Lotus Elan

Weber choke mechanism removal trap.

PostPost by: au-yt » Sat Apr 06, 2019 11:05 am

I hadn?t posted for quite some time but a recent problem, I wanted share and to post my finding for others.

My Twin Cam powered 7 developed an interesting and initially unfathomable problem.
It would not run on #2 cylinder..
Plugs replaced, leads replaced and compression all good , timing good.
I toatally eliminated any classic electrical issue as the primary cause
All weird as the engine ran but it wasn?t happy.
There was no change to the idle using the mixture screw.

I initial partially stripped the carb to find the blockage.
That didn?t find anything.
Engine run, again same issue, I even sprayed fuel into the intake .. no change.

Second strip I was looking for an air leak thinking there wasn?t any fuel.
Instead the second far more complete strip I discovered the choke piston on #2 jamed in the full up position = extremely rich.
Which when I looked back, I ran the engine up to 4000 revs there was serious black smoke but dismissed it as jetting yet to sort.

The Cause...
I had previously removed the starter mechanisms on the carbs and replaced them with Weber blanks, as I figured I didn?t need them.
What happened was all was good until #2 cylinder coughed badly on start (still getting mixtures sorted)
The pressure from the back fire forced the #2 choke piston into full rich and the spring had not enough force to push it down.
This didn?t present as rich but as just a missfiring cylinder, which caused much frustration....

The jetting I?m adjusting with an Innovate Motorsport LM-2 and even that showed an erratic reading and as it?s my first experience with this unit I dismissed it,
One day I hope this helps someone.
LOTUS Evora 400
Caterham 7 TC
66 one-off 2.6Ltr v8 Mid-engine Berlinetta Coupe........... in restoration
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PostPost by: nigelrbfurness » Sun Apr 07, 2019 8:29 am

That is a very useful post and the sort of thing this forum excels at. Use of a Gibson's opportune may well have revealed this problem too.
1970 S4SE/1760cc big valve/SA-AX block, L2s, 45DCOEs, 1978 Jensen GT, 1962 AH Sprite, Alfa-Romeo 159, 1966 Bristol Bus, 1947 AEC Regal bus.
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PostPost by: RichC » Sun Apr 07, 2019 5:57 pm

I know a lot of people say they never use the choke on their webered twink ...
From cold, even in the summer , i need to pull the choke out half an inch to get any life , and a steady idle .
it goes without saying i use couple or so pumps on the pedal beforehand.
I'm not aware I'm running lean ...... :roll:
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