Lotus Elan and Lotus Elan Plus 2 Forums

Thats more or less how i set it up with the stock Weber jets (although that was a little rich on the progression too) i admit it's a bit of a pain to get right, my biggest problem with the stock jets was tweaking the tip in to come in as the idle jets started to drop off while having the progression afr anywhere near and all that with no stumble / flat spot.

I need to really get some quality time with it, as it stands my best cruise afr is 10.8-11 with all the holes open in the W50's, after a fair bit of reading up i am starting to think the fat mixture is hiding the need for a fourth progression hole to cover an off idle lean spot which is preventing me from getting it running better in the high gear low rpm cruise range.

I also have a very slight twist on my back carb shaft which has introduced some popping on over run, just another thing to fix but i will get there in the end.

Very interesting read. I have just finished my engine overhaul (3rd attempt getting it oil tight) and I'm in the process of fiddling around with carb tune. This time I have fitted an AFR gauge to roughly know what's going on. My Plus 2S is fitted with QED 360 cams and I started with the jets recommended on QEDs data sheet for the cams. Since I have the cams (been in the car since my fist attempt of an engine rebuild approx. 10 years ago) the car never ran very good at idle and low revs. Also I had changed the recommended 125 main jets for 130 in the past, cause the car was running lean on full throttle. With the 45F8 idle jets I get roughly 12.5 AFR, but when accelerating, it drops to 15-16 and the engine clearly leaves out some ignitions. Spent hours and hours setting the idle mixture screw (up to 2 3/4 turns out), but couldn't really cure the stumble between 1000-2500 rpms.

Then I changed the idle jets from 45F8 to 45F6 (F6 draws less air, so mixture should get richer) and lift off from 1000 to lets say 2000-2500 revs has improved. AFR reading is between 10.5/11.5 at idle, but now stays between 12-14 above 2500 rpms. Only in 4th gear running downhill or cruising on an even road it may get to 14-15 AFR.

Then I came across this post and started to check the float level again. I had set the level to 12.5mm (plastic)float to cover incl. seal. This results in 48mm fuel level measured from the round top rim of the jet cover, taking one main jet / emulsion tube out. So minus 16mm, that´ll be 32mm level below top of the carb body.

Reading that 25mm might be ideal, I started raising the fuel level by bending the floats notch and floats itself. In the end I set it to about 10.5mm float to jet cover, resulting now in a fuel level of 44.5mm from round top edge of the jet cover. So that should be 28.5mm fuel level below top of carb body. For this raising of 3.5mm alone quite a lot of bending was necessary. If I would try to set the level to 41mm / 25mm below top of carb body, I would have to bend the floats totally out of their original shape. So I am a bit confused how people reach this 41/25mm level with standard floats AND given the facts nearly all books describe a float level of around 12mm float to cover as ideal, I'm not sure I should go any further here.

Now to the "improvements". After raising the fuel level from 32mm to 28.5mm, the car is running richer at idle. AFR shows more like 10.5. Overall not an improvement, acceleration hasn't really improved with that. So I went back from 45F6 to 45F8. Instantly got an AFR of 12-13 at idle, but then when accelerating, it drops to 16-17, causing lots of stuttering. Overall, raising the fuel level seemed not to help, feels more like it made things worse.

Went back to 45F6 to cure the lean mixture when accelerating from 1000-2500rpms. For now this seems the best I can achieve given the jets I have at hand for now. Went for a drive with mixed throttle situations, at the end full throttle uphill for 1.5km, then quite immediately shut off and looked for the plugs (BPR6S). All whitish.

Any suggestions welcome. Going to 50F8? Change emulsion tubes (F16 are in for now)? Raise level? Set the damn car on fire?

cheers, Matthias

marode wrote:Then I changed the idle jets from 45F8 to 45F6 (F6 draws less air, so mixture should get richer) and lift off from 1000 to lets say 2000-2500 revs has improved. AFR reading is between 10.5/11.5 at idle, but now stays between 12-14 above 2500 rpms. Only in 4th gear running downhill or cruising on an even road it may get to 14-15 AFR.

this sounded like a good way to run though : full idle is not very relevant and would depend on progression holes and the spring screws per barrel, but if you manage to stay within 12 and 14 AFR from 2500 up under load you're not bad at all (may need to check the amount of variation within that range). It is normal to get lean when crusing or lifting going downhill, I would not worry about that.
Then for fine tuning it's hard to do without a rollling road at some point.

marode wrote:f you manage to stay within 12 and 14 AFR from 2500 up under load you're not bad at all

obviously, yes, I would agree. Except the car doesn't feel very powerful at all. Timing is spot on, have checked that with a dial gauge while rebuilding the engine, also ignition is set to 12° BTDC at idle and 26° max at about 2500rpm.

Marode - I really feel for you - frustrating it certainly is. I enclose the jetting saga I have been going through with 450 cams. It may help point you in the right direction!! D

thanks for that! So your emulsion tubes have always been F16?

Hi TBG,
That's really useful... can you share how your carb float height was setup?

In my experience this has a large influence on curing flat spots, and overall fuel consumption.

Thanks
Iain

Measuring fuel level I just put the carb tops in a vice, vertically and measure the two distances, fully up and fully down. I have also used Keith Franks method with an acrylic rod and get the same results. I have had F16 emulsion tubes almost forever, but did dally with Keith's ones for time but eventually found no discernible improvement. Changing/buying lots of jets is frustrating and expensive! I now have a pile of jets sitting in my workshop! D

Tuning webers isn't easy. They have two circuits and the pump jet that all interact.

The "idle" circuit uses vacuum direct from the engine and operates at small throttle openings. The main circuit uses vacuum generated by a venturi which requires good airflow through the carb to generate a vacuum. The switch between circuits varies depending on rpm and throttle position.

The idle screw isn't a primary tuning method, you need to get the right jets in the carb. You need to tube for slow running low throttle. Fast rpm with throttle open and then compromise these to get the cross over period of the two circuits ok enough the drive ability is ok.

That can normally be done by a richer idle jet that gets you a bit more running before it runs out. Or getting the main circuit to work sooner, this is often raising the fuel level or trying larger sure correctors.

marode wrote:

marode wrote:f you manage to stay within 12 and 14 AFR from 2500 up under load you're not bad at all

obviously, yes, I would agree. Except the car doesn't feel very powerful at all. Timing is spot on, have checked that with a dial gauge while rebuilding the engine, also ignition is set to 12° BTDC at idle and 26° max at about 2500rpm.

well, I have about 8° more on my S4se for a total advance of 34° (though later than 2500 : something like 12°@1000 18°@1400 25°@2500 32°@3500 34°@5000 34°@6500)...
as for AFR, my target was 12-13 (trying to stay below 14) when I tuned it on the rolling road

What type of ignition/distributor do you have? I have the original 23D4 distributor, have cleaned and oiled all moving parts. Ignition is by Powerspark. 14 degree max advance is stated in the manual and this is exact what I get with setting static (and idle) ignition set to 12* (12+14=26).

But I´d like to keep the topic more on fluel levels and float bending. So, has anyone here ever gained any significant improvements with altering the fuel level while setting up caburettors or is this only theory?

For myself over many many years I have kept the float levels as stated in the manual. It has worked for me but I am sure there are some of us out there who fiddle with the settings and may have found improvements. D

Setting the fuel level is part of tuning the carb, it changes how far the fuel needs lifting to get in to the engine. So effects the vacuum level needed to lift the fuel and get in into the engine and it interacts with other tuning of the carbs. So it generally set the fuel level and re jet from there.

Imo you need to figure where your lean and from there look at changes to make to address that. Your unlikely to get it in a 12-13 range everywhere, espicially using standard jets, so you need to be prepared to compromise in some areas to get best mixture at others.

I run my carbs are 25mm and tune around that level and use Keith' Franks jets. I also run a 123dizzy with 34 deg advance and aggressive ramp on the curve.

So far so good, I think I understood the theory of lifting fuel and consequently altering the vacuum needed to let the main circuit begin to take over. But what's with my question about the obviously desired fuel level of 25mm and the actual level I got resulting in setting the floats as per the manual. This is quite a difference. Does anyone else have difficulties like I had setting the fuel level between 25 and lets say 30mm below carb body and staying somewhere near the manual recommended figure of 12mm float to cover (vertically)?

marode wrote:Reading that 25mm might be ideal, I started raising the fuel level by bending the floats notch and floats itself. In the end I set it to about 10.5mm float to jet cover, resulting now in a fuel level of 44.5mm from round top edge of the jet cover. So that should be 28.5mm fuel level below top of carb body. For this raising of 3.5mm alone quite a lot of bending was necessary. If I would try to set the level to 41mm / 25mm below top of carb body, I would have to bend the floats totally out of their original shape. So I am a bit confused how people reach this 41/25mm level with standard floats AND given the facts nearly all books describe a float level of around 12mm float to cover as ideal, I'm not sure I should go any further here.

Hi Marode,
I think that if you use the recommended settings by measuring the float position then the fuel level will be
28 to30 mm.
Keith Frank in the states firmly recommends 25mm.
I believe this is correct if using his own improved jets but not necessarily the best if using Weber jets. I have followed Keith`s research and discussion for many years now and I have a great respect for his work and the reported results. I have learnt a lot from him.
However when I tried and persisted with the 25 mm setting with Weber jets I could not get a satisfactory result.
The mixture was always too rich and the life of spark plugs severely shortened.
Following suggestions from Alfadave on this forum I gradually lowered the level and it is now back at 29mm.
The plugs are a reasonable colour and I have not replaced them for a year or so.
I also went to a weaker idle jet.
I was then getting an off idle stumble so I followed Keiths advice and drilled a fourth progression hole downstream of the existing three. Keith sells a jig for this, but I just did it by dead reckoning and careful measurement, By that I mean about two days measuring, thinking and getting up the courage and then 10 minutes to do it.
The results are not perfect but I am happy with the performance and smooth acceleration which I am now getting.
Eric in Burnley
1967 S3S£ DHC