Weber Flat Spot versus Idle

[simonriley11]

I have recently completely rebuilt my Mk1 Twin Cam for my S3 DHC. The engine has standard valves and camshafts (not S/E spec) and has 40DCOE 31 carburettors with the standard jets. I have set the idle mixtures using a Colourtune and balanced the carbs too and achieved a very nice smooth tickover with correctly coloured plugs. However with gentle throttle input (not using the accelerator pumps) I have a flat spot from about 1500 to 2500 rpm. I have gradually richened the idle mixtures to successfully remove the flat spot (each idle screw is now richened by 7/8 of a turn) so that the car drives really nicely but the tickover is now quite lumpy and the plugs are quite sooty after idling for a while.
Should I be happy that the car drives really nicely but has a lumpy tickover or is it possible to alter the jets to remove the flat spot and have a nice idle too?

[mbell]

I am not an expert* on this but have done some reading.

Normally the flat spot is caused by the fuel mixture going too lean as you transition from the idle circuit to the main circuit. So to counter this you need a bit more fuel in that transition phase. If you adjust the jetting you may have some success or you may just mess the mixture up for the none transitional phases.

I think the common solutions are to rich-en the idle mixture as you have done or raise the fuel level slightly. This has the effect of starting the main circuit slightly earlier so increasing fuel flow in the transition phase. I'd recommend that you make sure the fuel levels are actually set at 25mm as a starting point. Then it's probably a balancing act of rich idle mixture and slight increase to fuel level.

* I am wary of most people who claim to be experts in this area...There is a Yahoo group "Sidedraft Central" where you can find some real experts.

[Chancer]

I would be looking at enlarging or adding to the progression holes, possibly chamfering the edge of the butterflies to get them to cut in earlier.

If your jetting is correct at tickover and above 2500 RPM I think the progression enrichment is the key.

Other possibilities are ignition advance curve and standing wave resonance but thats only likely with wild cams and a 4 into one exhaust, I had a highly tunes allsteel race X/flow that was dreadfull and wouldn't accept full throttle between 1500 and 2250 yet would pull from tickover to 1500 rpm on a wide open throttle in top before bogging down, a correct expansion volume in the silencer immediately after the manifold made it as sweet and tractable as a standard unmodified 1600 non GT engine.

[nigelrbfurness]

If you use the original carburettor settings for S/E cams you will get the flat spot you mention. There was a Lotus service bulletin issued to address this with a minor change in jet sizes and smaller (30?) chokes instead of the 33s that are specified in the workshop manual. I think the cost was 2-3bhp further up the rev range. Be best to check choke sizes as I'm speaking from memory. Later issues of the workshop manual mentioned the the changes, so perhaps you have an early manual. It does fix the flat spot as I did the job on an SE engine with the same problem last year. Incidentally the same thing occurred with the first 907 engines in Jensen Healeys - the fix was similar, though they were on Dellortos.

Nigel F.

[quaybook]

You could try slightly richer idle jets. This will not give you excessive richness at idle because, within the range of the jet, idle mixture is determined by the mixture screw, so you should still be able to achieve your sweet idle, but it will give you a richer mixture over the transition phase. Don't know what your current idle jets are, but the sequence from lean to rich (of jets most probably relevant) is 45F8, 45F9, 50F8, 50 F9.

I wouldn't modify the progression holes if I were you, they are very carefully designed and if you alter them at all and don't get it right there is no way back, you've wrecked your carburettor bodies, an expensive error.

I'm struggling with the same problem, but going to a one step richer idle jet has certainly helped.

Hope this helps.

Vernon

[Craven]

Hi,
Check you have the correct throttle plates in you 31?s, the small hole was added in an attempt to retard the progression, a basic problem with 31?s on a Twincam. Apparently it improved things when engines were new but later the fundamental fault returned giving the lumpy over rich tick over.
FWIW
Ron.

[vincereynard]

As someone has already mentioned somewhere else, these are the chaps to contact for expert Weber chat.

https://groups.yahoo.com/neo/groups/sid ... ntral/info

I seem to remember someone mentioning a company that specialises in modified emulsion tubes and jets to correct the problem you have.

I cannot be ant more specific as it was just something I browsed over, but its in there somewhere!

Vince

[vstibbard]

I think your are referring to the Hypojets, e tubes and an amazingly technical source of sidedraft knowledge.

Keith is the guru who makes them is on Sidedraft central. I've lost the details but a nu,her of members use them on their cars.

Cheers

V

[simonriley11]

Firstly I should correct an error in my original post, my carbs are 40DCOE 18's not 31's. I have now fitted slightly larger idle jets as suggested, I've gone from 45 F9 to 50 F9. I have reset the idle mixtures using the colourtune and each mixture screw has gone in (weakened) about 7/8 of a turn from their position with the original jets. The tickover is now much smoother and there is no flat spot so I have not had to adjust the idle mixtures any further.
I bought the jets from D and B Carburettors in Nottingham UK who were very helpful.

Thanks to everyone for your help.