Lotus Elan

Running issue, advice needed

PostPost by: SeanB » Wed Apr 08, 2020 4:48 pm

Hi Folks, I hope we are all well and staying safe in these strange times.

So my Elan has a slight running issue that I would like some help with. When cruising (very light steady throttle) in 4th at low speed, say up to 50 mph everything is fine. When pushing on through the gears and using a lot of throttle everything is fine. However, If I am cruising at say 70mph with a very small amount of steady throttle, the car starts to hesitate a bit. It feels like there isn't enough fuel. I'm not saying that is the problem, that's just what it feels like. If i put my foot down and push on all is well again. Any ideas?

The engine is a fresh rebuild, super unleaded fuel that isn't old. Brand new 40 Webers, that I balanced at the weekend, new electric fuel pump setup. The fueling was also recently setup by Northampton Motorsport on their rolling road. But when I think back I don't think we did any 70mph in top, small throttle opening runs.

Any help greatly received, thanks in advance

Sean..
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PostPost by: JohnCh » Wed Apr 08, 2020 5:41 pm

Given they are new Webers, how do the progression holes line up with the original fitment DCOEs? I had a similar problem when using new 45 DCOEs on a built up Alfa engine many years ago. The modern DCOEs were one-size fits all and used progression hole alignment that didn't work well with the engine. At light throttle at higher speeds it would go through a lean phase as throttle opening wasn't sufficient to bring on the main circuit, but the idle circuit couldn't handle the fuel requirement.

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PostPost by: SeanB » Wed Apr 08, 2020 5:58 pm

Thanks for the reply. That sounds like exactly the issue, but I’m not much of a carb man I’m afraid. I’ll give Troy at Northampton Motorsport a call and see what he thinks. Maybe just going up one size on something in the idle circuit will do the trick.
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PostPost by: JohnCh » Wed Apr 08, 2020 6:10 pm

I played around with a lot of jets and emulsion tubes and finally reached a stage where it was acceptable. However, I was told the correct fix was to redrill a progression hole in the right spot. That wasn't something I was comfortable doing, and no local shops had experience, so I never tried it.

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PostPost by: benymazz » Wed Apr 08, 2020 8:49 pm

I am going purely off of what I've read as I have the original 40DCOE18s on my S2, but it seems like this is a common problem with the newer Webers - usually described as the "off-idle lean stumble". Apparently the problem is that the progression holes are too far upstream and, as John said, the only "true" fix is to drill another progression hole downstream. You can mask the issue pretty well with a wetter idle jet but beware spark plug fouling if you go down that path.

If you want to really dive into this and get proper information I would recommend joining the "VintageTechnologyGarage" group on groups.io. Keith and others on that group are wizards with this stuff and can provide much better guidance than I can.

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PostPost by: MarkDa » Wed Apr 08, 2020 10:06 pm

It's going to be hard to do much on the road testing at steady 70 mph during the lockdown.
Can't see plod going along with that as an essential journey unless your nearest food store is the other end of a dual carriageway :D :D
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PostPost by: Grizzly » Wed Apr 08, 2020 10:15 pm

I had a very similar issue when i set the idle up to "BEST LEAN IDLE", there are a few methods to fix it but the one that worked for me was to set the idle mixture to 12.5afr which adds enough fuel to cover the big lean spot created. It usually manifests in a off idle sputter but i noticed if your cruising in a high gear with minimal throttle input (the throttles almost closed) your afr will be very lean.

Keith Franks white paper is required reading when your trying to set up Webers.
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PostPost by: Quart Meg Miles » Wed Apr 08, 2020 10:43 pm

I had a misfire at about that speed for years (on Weber 31s) and bought new151s, but this didn't cure it and I my notes say "clearly running weak" and I tried richer main jets without any improvement. So I started modifiying the idler jets, restricting the air holes (with solder and redrilling) and made progress. Am still using them 20 years and many miles later! It was cheaper than buying sets of jets.
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PostPost by: SENC » Wed Apr 08, 2020 10:52 pm

Having just finished Des Hammill's book on Webers and Dellortos in learning about them for my Seven, I can recommend that to you, as well. It is available for Kindle and Kindle apps, too, and quite worthwhile.
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PostPost by: alan.barker » Thu Apr 09, 2020 8:25 am

Hi Sean,
do you still have the old Webers so you can compare the Jets.
Also compare the position of the progression holes
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PostPost by: Grizzly » Thu Apr 09, 2020 9:42 am

Maybe take it back to who ever tuned it?
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PostPost by: SeanB » Thu Apr 09, 2020 10:03 am

Grizzly wrote:Maybe take it back to who ever tuned it?


Yeah that will be my plan. Gonna have a look into this progression hole thing though.
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PostPost by: SeanB » Thu Apr 09, 2020 10:06 am

alan.barker wrote:Hi Sean,
do you still have the old Webers so you can compare the Jets.
Also compare the position of the progression holes
Alan


Yes I do. They are actually up for sale if anybody is interested. I will have a look at the jets and progression holes.

Does anyone have any pictures of their progression holes? Also what jets are folks running on a standard 100bhp engine?

Thanks and stay safe people.

As far as me using the car during lock down, I'm currently using it as my daily driver when going into work as my regular daily driver is only a few hundred miles away from needing it's first running in service and with the dealers all closed, I'm trying to ration myself.
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PostPost by: Andy8421 » Thu Apr 09, 2020 12:24 pm

Slightly off topic, but I have an old book (mid 60s) on carburettor design and tuning. Apparently, high speed light throttle misfire was the bugbear of carburettor designers - to the point it had its own name 'Harry and Willie' - I can only assume Charles and Diana had read the book when naming their children.

The chief cause was a combustion problem, particularly in higher performance engines caused by the dilution of the mixture in the cylinder by exhaust gas . The high levels of inlet vacuum would draw exhaust gas back into the cylinder during the valve overlap period, adversely effecting the relatively small amount of burnable mixture admitted into the cylinder at light throttle. This effect could be overcome by suitable carburation, but was asking even more of the progression circuit than simply maintaining the correct mixture.
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PostPost by: Citromike » Thu Apr 09, 2020 4:34 pm

I don't have direct knowledge of the Webers you are using; my Elan has Zenith Strombergs.

However, a common contributor to this problem is ethanol in modern fuels. It has a cooler burning temperature and less energy density; thus cars that are correctly tuned on the edge of lean can go quite a bit leaner with a 10% or so mix of ethanol, causing the light-throttle symptoms you describe.

On several of my Citroens, with advice from a UK tuner who had the same problems AND a huge box of jets to swap, I was able to resolve the problem by drilling the main jets about 5-8% larger. I do watch repairing so am fortunate to have a tiny drill press and lots of drill bits of the proper size. And I have the jet sizes and instructions.

As mentioned by others, there are multiple variables so be sure you are working in the right area of the carbs! Good luck.
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