Weber Tweaks

[type26owner]

Listers,

I've been sorting my 40 DCOE for several weekends because of a

persistent problem with the idle speed changing from 1000 rpm when cold

to 1500 rpm when hot. Blipping the throttle would usually reduce the rpm
to 1300 only.



I first had to sort several problems which prevented the throttles from

closing completely. The throttle shaft endplates and linkages where

rubbing and causing a sticking situation which made the actual closed

position erratic by about a degree. Another problem which is not so

obvious is there exists a classic over constraint situation with the

rather dodgey linkage design which couples the two throttles together

that is stock on the Elan. To put it simply, the spring loaded button

which opposes the idle adjustment screw on the front carburetor has

enough spring strength to lift the rear carburetors' throttle off of

it's idle adjustment screw which also happens to be the MAIN IDLE

ADJUSTMENT. As you might surmise this is could be bad. What to do? Well

you can bend the linkage to a limited extent to prevent this effect or

stack up washers under the cotter pin to retract the button so it just

clears the tang when both throttles are balanced for air flow rate and
to the desired idle speed. Retract the button fully when adjusting the

idle setting. Adjust the rear carburetor first. BTW, the surface of the

tang on the linkage of the rear carburetor that the idle screw seats

against usually needs to be cut back about 1/16" or the tang fouls on

the carburetor body before the throttle is fully closed. The last issue

is the throttle cable sheath rubbing with several pounds force against

the underside of the bonnet. If you want to see this effect for

yourself, then open the bonnet and have someone open the throttle with

the pedal then push the cable down like when the bonnet is closed. This

rubbing is aggravated by the tangential orientation of the sheath to the
linkage and a severe interference with the bonnet. I spent all of Sunday
modifying the linkage and cable to lessen the interference with the
bonnet. I was able to reorient the cable so it just touches the choke
cable at idle, it seems to be barely acceptable now. Now when the
carburetors are hot and the bonnet is closed the idle is 1200 rpm and
blipping the throttle doesn't make any reduction in rpm. The
modifications I made are way beyond the capabilities of the normal owner
though.



I suspect the mixture is being changed by the carburetor body reaching
their steady state temperature at about 160-180 F. This temperature is
estimated. I can't keep my hand in contact with the carburetors for more
then a couple of seconds. I would not be surprised to learn the fuel is
boiling and the latent heat is preventing the temperature from going
even higher. I dimly remember someone on the list mentioning this
problem but I can't recall a solution being mentioned. I'll try driving
it with the bonnet off and see if that solves the problem. I've got some
bubble sheet which is aluminized on both sides to reduce it's emissivity
to wrap the carburetors and try to reduce the heat load from convection
and radiation. The ambient air going through the carburetors can be
considered a cooling source. Does someone else have a better idea to
cool off the carburetors besides hacking a hole through the fender or a
hood scoop. I really don't want to cover up the carburetors with
smegging bubble wrap. What I need instead is aerogel in a rattle can!

--
Keith Franck

LBNL Engineering

***@***.***

ps: Some construction workers bored a hole through the some of the phone
lines in my town last week. My home ISDN line is still intermittent. So
excuse my use of the lab email.

[gobw2]

How about using a lab thermometers, or your wife's meat thermometer and
some dum - dum to hold it on carb. I would be surprised at 180 unless
sitting in traffic. Which Elan do you have? Mine does not have button or
front screw. How exactly did you modify linkage?

On Mon, 28 Aug 2000 10:01:09 -0700 Keith Franck <***@***.***>
writes:

[type26owner]

I should have a thermocouple laying around somewhere. I'll try to attach
it this week and monitor the temps.

I'm located in the SF Bay Area. We tend to get stuck in traffic a bit
here!

I notched the linkage, bent it and used my gas fluxer and eutectic
solder to change the linkage where the cable attaches in angularity
relative to the throttle axis. Viewed from the front of the car looking
towards the rear, I rotated the the linkage clevis by about 20 degrees
clockwise. Like I said the sheath just clears the choke cable now. I
also relocated the cable end on a swiveling bracket. in addition, I
built a smooth rubbing strip out of 3/8" diameter tubing for the sheath
to rub on instead of the ribs on the underside of the bonnet. This I
fastened to the cam cover studs farthest away from the carburetors. One
might be able to fasten on teflon sheets over the ribs on the bonnet and
do as well. It reduced the bonnet to sheath rubbing enough so it doesn't
hang up noticeably.
--
Keith Franck

LBNL Engineering

***@***.***

[type26owner]

Sorry, I've got a 66 S2.
--
Keith Franck

LBNL Engineering

***@***.***

["William \" Ski]

Keith...

Don't know if I can help much with this .... but one thing I'm pretty sure
of the that there should not be a idle speed screw on the front carb. The
idle (balance) for the front carb is set with the rear carb idle speed screw
and the screw opposite the spring loaded button. Whenever I have had the
same problem with hot idle (Yuma, Phoenix) it has always been caused by drag
in the system. I lube the cable and repack the spindle bearings. I have
also repositioned the lower end of the return spring to give it a little
more umph. Hope this helps a little.

Skip King

'69 Elan S4SE (Weber powered)
'61 Seven S2 (BMC powered)

Measure it with a micrometer
Mark it with a grease pencil
Cut it with an ax

[gobw2]

Yeah, well I forgot to sign off ! George '67 Elan in pieces. (s3) 2-
40DCOE18

On Mon, 28 Aug 2000 17:09:47 -0700 Keith Franck <***@***.***>
writes:

[type26owner]

Skip,
The linkage system is comparable to an electrical circuit which is in
series. The front carburetor does have an idle speed adjustment which is
part of the coupling between the two throttle shafts. It adjusts the
angular phase of the front carburetor throttle shaft relative to the
rear. The screw idle on the rear carburetor adjusts BOTH throttle shafts
simultaneously.

There are two main problems that this setup was to designed accommodate.
First, is the positional errors that occur when fixing each carb body to
the manifold. You should pay attention to the coaxial alignment of the
throttle shafts by adjusting the nuts with the compliant washer
accordingly. Secondly, the actual passage created by the throttle
butterflies to their throats may vary between carb bodies along with the
positional errors of the linkage pieces mean that angular position
between the the throttle must be adjustable over a small range and be
able to uncouple at the idle end of travel for fine idle tuning. My
front carb throttle would close first but the rear throttle needed to
travel about .010" farther but couldn't because the the spring loaded
button or plunger on the shaft to shaft coupling was holding it off of
it's idle screw (over-constrained). The button position must be adjusted
after balancing and idle speed adjustments are made.

If you make these changes as I've described and then readjust the
mixture screws you be pleasantly surprised at how well it purrs. You'll
never get there by applying more force to a system which is sticking and
out of adjustment. I've done that too!

--
Keith Franck

***@***.***

Aston Martin 55' DB2/4 Mk1

Lotus Elan 66' S2

[Lincoln62]

Keith

I had sticking throttle 12 years ago when I bought my S2. My first
thought was to replace the cable. I bought an after market one from a
speed shop. It didn't fix the problem, sticking at idle. I pulled the
Webers apart to see if something was sticking inside...it wasn't. I
tried it with the bonnet off and it was fine so I suspected the cable
again. I decided to try another cable which I bought from a Ford
dealer, it was no better. I made up brackets for extra springs and
spent many hours fiddling to get it to idle, still the same problem.
Would always work fine with the bonnet off. I ended up getting another
cable from a Rover which seemed to be a little more flexible than the
Ford one. Bingo, it worked. I haven't had a problem in the 10 years
since.

I think that because the outer cable moves (with most cars only the
inner moves) it is critical to have a very flexible outer cable. I
didn't realise they vary from brand to brand but they do.

Hope this helps.

Peter
Sydney, Australia

[type26owner]

Peter,
Or the friction coefficient of the plastic covering the cable sheath
against the bonnet was less then the stock cable but that is highly
unlikely. I'm guessing where the extra flexibility really helped was
lessening some binding of the cable in the area of the sheath to linkage
connection because of the interference with the bonnet and/or has
reduced a slip/stick interaction with the bonnet. That is a very clever
solution. Thank you!

BTW, mine was sticking when the bonnet was open. My stock cable had a
bare patch worn through the cover directly adjacent to raised
reinforcing rib on the underside of the bonnet, the one over the exhaust
manifold.


--
Keith Franck

***@***.***

Aston Martin 55' DB2/4 Mk1

Lotus Elan 66' S2

["William \" Ski]

Keith....

Thanks for the explanation ... The part I don't understand is the uncoupling
at idle. When I look at my setup the rear carb seems to always be in
charge. When I try to duplicate your situation of the front throttles
closing first by rotating them open slightly, there is no effect on the rear
throttles since the main throttle return spring is connected to the rear
carb and is much stronger than the button spring on the shaft to shaft
coupling . This action simply compresses the button spring.

Skip

[type26owner]

Skip,
Yours is behaving as one would expect by visually inspecting it.
However, the displacements are so small though they are almost not
discernible to the eye, it's more a matter of feeling if the adjusting
screw solidly homes overtime when you nudge the linkage. You've got to
have the engine running and be able to correlate the nudge with a
noticeable change in rpm or not. This must be done repeatedly with a
blipping of the pedal and with the bonnet closed between each nudge
preferably to determine an average. Opening the bonnet to nudge each
time screws up the results! This is the reason I built and installed the
rubbing guide for the cable and realigned the cable to eliminate the
bonnet as a tuning feature. I've a new appreciation for the level of
delicate adjustments (microns) necessary to get the Webers tuned to
perfection.

Do me a favor and measure the cross sectional diameter of the wire on
the button spring. Also the amount of turns. I can estimate the free
length. You don't happen to have a spring compression gage by chance?

The function that the button primarily does is ensure that the front
throttle closes fully in case the internal throttle closure spring were
to break inside the carb body. I can make the argument it's not really
needed. One should check for a broken internal spring occasionally
because you may not be able to detect a change in the pedals' feel if it
happens. I'm wondering now if a PO or mechanic improved mine with a
super heavy duty spring?

--
Keith Franck

***@***.***

Aston Martin 55' DB2/4 Mk1

Lotus Elan 66' S2

["William \" Ski]

Keith...

Approximately 2.5 turns on the spring. I'll measure the wire diameter at
work tomorrow. Sorry, don't have a spring compression gage for anything that
small.

Skip

["William \" Ski]

Keith....

Sorry, unable to take measurement today.... to many airplanes, not enough
time. Will try again tomorrow.

Skip

[type26owner]

Skip,
No rush. Did my first cold start today. I needed about 1/2" of choke
applied for about 3 blocks, that's new. Got to reprogram my head not to
blip the throttle pedal now. Wow, what a wonderful difference!

I'll bet if I were to wrap the exhaust manifold and expansion chamber to
the bottom with insulating tape it would reduce the temperature of the
carburetor bodies significantly. 75% of the heat load in the engine bay
is contributed by the exhaust, I'm guessing. Does anyone have first hand
experience with the coatings that are advertised for exhaust manifolds
which are supposed to be super insulating?
--
Keith Franck

***@***.***

Aston Martin 55' DB2/4 Mk1

Lotus Elan 66' S2

[gobw2]

Kieth -you are bringing up something I never considered.
Elan exhaust and resonator are on right side of engine, carbs on left -
do you think there is that much heat transfer to the other side, or are
you running another engine with exhaust under carbs? George

On Wed, 30 Aug 2000 23:04:27 +0100 Keith Franck <***@***.***>
writes: