
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PostPost by: DrEntropy » Sat Mar 20, 2004 1:01 pm

I'm currently sitting in th' garage, leanin' on an S-3 Elan, starin' at a poster of Weber carburatori... "a., b,. c,. d.," It all looks sooo ~straightforward~ on th' poster!

More Voodoo stuff.


heh.

Told ya so.
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PostPost by: type26owner » Sat Mar 20, 2004 4:55 pm

Dr E-,
Have the same poster hungup in my office at work. Now I can glaze on it with admiration and no fear! :)
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