Weber idle jets

[Brett C]

Well, I have reinstalled my Weber carburettors (40DCOE 76 & 77) following a professional rebuild, and I have adjusted the idle mixture screws to obtain a smooth transition from the idle circuit to the main circuit. This requires 4.75 turns out from closed. This is less than the 5.5 turns that was required before overhaul but more than the 3.5 turns max that the Weber literature suggests. And the car seems overly rich at idle – lots of soot and carbon.

Other details:

•   '73 Australian delivered Spider 2000
•   Ignition system is healthy with timing correct and advance functioning
•   Cam timing and valve clearances are correct
•   Compressions are all good (200 – 210 psi hot)
•   The carburettors are synchronised
•   All jetting is standard (idle jets 0.55 – F17)
•   Idling around 700rpm

My question is; is this idle jet setting typical/normal, should I change the idle jets - if so, to what?

My reading suggests that larger jet would reduce the number of turns required, and a smaller air bleed hole would enrich the transition – am I on the right track?

Or am I over-thinking this and should stop worrying and just drive it!

Brett

[Paul Byrne]

Hi Brett, I hesitate to get into the quagmire of advice on adjusting DCOES. But-- here a a couple of thoughts anyway!
I am assuming that all jets are standard ALFA, as is the emulsion tube. This latter is very important because it is what controls transition to the main circuit- not the so called mixture screws.
First I would get the two carbs synchronised by doing the following:

Unscrew the throttle arm position adjusters so the butterflys can close completely

Remove the four brass caps covering the progression holes

Screw in the adjuster on the rear carby until you can see the edge of the butterfly (throttle plate)     just block the first progression hole (this should be the same for both plates on the one carby-otherwise you have a twisted throttle shaft or plates not centered when tightened to shaft).

Screw further until back edge of plate just clears the hole

Check other carby and adjust linkage joiner screw until plate just clears hole by exactly the same amount (we are talking about just being able to see that plate has cleared first progression hole)

Carbs are now synchronised and are probably set about right for idle.

Now start engine with mixture screws about 2 turns out and adjust for smooth idle.

Only revisit speed adjuster screw if engine will not keep running, or for final trimming of idle speed.

 

[Brett C]

Hi Paul,

Thankyou for your thoughts. Yes all the jets etc are standard for this installation. I set up the two carbs in accordance with the manual then dynamically balanced them with a carb airflow synchroniser, but I will check them with the method you suggest this weekend.

Cheers

Brett

[Paul Byrne]

Hi brett,
Hope it helps. Just to clarify: When I said "Screw further until back edge of plate just clears the hole", I meant that the back edge of the plate should be just coming into view when looking through the hole. ie the first progression hole is still almost completely blocked by the plate, but the back edge has just become visible. Shining a torch into the progression holes area helps, as does very good eyesight, or reading glasses/magnifier etc!
Hope this clarifies what was meant. This approach does seem to work well with our Targa prepared cars but should work ok on a standard setup just as well.
Cheers,
Paul

[El_4_Romeo]

Hi, Brett. You might have jets for the later 'emissions' Webers which are set approximately 4.5 (edit: 3 to 4 turns) out as a baseline. The emissions Webers are distinguishable from the old ones in that the idle jet screws are enclosed. Your idle jets might be either 55F17, 57F21 or 59F21. It might be that the carb covers say 76 and 77 but they could be later type carbs underneath.

[shaggy]

Hi, Brett. You might have jets for the later 'emissions' Webers which are set approximately 4.5 turns out as a baseline. The emissions Webers are distinguishable from the old ones in that the idle jet screws are enclosed. Your idle jets might be either 55F17, 57F21 or 59F21. It might be that the carb covers say 76 and 77 but they could be later type carbs underneath.

I've always heard DCOE40 76 & 77 carbs as emissions Webers, they do in fact use the above mentioned jets (well the 2 sets I have do anyway) and F34 emulsion tubes.

One of the problems I have found is that almost all the literature on Webers refers to the 'classic' webers and doesn't necessarily translate to the emissions versions. Also if you have spare F34 ETs and any of those jets, hang on to them...they are almost impossible to find on the open market these days, I've been looking for jets for these carbs for about 18 months without luck.

[El_4_Romeo]

Ok. Then 4.5 turns out is correct as a baseline. I actually got my F34 ETs and 55F17s from a guy in the US as nobody else had them . I also have the later type carbs (138 and 139) which were standard issue on the '80s cars in South Africa. They had 59F21 idle jets and F41 fitted which were unsuitable for my mods.

[Brett C]

Hi Guys,

Weber applicability info that I have (Dated 1977) list the 76/77 as  2000 GT Spider Europa and specify  55F17 idle jets, which are what is fitted. I've seen 'emissions' carbs referred too but mine have no extra plumbing and look like all the diagrams in the literature. The Idle screws stand proud.

I know the engine in the car is not the original and, but it is the correct prefix and slightly earlier than the one that the car left the factory with. The car only had two owners before me and the second owner had it from '84 and claimed no knowledge of an engine swap so I have no idea of the engine or carbs history.

The more I research Webers the more complicated the subject becomes!

[El_4_Romeo]

Edit: 3 to 4 turns as a baseline (I'll correct above as well). Have a look at post 43 in http://www.alfabb.com/bb/forums/carburetors-fuel-injection-air-intake/328705-what-emmission-weber-4.html#/forumsite/20551/topics/328705?page=5 specifically. I wouldn't worry too much about the number of turns to get a good idle unless you have air leaks somewhere. There are also different air-fuel mixture screws which influence the number of turns to get a decent A/F mixture. As long as it doesn't backfire on the overrun and does not run way too rich, I'd be happy.

If you want good info about emissions type Webers, look for posts by alfaparticle on alfabb.

[shaggy]

Hi Guys,

Weber applicability info that I have (Dated 1977) list the 76/77 as  2000 GT Spider Europa and specify  55F17 idle jets, which are what is fitted. I've seen 'emissions' carbs referred too but mine have no extra plumbing and look like all the diagrams in the literature. The Idle screws stand proud.

I'm no expert, but I think all 2ltr Alfas in Aus that were delivered with Webers have 72/73 or 76/77 carbs. Everyone I've seen has 76/77 actually. I don't think the Aus delivered cars differed much from those in Europe. There are people around locally who know these carbs well, they just don't put the info on the internet...I'm sure most of the club sponsors could point you in the right direction.

That link that El_4_Romeo posted is a good source of information, particularly the tip on sourcing jets etc from ALFA1750 on eBay (looks like he has 55f17 and 55f21 idle jets in stock at the moment)...still looking for some spare f34 tubes tho