Lotus Elan

Weber 40DCOE - Crossflow Engine (Lotus Seven)

PostPost by: TheKid#99 » Thu Nov 09, 2023 10:44 pm

Hi everyone,

I got my weber carbs rebuilt and having an issue with the idle. When I start the car, it revs up to over 3000rpm's, it runs smooth though, no back fire through the exhaust or the carbs.

Current jettings:
120 Main
150 Air
45 Idle
30mm Venturi
F15 Emulsion Tube
40 Pump Jet

My Lotus Seven S4 has been bored out. 1700cc crossflow kent engine 135HP-140HP. My father and I are lost, we have called Ray at RD, Ken at Dave Bean and everyone we know to try and help us. Have any of you experinced this with webers on the Twin Cam or other engines?
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PostPost by: h20hamelan » Thu Nov 09, 2023 10:55 pm

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PostPost by: gjz30075 » Fri Nov 10, 2023 12:35 am

Sounds like a massive vacuum leak. How are the 'O' rings on the mounting plates?
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PostPost by: TheKid#99 » Fri Nov 10, 2023 12:54 am

I tried the new metal mounts with the o ring and I tried my old plastic mounts. I'm using cup washers and cushions. Same result, 3000 rpm.

Their isn't any leaks as far as I know around the manifold. It may be the timing...? I'll check that tomorrow.
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PostPost by: nmauduit » Fri Nov 10, 2023 10:17 am

TheKid#99 wrote:I tried the new metal mounts with the o ring and I tried my old plastic mounts. I'm using cup washers and cushions. Same result, 3000 rpm.

Their isn't any leaks as far as I know around the manifold. It may be the timing...? I'll check that tomorrow.


before messing with the timing (esp. if running correctly before) I would spend some time looking for leaks (e.g. using a spray can of starter fluid) or if both butterflies of each carb is actually closing fully and symetrically.
Last edited by nmauduit on Fri Nov 10, 2023 2:43 pm, edited 1 time in total.
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PostPost by: gjz30075 » Fri Nov 10, 2023 10:33 am

^^^^^ I agree. Are these carbs the 151 type? If so, make sure each throat's air bleed is fully seated.

Check this thread:
viewtopic.php?f=40&t=49536
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PostPost by: Craven » Fri Nov 10, 2023 12:52 pm

Check your cold start/choke is fully closed, clearly lots of air coming from somewhere.
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PostPost by: promotor » Fri Nov 10, 2023 12:57 pm

First things first:

Set both carb butterflys to be open exactly the same amount as each other.
Balance throttle linkages so that the carbs match each other in terms of synchronised opening.
Check throttle stop screw - carb butterflys may be open more than needed causing high-speed running.
Check idle mixture screws.
Check ignition timing.
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PostPost by: elans3 » Fri Nov 10, 2023 1:09 pm

As above, sounds like a balancing & throttle stop issue. One carb's butterflies are opening more than the others, ie two on one carb slightly open at idle. Even if you remove a pair of carbs, and i minute later put them back on without touching anything, they'll need rebalancing , due to re-tightening of the mounts, and not being able to get them exactly the same tightness as they were.
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PostPost by: TheKid#99 » Fri Nov 10, 2023 4:57 pm

They are real Italian made webers. I took them off and took them down to a friend of mine who is the expert weber guy at a restoration shop about 30min away from my house. He said he would look at them this weekend for us.
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PostPost by: SENC » Sat Nov 11, 2023 1:15 pm

I've also found the idle mixture screws can be a source of air leaks. First, confirm all 4 are the same shape (differences occurred over the years). Then, use some fuel lube (like EZTurn) on the threads. I also use a light smear of EZTurn on the mounting o-rings
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PostPost by: TheKid#99 » Tue Nov 14, 2023 8:31 pm

According to my mechanic who is very familiar with webers. The throttle stops were holding it open and they had to modify them to make them close. Not exactly sure what they did since I haven't picked them up yet, but I knew something wasn't correct.
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PostPost by: TheKid#99 » Thu Nov 16, 2023 11:15 pm

My mechanic trimmed the throttle stops in order to have the carbs close all the way. Problem solved, just need to get the mixture correct now.
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PostPost by: promotor » Sat Nov 18, 2023 5:50 am

TheKid#99 wrote:My mechanic trimmed the throttle stops in order to have the carbs close all the way. Problem solved, just need to get the mixture correct now.


Can you post photos of what was done as I'm confused why the throttle stops needed to be trimmed, rather than the throttle stop screw just being adjusted/backed off, as should be the case?

My initial thought: it seems like there may be a mismatch of parts somewhere.

Otherwise, if not, perhaps there is an opportunity for me to learn something!
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PostPost by: TheKid#99 » Sat Nov 18, 2023 6:07 am

Yes, I do believe that the company I used to rebuild them used their own throttle stops and not the ones we had on it.

My mechanic just shaved off the top of the throttle stops to make them close, that’s it.
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