Weber jetting

[daverubberduck]

I have an early version of the +2 with C type cams (single groove) and 40 DCOE31 webers. I bought it as a half complete project so have never had it running perfectly. When I first got it on the road it was running terribly so I checked all the jets, chokes etc. They seemed to be wrong so I changed them based on things I read on this forum. It turned out that my main problem was too much fuel pressure and then worn needle valves. Having fixed both of those it is running much better, though there is a hesitation off idle when blipping the throttle. Plugs seems black even though I have set the mixture with a colour tune. Have also set balance and fuel height at 25mm. So now I am looking again at the jets etc. Below is what was in the carbs and what I changed them to:
________________then____________now
choke:____________30_____________32
main jet:__________130 ___________115
air corrector:_______???___________150 (number was erased on originals)
idle jet:__________45F8__________50F8
accel pump jet:______35____________40

My first thought is to go back to 45F8 idle jet, I will try this. I was wondering what setups other people use because I have found some inconsistencies between the manual and what others have said on here. Below is the relevant page from the manual. Wondering if I should go back to 35 on the pump jet too.

Dave

[seniorchristo]

Hi Dave
I am struggling with off idle hesitation in my Elan as well. In addition to experimenting with the jets, It was suggested to me there is a modification to eliminate the relief valve on the accelerator pump which will increase fuel fed on acceleration. There is also a paragraph in the Lotus Elan shop manual which suggests going to the smaller chokes (30mm?) to eliminate hesitation. Also sidedraft central is a good forum where this is often discussed.

[mbell]

For off idle stumble first thing I'd check is throttle plate position relative to first person hole. You should be able to see the plate edge but the hole shouldn't be exposed on the engine side.

If that's ok, I'd tune for a slightly rich idle rather than perfect idle. I'd run the idle screw out half a turn as a quick test. If not you could use the color tune and watch the could change and pick something blue witha but of orange.

[daverubberduck]

Hi Guys
I found a comment in this thread:
lotus-twincam-f39/rough-running-help-needed-t28637.html
about going to smaller chokes as you say, but I have not been able to find that paragraph in the manual. I have the full +2 manual and I also have the hardback Autobooks manual. Can you point me to the relevant paragraph.

I have checked the throttle plates and they are perfect. I could tune slightly rich as you say, but the plugs are already black which leads me to think that is not the problem.
Dave

[ericbushby]

Hi Dave,
I found it in the Elan manual, (Weber flat spot, S/E engine), Section L, Fuel System, page 54, but of course it may be in a different place in the Plus 2 book.
It suggests fitting 30mm chokes and 200 air corrector jets to remove the hesitation on rapid throttle openings.
They say that it gives better flexible low speed performance at the expense of a small reduction in maximum power. You will lose about 3 HP above 5000 RPM, but I don`t go there much.
I did this some time ago and it worked for me. No flat spot and pulls well at low revs.
My carbs are 151`s
Hope this helps
Eric in Burnley,
S3SE DHC

[nmauduit]

the way I understand it (which is likely far from exhaustive) is, for static conditions (or slow varying)
- "idle" jets govern low revs (via the progression holes, that is when the butterfly is not so opened that suction there is still significant) - yes, they also govern idling, indirectly, via the spring screws, but you usually always can adjust that if the rest is in the correct ball park
- then the main jets take over, usually around 3000rpm (+/- couple hundreds depending on engine specs)
- at higher revs, richness is corrected by air correctors (larger = more air, leaner at high revs)

there is also the accelerator pump, which is to supply extra fuel when pressing suddenly (constant amount, to be added faster if the jet is bigger)

In the case at hand, I would first reduce the "idle" jet from 50 to 45, and retune the idling (not too low even if it would - say 1000). I would not run it overly rich at idling (via the spring screws) since on the road it is mainly the "idle jet" governing low RPM, not the spring screws. Then I would double check with the 35 accelerator pump (in case it gets overly rich when accelerating suddenly).

all that with perfect sparks and timing (I understand the butterflies have been checked for proper idle balance between all tracts, e.g. using a flow meter). I find a lambda sensor quite helpful in that process, but they work a lot better when installed up the exhaust line, which requires a bung.

good luck !

[daverubberduck]

Hi Eric & nmauduit (sorry don't know your name)
I still can't find it in the +2 manual about smaller choke & 200 air corrector, but I think I may try it. Like you Eric I would prefer more torque at lower revs at the expense of a few HP. I may also try to reduce the idle jet to 45, and the accelerator pump to 35. So a few things to try. I have checked the balance with a flow meter and also by looking at the progression holes.
Thanks for your help.
Dave

[daverubberduck]

Just to finish off this thread. I reduced the idle jet and the accelerator pump jet, so my set-up is as per the manual: 32 choke, 115 main jet, 150 air corrector, 45 idle jet and 35 accelerator pump jet. The car now runs well with no hesitation from idle. I should have started with this but I think I had mistakenly used the set-up for baby Elans (see the page from the manual in my first post above). This begs the question why there is a difference between baby Elan and +2.

I am still interested in the option of boosting low down torque by reducing the chokes to 30mm and increasing air correctors to 200. Thanks again Brian for sending me the relevant page from the manual. I think what I will do is run the car in some more and then go to a rolling road and do a comparison of the 2 set-ups. When I do that I will post the results.

Thanks everybody for your input. I'm planning to go to Castle Combe so hopefully see some of you there.
Dave