Lotus Elan

Weber Tuning Questions

PostPost by: Matt7c » Mon Apr 17, 2006 11:18 am

I was tuning my carbs today (twin Weber 40DCOE18). Its amazing how you can get the engine running quite smoothly on just 3 pots. :?

That wasn't the intent of course, but after noting that I could produce no response when adjusting the idle mixture screw on pot 4, I pulled the HT lead and even that produced no difference at all in idle speed. So pot 4 isn't firing (at least at idle). The plug is wet and the spark is clearly present. All the jets are clean and the idle and progression holes are clear. It does seem like fuel is getting to the pot, but maybe not enough. Any suggestions?

Different issue: when using a synchrometer to adjust the throttle spindle setting between the twinned carbs, I found that I can produce an identical volume of air (3.2 kg/h) through pots 2-4, but pot 1 takes significantly more (4.8kg/h). Now since pots 1 and 2 are on the same spindle, I can only assume that the butterfly on pot 1 is somehow set differently to those for pots 2-4. Would you agree with my train of thought?

Matt
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PostPost by: Verve » Mon Apr 17, 2006 9:50 pm

Hi Matt .

I would suggest your spindle needs a very slight "tweak" on your front carb`, to match your number 1 & 2 pots perfectly together ...on memory , it does tell you to do that in the Lotus manual ...it`s simple & easy .

Once you`ve done that ...and calmed down your runaway number 1 , you will have to raise the idle back again , which in turn may help struggling number 4 .

Finally , assuming everything is working perfectly as you`ve said , including valve clearances & compressions ...I found that by taking the head breather to a catch tank , away from number 4 off the airbox ...made everything spot on .

I don`t know if yours is still connected to the airbox ...but number 4 always gets the lion`s share of rubbish from the breathing system , particularly at idle...which in my experience didn`t help it`s cause for great combustion ....which it has now .

Hope this helps ,

Cheers ,

Verve ( Enthusiast not Engineer ; ) .
Matt7c wrote:I was tuning my carbs today (twin Weber 40DCOE18). Its amazing how you can get the engine running quite smoothly on just 3 pots. :?

That wasn't the intent of course, but after noting that I could produce no response when adjusting the idle mixture screw on pot 4, I pulled the HT lead and even that produced no difference at all in idle speed. So pot 4 isn't firing (at least at idle). The plug is wet and the spark is clearly present. All the jets are clean and the idle and progression holes are clear. It does seem like fuel is getting to the pot, but maybe not enough. Any suggestions?

Different issue: when using a synchrometer to adjust the throttle spindle setting between the twinned carbs, I found that I can produce an identical volume of air (3.2 kg/h) through pots 2-4, but pot 1 takes significantly more (4.8kg/h). Now since pots 1 and 2 are on the same spindle, I can only assume that the butterfly on pot 1 is somehow set differently to those for pots 2-4. Would you agree with my train of thought?

Matt
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PostPost by: marcfuller » Mon Apr 17, 2006 11:06 pm

Have you had the carbs apart and inspected the areas which could be contributing to the two problems you mention, #1 and #4. You could have some significant internal mechanical problems in the carbs related to wear and DPO actions.

I think that trying to acurately tune older carbs without being familiar with thier internal condition is risking a great deal of frustration and compounding previous problems.

If you know carbs are in good condition I would follow the step wise procedures in Keith Franck's recently published DCOE tuning whitepaper. Available at - http://autos.groups.yahoo.com/group/sidedraft_central/
-Marc '66 Elan DHC (36/6025)
http://www.lotuselan.us
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PostPost by: alaric » Mon Apr 17, 2006 11:13 pm

Hi. These cars throw up some teasers don't they. My suggestion is to check that there are no air leaks, as this generally makes the mixture setting insensitive and could be the problem with pot 4. Once there are no air leaks, then you should be able to follow the procedure in the book to set the carbs up from scratch. I bought a book from Eurobarbs on ebay that deals with the Dellortos and Webers, and after following that found that I got the engine running very smoothly, and that a quarter turn of any mixture screw was noticeable. Has someone perhaps adjusted the butterfly offset of pot 1 to comensate for pot 4? Just a thought. If you aren't sure of the condition of the carbs I'd get them off and strip them, replacing all the seals etc, then start from scratch with all the settings.

All the best.

Sean.
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PostPost by: Matt7c » Tue Apr 18, 2006 7:45 pm

Thanks for the tips, guys. The carbs were bought fully refurbed (allegedly) about 12 months ago and they do look in top shape. I spent quite some time yesterday studying what was going on, and in the end decided the pot 4 was intermittent at idle, rather than completely defunct. Setting the revs at about 950ish, I found that the revs would drift upwards to 1050 and then back down again. By completely closing the idle screw, or by removing the HT lead for pot 4, the engine would settle at 950 on 3 pots. Only when pot 4 was 'live' did the drift up and down occur. So I think that pot 4 cuts in and out. I'm pretty sure that it isn't ignition as the spark is there and the plugs are in good nick (save for some fouling (probably caused by this problem)). My current working theory is that it is an induction problem causing the mixture weakening - probably an air leak. I'll have the carbs off and have a good look around. If Keith were here, he would probably tell me to check the air seal on the spindle, so that's what I'll do first!
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PostPost by: rgh0 » Wed Apr 19, 2006 11:10 am

Matt

My understanding of Keiths methodology for the DCOE18's idle setup would be as follows.

1. Set up the butterflies so they are all postioned just before before the first progression hole.

2. Adjust the idle mixture screws to get sufficent fuel for a slightly rich idle

3. If the idle speed is OK at these setting then your lucky

4. If idle speed to high then look for air leaks at spindle shaft, vibration mount O rings or around the idle screw.

5. Finally if idle speed still to high investigate different closing angle butterfly plates and drill a hole in the plates to get the combination of right idle mixture, right idle speed and good transition from idle to progression holes.

regards
Rohan
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PostPost by: john66s3 » Fri Apr 21, 2006 8:29 pm

I had a similar problem when I did my Webers. It turned out to be a leak in the vacuum take-off port in the #4 intake runner. It is really hidden back there next to the fire-wall. - John
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PostPost by: Matt7c » Sat Apr 22, 2006 5:54 am

Thanks Rohan and John,

Have house guests this weekend, so the carbs will have to wait until Sunday. I'll report back any findings. However, I already discovered an issue with the vacuum take-off which I need to resolve first, although that is on pot 1. I tried to remove the drain hole on the vacuum (I have a Spyder chassis) so that I could check its seal and the hole valve came away in my hand along with several chunks of filler! Something else the PO omitted to mention. Having removed all the filler, I now have a humungous dent in the vacuum tank and a large hole where the valve was obviously ripped out by the collision. :shock:

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PostPost by: rgh0 » Sat Apr 22, 2006 7:25 am

Matt

Its not nice finding a bodge like that. Have a good look at the welds of the front cross member to the front uprights and the streering rack brackets. The impact that caused the dent in the cross member may have lead to cracking in these.

regards
Rohan
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