Lotus Elan

Weber Carbs Twin 40dcoe

PostPost by: JACKJABBA » Sat Feb 07, 2004 2:48 pm

I have my carbs off at the moment and have noticed that they do not have the accelerator pump opening cover plates and gaskets fitted. Is this normal or could it be the cause of my running problems?

:unsure:

It's the plate that is under the stater device, and is between the two barrel's, facing the inlet manifold.
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PostPost by: fatboyoz » Sat Feb 07, 2004 11:45 pm

My carb's have the plates fitted.
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PostPost by: type26owner » Sun Feb 08, 2004 5:02 pm

Those covers should be installed and air-tight. That cavity is connected via the clearance hole for the accelerator linkage and the internal throttle springs if remember it correctly to the airspace on the top of the float bowl. So are all the jets which draw in needed air to premix with the fuel before being presented to the induction area. Since the carbs are designed to react to minute pressure gradients in that area. It's connected to the airbox by a large aperature through the airbox metal flange. The interior air pressure must be coupled to the induction dynamics for best operating performance. While addressing air leaks on the carb bodies let me point out there is another type to check on. The lead plugs which seal the fuel passages on the outside are prone to suck in air but not leak out any fuel. It doesn't take much of a leak there to completely screw things up.

All the paper gaskets on the DCOE are marginal at best. I highly recommend replacing them with elastomer ones. I make my own up. Including the top round jet cover. There you have to be careful not to allow any compliant gasket to droop down and block off the air passages on top on the main jets.
Good Luck!
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PostPost by: khamai » Sun Feb 08, 2004 5:43 pm

<!--QuoteBegin-JACKJABBA+Feb 7 2004, 02:48 PM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (JACKJABBA @ Feb 7 2004, 02:48 PM)</td></tr><tr><td id='QUOTE'><!--QuoteEBegin--> I have my carbs off at the moment and have noticed that they do not have the accelerator pump opening cover plates and gaskets fitted. Is this normal or could it be the cause of my running problems?

:unsure:

It's the plate that is under the stater device, and is between the two barrel's, facing the inlet manifold. [/quote]
Bit confused here... If it's the cover you've described it does not cover the accelerator pumps, but access to the "sliding valves" for the cold start circuit. If that's the cover you are asking about then it simply keeps dirt & grime from collecting around the cold start sliding valves. If they are missing it won't affect the running immediately. However allowing dirt & grime around the sliding valves may cause them to bind. A lot of the Webers I've rebuilt, these sliding valves are seized due to in-activity. Also, if you do use the circuit a lot, the dirt & grime will cause the sliding valve to wear allowing air to pass by them, thus leaning the mixture.

Bottomline, best to have the covers on, but likely no short term issues.

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PostPost by: JACKJABBA » Sun Feb 08, 2004 7:59 pm

Thanks for the replies. I almost ordered a set of new carbs today, but will now wait.

The carbs have been like this since I baught the car last year. I drove it from St Albans back to Darlington with no problems other than a flat spot at around 2000 rpm and a poor idle. I should have flaged this as the most likely culprit but can not belive that they are missing. Now I just need to get the covers or make some up.


Anyone got some spare? I now just want to get things fixed.

Kiyoshi, it is the metal plate that covers the pump control lever and throttle spindle return spring. It is not the plate that is fitted in place of the starter device, as on some Webers.
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PostPost by: type26owner » Sun Feb 08, 2004 8:07 pm

Since I'm drilling passages through throttle plates today I opened up the cover plate in question. Kiyoshi is correct. There is no passageway connection to the float bowl area. It's sealed off by the float bowl gasket. However there are also no seals on the throttle shafts clearance holes through the carb body. Just a close fit. It could be an issue leaking there. I presently have a buildup of gooey residue just by the clearance holes on the shaft inside the cavity. My apologies for my faulty memory.
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PostPost by: type26owner » Sun Feb 08, 2004 9:16 pm

Upon closer examination of the sealing capablities of that coverplate I just discovered a casting flaw which is identical on both my 40DCOE_18 carbs. On the land which goes around the retangular perimeter upon which the gasket is suppose to home against and seal. Most all away around it's 1.5 mm wide. There is section about 8mm long in the lower left area which is only 0.5mm wide. No gasket is going to stay in place on such a narrow land. I'm concerned because looking on the engine side of the throttle butterflys I've noticed a clean area in a chevron pattern on the wall of the carb body which spreads away from those clearance holes. I've evidently got air leaking in there. Never noticed it before now. Could have used some mentoring from someone else myself.

Bet I'm not the only one with this problem!
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PostPost by: JACKJABBA » Mon Feb 09, 2004 4:16 pm

I have just stopped off at my nearest Weber stockist and he showed me a set of DCOE's. The plate on these actualy had a hole in the centre, the same size as the one in the gasket. He then told me that it is just a dust cover. Back to the drawing board. I was trying not to spend the money on Dyno time but it looks like the only way forward now. :(

Thanks for the input.
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PostPost by: khamai » Mon Feb 09, 2004 5:14 pm

<!--QuoteBegin-JACKJABBA+Feb 9 2004, 04:16 PM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (JACKJABBA @ Feb 9 2004, 04:16 PM)</td></tr><tr><td id='QUOTE'><!--QuoteEBegin--> I have just stopped off at my nearest Weber stockist and he showed me a set of DCOE's. The plate on these actualy had a hole in the centre, the same size as the one in the gasket. He then told me that it is just a dust cover. Back to the drawing board. I was trying not to spend the money on Dyno time but it looks like the only way forward now. :(

Thanks for the input. [/quote]
The hole in the center of the cover plate is the mounting hole (screw goes through it).

Are you chasing the 2k rpm flat spot mentioned in your previous message?

Have you checked the carb soft mounts (leaks, tension)? Syncronization?

2k rpm is at the beginning of the transition from the idle circuit to the mains. Flat spots and stumbling usually indicate a change in the emulsion tubes.

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PostPost by: JACKJABBA » Mon Feb 09, 2004 7:42 pm

Kiyoshi, yes I am trying to get rid of the 2k stumble, but also the car idle is not that great. The best I have had it running caused it to backfire when on the over run usually on downhill sections, but the flatspot was not as pronounced. I have replaced all the o rings on the mounts and checked the gaps on the washers. I have synchonised the carbs using a synchrometer (?50) and they are well balanced. I tried the propane method of leak testing with no change in rpm, and have tried rocking the carbs while running, also with no change. The headlamp vac is good with no leaks, but I will blank it off just to be sure.

I will strip and rebuild the carbs at the weekend and fit the cover plates that were missing (?9 for the two + p&p for the type with no hole) and then try again.

Then it is Dyno time. Its got to be cheaper than new carbs and a new dizzy. :unsure:
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PostPost by: type26owner » Tue Feb 10, 2004 6:26 pm

Jack,
I've probed my induction system with propane too (Weber Manual recommends spraying gasoline) and found no leaks. It is only upon a visual inspection I found it's been leaking air from both the outboard leather seal and that coverplate sides. Obviously this impacts the idle mostly and I'm 100% now sure it also causes the stumble. By idling on the air leaks with the throttle plates completely closed there is a gap of 1-2mm where the plates have to open across before causing any venturi effect upon the first progressive hole. Guaranteed lean mixture stumble if this is the case which cannot be cured by swapping idle or any other type jets. If you fix the air leaks and the plates are still out of position to the first progressive hole then there are replacement plates which close at different angles. I have the stock 79 degrees 30 minutes ones installed now. I just ordered a set of 78 degree ones which should further reduce that gap more if needed. The hardest part of this process is determining the actual problem.

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.
Good Luck!
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PostPost by: cdraper » Wed Feb 11, 2004 5:42 pm

I sufferred from a "hole" at around 2000rpm on a 130s big valve. Changed to 30mm chokes from 33mm and went down a size on the main jet. Much more pleasent to drive, less fuel and the Weber agents tells me I should not have lost more than 3-5 bhp over 6500 rpm. For me a good trade!!
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PostPost by: JACKJABBA » Wed Feb 11, 2004 7:58 pm

I have also dropped down to 30mm chokes and adjusted the jets as recomended by Lotus. But the spot is still there. Rebuild kits came today so will do the carbs Saturday and get it on the Dyno next week.

I will let you know what the discover. Hope its not going to be expensive to put right, but I bet it will be. :unsure:
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PostPost by: khamai » Wed Feb 11, 2004 10:51 pm

The synchronization still bothers me... I'm not familar with the device you used. By experience has not been good with nearly every device that covers the trumpets to do synchronization. Most mask the intake that the rpms fall, giving anything but a good reading. I've resorted to 1 of 2 methods, the old fashion inches of mercury or a mechanical means that allows me to get very close in the paddock at a race.

The later is one that I've never seen written up (guess I should do that!). This method is to remove the all bleed hole caps (brass plugs on the top of each throat) on both carbs. This reveals 2 or 3 bleed holes beneath the caps. Now turn idle speed adjustment screw to open up the butterfly. Looking down through the bleed hole nearest the manifold as you turn the idle screw you'll see the edge of the butterfly pass beneath. Adjust the idle speed screw so the leading edge of the butterfly is aligned in the middle of the bleed hole. Now check the each of the other throats to see if the alignment is identical. If it's off from one carb to the other you need to adjust the synchro. If it's off from one throat to another on the same carb then your butterfly shaft is twisted or the bleed holes are drilled incorrectly.

I've been using this method for years at the track and it's never failed. I've shown it to others and they've come back after using various devices and told me that it's worked better than the devices.

Anyway, I share here for comments...

Cheers!
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PostPost by: JACKJABBA » Sat Feb 14, 2004 10:16 am

Kiyoshi, do you fit new brass plugs after, how did you get them out in the first place?

The synchrometer is on page 195 of the Demon Tweeks 2003 book.

Regards Jack
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