Lotus Elan

Weber problem.

PostPost by: rmd24 » Fri Jan 28, 2011 11:56 am

Not a huge problem but a bit irritating. When opening the throttle gently between 2000 and 3000rpm the engine seems to bog down and stutter before clearing. It only happens in this rev range and only when using gentle throttle opening. Using full boot gives clean acceleration but this can be a bit embarrasing in slow traffic. At the suggestion of a friend who has Webers on a Sunbeam Alpine I have used half a tank of fuel with a carb cleaning product added which seems to have helped somewhat.
I'd welcome some input from anyone with a more intimate knowledge of Weber internals.

Regards,

Roger.
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PostPost by: ardee_selby » Fri Jan 28, 2011 12:55 pm

Roger,
The archives contain all sorts of suggestions to cure similar symptoms. Almost everything has been suggested in both the fuel & ignition systems, including wrong jetting by previous "tuning", dirty fuel, faulty coil, condenser etc. etc. etc. Have a look see.

However, have a look at this thread first:

elan-f14/tune-day-for-webers-t20007.html

I'm sure the guys who posted on it can offer more first-hand advice, and ask the right questions on your paticular set-up/scenario.

Best of luck - rd
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PostPost by: trw99 » Fri Jan 28, 2011 2:47 pm

Roger

Keith Franck used to post on lotuselan.net and you would have a fascinating hour or so reading of his experimentation by doing a search for 'type26owner' on here. It was a great shame that he departed our forum but he seems to have a found a natural home for his enquiring mind.

Tim
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PostPost by: bluesbrother30 » Fri Jan 28, 2011 6:33 pm

Roger.......

Keith has started a forum of his own. If you go to yahoo groups and search for Sidedraft_Central. That is Keiths new home..........well worth the sign up. I drove Sartos weber S-3 car not long ago with set of Keiths ET's. Forth gear roll on from about 1500 rpm.........pulled like a electric motor.

Mike Summers
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PostPost by: elangtv2000 » Fri Jan 28, 2011 10:29 pm

Roger,
What you describe is also commonly associated with the transition circuit in Weber carbs. There are 2-4 (depending on model) very small transition holes in the throats that the throttle plates uncover as they open; this transition to the main circuits is often unbalanced, and can take some time to correct, or improve as best as possible. Any tuner of Lotuses, Alfas, or other cars using Webers is familiar with this and should be able to improve your situation if that is what's ailing you. The current alcohol blend fuels don't help matters, by the way.
Keith Franck's experiments have focused on the transition circuit, emulsion tubes, main jets, and other details, in the attempt to balance each of the progressive stages of the carbs during operation.
Cheers,
Greg Tatarian
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PostPost by: mac5777 » Sat Jan 29, 2011 7:06 am

ardee_selby's link above tells the earlier story of Keith's scientific mind at work.

Last weekend, Keith installed his latest E-Tubes into my webers. They do not look at all like the first set I was running. I thought it could not get any better. The new E-Tube's performance blew me away and on the way home I got 14% better gas mileage even when playing around with the new found power. Keith said that he can improve the MPG and power with further testing and adjustments.
I now can lug the engine in fourth gear to under 900 rpm and give it gas and it pulls smoothly without any hesitation.

Sarto
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PostPost by: gjz30075 » Sat Jan 29, 2011 11:44 am

Sarto, I've read that over at Sidedraft Central and that's fantastic. I'm waiting for him to market his e-tubes so I can experience the same but I think it's not that easy due to all the variables involved. I'll keep following along for now :D

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PostPost by: 65 Lotus » Mon Jan 31, 2011 12:20 pm

Is this somthing that came on suddenly, or a problem that has been happening for a long time? I'd change the main jets to 110 and see what that does.

If the engine "bogs" then then there's too much fuel and it's having to eat it before the car gets on the proper mixture again. It it "stumbles" then it's loo lean and looking for fuel. The difference in the two can sometimes be subtle, but you begin to recognize the difference after a while.

At 2000 - 3000, you're on the main jets, but the air correctors aren't quite in play at those speeds. Changing the jets one direction or the other should tell you what's happening.

I had rich issues at that speed and had to go to 110 mains to fix the problem. I use 145 or 150 air correctors depending on air temps and it runs great.
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PostPost by: Brit01 » Tue Feb 01, 2011 7:23 pm

I had exactly the same issue but on a pair of Dellorto DRLA's from 1988.

I totally refurbished them with a new kit, new washers, new jets, new gaskets, cleaned all the progression holes etc.

Now they work like a dream.

Also fitted a lambda and gauge to see the AFR while driving on the different jets. Very easy to tune after that.

p.s My car is an alfa. sorry. but the issue is the same on whatever car the carbs are on.
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PostPost by: rmd24 » Wed Feb 02, 2011 5:44 pm

Thanks for the input guys. The problem has been there since I acquired the car seven months ago but I've been sorting more pressing items. The car has a big valve head and Vulcan reprofiled cams with light weight cam followers. The PO tells me that the cams are very similar in spec to Sprint cams. The carbs are fitted with 34mm chokes and 125 main jets and his recommendation was to fit 130 jets if the air box and cleaner were removed. At the moment the air box and cleaner are still in place. If there are any other thoughts in the light of this info I would be very glad to hear them.

Regards,

Roger.
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PostPost by: gjz30075 » Wed Feb 02, 2011 6:44 pm

Those chokes are pretty big. On a most stock to Sprint spec engine, you should have anywhere from 30 to 32, maybe 33 chokes. Anything on the engine that warrants 34s?

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PostPost by: billwill » Wed Feb 02, 2011 6:54 pm

No point in trying ANY changes to your jets until you are sure that all the existing holes are blown clear with compressed air and that your float levels are set correctly and that you do not have any air leaks around o-rings and gaskets.
Bill Williams

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PostPost by: elans3 » Wed Feb 02, 2011 7:29 pm

Chokes way too big in my opinion. Should need no more than 32mm. I believe that if you drop to 32mm chokes your bogging down will go away, but your mains and air correctors will need tweaking.

Sounds rather like the S3 S/E problem that Lotus had from the beginning with the jetting of the SE engine.
Very, very fast when revved, but always had a flat spot on steady acceleration.
Got so many complaints that Lotus had to put a cure in the Workshop manual, (drop the chokes from 32mm to 30mm , and change the air correctors).

Steve
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PostPost by: ardee_selby » Thu Feb 03, 2011 10:16 am

Roger / Guys

Ah...Vulcan Engineering....

"The car has a big valve head..."

A Lotus Sprint spec head? or is it one of Vulcan's BV heads? If so, do you know what inlets are fitted? 1.6"? Larger exhaust valves?

"...and Vulcan reprofiled cams with light weight cam followers.The PO tells me that the cams are very similar in spec to Sprint cams"

Could they be "VL107"'s? Whatever they are, they should be marked on the ends.

The carbs are fitted with 34mm chokes and 125 main jets and his recommendation was to fit 130 jets if the air box and cleaner were removed.

Eh? Don't remove the air box/filter! Apart from hot & dirty air getting into the motor, an even worse scenario is fuel getting out of it (spitting fuel if running lean) onto distributor/HT leads!

FWIW, Vulcan have, in the past, recommended the following carb mods to suit a head/cam combination as follows.

34mm chokes, 130 main, 180 air, 45F.9 slow running, 45 acc pump, F16 emulsion.

Vulcan Engineering "Stage 3" head, 1.6 inlets. Vulcan VL107 cams (referred to, by them, as "Sprint) (Spec: 0.345, 0.008"/0.010" 46/70-70/46)

Maybe their recommendations were to get the claimed (140bhp) max power, rather than tractibility. The 34mm chokes are a bit suspect and, as Steve has said, the symptoms are very like the S3 S/E scenario.

Before changing anything on the carb specs I'd check the balance of the carbs and, as already said, carb mounting "o" rings. Then ensure the ignition system is A1 (plugs, timing, advance, etc).

I hope this helps. It sounds as though the PO has tried to, but I don't think removing airbox & filter is a good starting point.

Cheers - rd (BTW - Dordogne? Have you driven up to Domme?)
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PostPost by: rmd24 » Thu Feb 03, 2011 12:11 pm

Thanks for all the detailed info. The PO is an Elan racer (not with my car) so I guess that explains the power as opposed to tractability set up and the idea of removing the air filter was for extra power if required. Shan't do that.
Domme is about half an hour from us, superb views but hideous crowds in summer. The road up to Domme is a bit of fun if there is nobody about. I haven't been up there with the Elan but I had a slightly naughty blast up in a Caterham Fireblade a couple of years ago!
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