Dellorto Idling Frustration - S4 Elan

[William2]

Spent ages setting the float levels to 16mm instead of the previous 14.5mm. It hasn't cured my fast running idle problem!! As a point of interest I couldn't detect any change in overall engine performance with no hesitation or flat spots. I am a bit surprised that changing the float levels didn't manifest itself in some way. I am not sure now whether to leave them at this setting or not. For the record I have 10 gram floats.

[rgh0]

Have you checked the butterfly location versus progression holes through the screw plugs above the progression holes yet ?

cheers
Rohan

[William2]

Hi Rohan, yes I have checked the butterflys with respect to the progression holes and they are all in the same position. I have booked the car in with Max over in Guildford to get him to tune the carbs properly with a manometer, etc and see if he can get to the bottom of it. He said he liked a challenge. It's a strange problem as it is erratic. If you take your foot off the gas pedal sometimes the idle speed will drop back to 800/900 rpm after a few seconds. I am assuming it's not the idle mixture screw settings but I guess he will adjust those as part of the tune up. Would you expect the float height to change the performance??

[alan.barker]

A wild shot.
What if the advance weights are sticking in the dissy.
Alan

[Chancer]

Plausible, you beat me to it!

I had a similar problem with twin 40's with seperate air filters, ie no backplate linking the two, when the throttle was closed sometimes one, sometimes both butterflies would close completely if just one the balance was thrown out and the idle slower, I was constantly rebalancing plus adjusting mixtures sometimes I would get the fast idle problem sometimes stalling.

Intermittent leaks from the O rings or misab plates can do the same, once again when the throttle is closed the carbs may be in a different position to either leak or otherwise.

I had twin dellboys on a Westfield, I could not find O rings for the manifold (later) so in desperation made gaskets and ran the carbs solid, it ran like a jewel, far better than any "properly" mounted carb set up, wnen I sold the vehicle the buyer said he worked in the day for the company that made the manifold (C&T) and told me the O rings were for a Hoover junior dustbag .

Many years later I encountered the vehicle at a club meet, it had been resprayed but I noticed a couple of parts that I had fabricated AKA bodges, the major bodge, the carbs, were still solidly mounted and still in perfect balance!!!!!

[mbell]

Have you checked with the throttle cable removed?

It's common on the +2 rhd for the bonnet to push on the cable and cause drag. Which can cause issues going back to idle. I had to shorten the cable to avoid this. You won't have this issue but might be worth ruling out drag on the throttle cable (and making sure you have good return spring).

[William2]

I am pretty sure the dizzy weights aren't sticking as I have checked it over and made sure they are well oiled. I have also ruled out the throttle cable sticking having carried out lots of checks.

[alan.barker]

You can check when engine is running using a strobe to see if timing returns back.
It will only take 5 minutes to check and no removal of parts. At least you can then tick it off the list.
Alan

[William2]

Lovely sunny day so took the car out for another road test having replaced all the Misab O rings in case I did have a leak at the carburettor flanges. Idling still sticking though, so no improvement. However I stopped the car several times, switched off the engine and immediately restarted it. Every time I did this, the engine went back to the correct 800/900rpm idle. Might this help in trying to get to the bottom of this problem?

[rgh0]

I still believe it is probably related to the throttle position versus the progression holes at Idle. On the early Webers people have drilled holes in the throttle plates to enable them to close fully and cover the progression holes and still admit enough air for idle. I believe Weber have a couple of different throttle plate closing angles and the early ones closed to much. I have not seen the same problem in the Dellortos on my Plus 2 however. s4's did not come with Dellortos orginally so are you confident they are all to the correct Lotus specification. Later Dellortos ( not the ones used by Lotus in the early 70's) had a throttle bypass screw adjustment to enable independent adjustment of throttle plate idle stop position and idle speed to over come this problem

I would try backing off the idle speed adjustment for the butterflies and try opening up the idle mixture screws a little on each throat. it still sounds like you have the butterflies to open and the first progression holes are partially exposed to the manifold vacuum when the butterflies return to the idle stop position. When returning from a higher speed the high manifold vacuum keeps the flow from the first progression hole going and the idle at a high speed but when starting the car the vacuum at the lower speed is not enough to start it flowing so the idle stays at a low speed.

you could test this by backing of the throttle idle screw position a fraction when the car has the fast idle situation and the idle should drop when the progression holes are fully covered

cheers
Rohan
I should be working on my next engine build but its to cold in the garage today

[Craven]

Hi,
I read over and over again, check the position of the butterfly?s, can it be seen through the progression hole.
What these contributors fail to realise the POSITION of the progression drillings within the body varies from type to type.
An example type 18 Weber, the butterfly angle at the 1st progression hole is 77.6 deg. A type 31 Weber, the butterfly angle at the 1st progression hole is 79.5 deg.
Clearly then the air volume passing the butterfly when set at the 1st progression hole, hence engine speed, will be considerable different in the two types of carburettors.
Sighting of the butterfly edge through the progression is a good way to check for matched position with regard balance or twisted spindle.
Ron.

[rgh0]

The DCOE18 butterflies where designed to be fully closed at a different angle to the DCOE31 butterflies to match the different progression hole location. In both cases when very close to fully closed all the progression holes are upstream of the butterfly so that they are not exposed to the manifold vacuum. The first progression hole should be covered by the edge of the butterfly or just upstream of it when the butterfly is a fraction open at idle speed setting.

cheers
Rohan

[Craven]

WRONG. Both use 79 . 3 deg throttle plates.
Ron.

[William2]

Hi Ron and Rohan, thanks for your helpful advice. I have carefully checked the butterfly positions with regard to the progression holes and they do cover them all when shut. The carbs are 40Es' so they have the additional idle bypass circuitry. As you say Rohan maybe adjusting this and the idle screws will cure the problem. The car does also seem to pop a bit at idle. I am taking the car to a Lotus professional mechanic on Wednesday to try and adjust the carbs with proper equipment. I will let you know what transpires. Regards, William

[rgh0]

I maybe wrong in terms of what was originally built into DCOE18 for Lotus by Weber I have not seen enough unmolested DCOE18s if such a thing exists to know---- but to work correctly the DCOE 18 needs to use a different angle throttle plate that matched the progression holes. This confusion is the reason for lots of the issue with hard to set idle and the off idle stumble that many experience.

cheers
Rohan