40DCOE major idle problem

[Biba69]

The pair are on my '75 Alfetta GT. I have an Euro intake set up, 10:1 pistons, and a fairly aggressive 12:1 set of cams.

The problem goes back 28 years. Coming to a stop light the idle is incredibly high even with foot off the accelerator. It is slightly better now, but could be as high as 2800 rpm before very, very slowly returning to around 1100 where this engine prefers it.

I started with a complete Shankle kit (in '80). I rebuilt the engine in the early 90's, no change. I converted to a complete Euro intake, a few years later. No change. About five years ago I drilled a small hole in each of the throttle plates which helped considerably...by comparison. Went only up to 1800 (or so) before slowly returning to idle. I switched to a new Bosch distributor with slightly modified springs for the automatic advance. No change. Later I switched to a MarelliPlex system which overall seemed to give more life to the engine and 'might' have helped the idle a bit.

Less than a year ago I built up another Alfetta engine and installed it. It has 10:1 pistons and I did switched in the 12:1 cam. It now has a stumble at initial acceleration - and I don't mean stomping on it - along with the high idle situation. I've also switched back and forth from the original F8 idle jets to F9's and back. It requires different settings for each, but the results seem to be about the same.

I've ruled out weak springs on the distributor's advance since I've installed two different distributors. I'd sent the MarelliPlex to Jim Steck to reconfigure the advance so as to give it a wider advance curve. Jim knows what he is doing.

I've also ruled out a weak return spring since, with the engine warm then revving it, it sticks around 1800 rpm (though varies) I then pull up on the center shaft and the idle still takes its sweet time returning to a decent idle.

I've also ruled out leaks in the inlet which of course also includes the throttle shafts by using the propane test of opening it and moving it around the intake. It does not accelerate.

Tom Sahines sent me to a Triumph web site and a fellow has gone to great lengths to describe the entire synchronization idle adjustment process along with what to do in a situation like this - which is to drill more holes in the throttle plates.

Yeah, I can drill then solder the throttle plates if it doesn't help or makes it worse, but would be interested in your opinions as to why the carbs simply Will Not allow the engine to return to idle - like it should.

Biba

[pathung]

Were these carbs ever rebuilt/thoroughly cleaned under your care? If not I'd start from there, and make sure all of the carbs' components work as they should, and start tuning from scratch.

[Ennio]

I experienced something like that when I was tuning the carbs (Dellorto 40DHLA) on my car...seemed that I couldn´t get it idle smoothly even though I adjsuted the idle screw. If I accelerated the car would hold a high idle, my mechanic told me to play a bit with the mixture screws, the problem ended. Then as I had a stable idle I used a synchrometer to balance the carbs, then used a colortune to adjust the mixture and viola! the car seems great.
You should rebuilt the carbs (clean them and change the accelerator pump diaphragm, the needle valves all the gaskets) and then balance them using a synchrometer or carbtune, then adjust the mixture on each cylinder. Then I think that by trial and error (better if you have a dyno) try different jets.
Also, before taking the carbs to rebuilt, adjust the timing using a timing light since it would be useless to adjust carbs if the timing is bad.

[Biba69]

I have stripped the carbs down and cleaned them and replaced all gaskets. I will say they looked pristine. I didn't say it but this is my one and only daily driver, so rarely have the carbs sat for long. Please keep in mind, when I first installed the carbs they were brand new, and would not idle decently. So this is not a situation of them getting worse over the years. I really don't put a lot of miles on my Alfetta, though obviously over the years the mileage has increased a fair amount. I do have a synchrometer with a 'snorkel' fitting so I can get it into the Euro lower air box housing. While the needle jumps around a lot, I have them very even across.

There is no diaphragm in the 40DCOE's. I have a good timing light and have it set so at high rpm it goes to the max advance mark.

Trust me, I'm not trying to poke holes in your suggestions, but just explain to you what I have done to try to rectify the terrible idle.

Biba

[Alfar7]

It`s probably the cam timing. I`m sure your cam builder can help with the issue.
As well the Distributor probably needs stronger springs to return the weights to center (non advanced)
Then there`s always the mechanic........

[Chsmadden]

Note: this post assumes a Euro intake as a DCOE/DHLA setup

I've had this exact problem, (except it would run as high as 3500-4000revs/min, and never come down). It turned out to be the balance screw was screwed out quite a distance, allowing cylinders 1 and 2 to stay cracked open a small amount, screwed that back in and that worked, for a little while. Mine will auto-maladjust and after a couple mile drive it's back out of adjustment. Right now a plastic zip-tie is holding them in the proper position so when the screw DOES come out of adj, it can't idle high. In short, for your problem, make sure the throttle closes as smoothly and quickly as when it opens and make sure that the balance screw is turned in until it contacts the other bellcrank on the carb.

This video contains evidence of MY problem(idle high, never come down, similar to yours):
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Just listen at stop signs(most obvious) and at about 7:10-20

[Biba69]

An update: For the first time in 28 years I now have a 'pretty darn decent' idle with my Alfetta GT. It was all done in about 45 minutes. I have always been aware that Spica throttle plates have to be completely closed at idle, but somehow had never realized it is also every bit as critical with carbs - in this case Weber 40 DCOE's. Tom Sahines had sent me to a Triumph (car) site saying not everything applies to Alfa's but there is some good information in (as it turns out) in the 17 printed pages.

This has nothing to do with adjusting carbs but is written by 'TerriAnn' who I assume is British. Since the British have a penchant for naming boys with what in the US we consider to be girls names, I'll let it go at that.

For me th most important paragraph in the entire 17 pages is: "Where just about everyone goes wrong setting up DCOE's is to use the idle lever adjustment screw to adjust idle RPM. More often than not this ends up uncovering the first progression hold at idle. This will cause you to pick the wrong idle jet, or if you have the correct one there will be a lean flat spot right off idle that you will be unable to compensate for."

I set out this time to do everything possible to not crack the idle lever adjustment screw, even a tiny amount. First I double checked that the first (closest to the head) progression hole was covered by the throttle plates and by using a small light verified the second progresson hole was evenly open across the four. Hopefully that makes sense.

I also have a Shankle 'pull type' throttle cable installed and perhaps the most important adjustments I made to make sure the throttle plates completely close is to stretch the compression type return spring to give a bit more presssure (I've since bought a heavier spring 'just in case' but haven't felt the need to use it). I then gave a bit more play to the the other end of the cable (which uses the stock Spica bracket, etc.) at the firewall.

I went in stages of opening the four idle adjustment screws ending up at two and a half turns and it still wouldn't idle properly. Yes, it is supposed to be one or one and a half turns with DCOE's. I'll repeat here that some time ago I drilled each throttle plate with a .77 mm drill bit. Virtually everyone is horrified that I did that, but Pat Braden in his Alfa Bible suggests it as something of a last resort. Jim Steck who I had reprofile (wrong term) my MarelliPlex distributor so I have a much wider curve from idle to max advance, also suggested the drilling of holes. The figures given are to start out at .5 m and if needed drill out to 1.0. .77 mm is about as large as a thick hair and seemed a good compromise. TerriAnn with no fanfare suggests the same eventually going up to a 1.0 and a .50 hole at the bottom of each throttle plate if necessary (do keep in mind they're pretty easy to solder up).

I've been changing back and forth from F8 to F9 idle jets and switched back to the F9's and then had a good idle. All other jets, etc. are stock Shankle circa 1980. I ended up with all idle screws at two and a quarter turns. I don't feel I have the ear or feel to tweak each one slightly to get the 'exact' perfect setting for each. I want to add that TerriAnn says to not press the idle jets all the way on so when screwed in they will seat properly. I for one appreciate information like that.

I have what I believe are Ricambi's AL5408A cams which they give a performance ranking of 10 and are said to be "Similar to an old (1960's) 10.5 Autodelta grind" and have a 12.0 cam lift. They definitely are not good for gas mileage. The cams have no markings on them whatsoever except for the cam timing marks.

Both engines I've had in my Alfetta have preferred a fairly high idle (1200 RPM). This is where it now idles. The engine now Always returns to no higher than 1300 and at long stops can go down to 800 rpm where it still idles, but not quite as smoothly. Cold (even on a 100 degree day) it won't idle at all until the temp is up. My shop/home is near a freeway which ends two miles north of me. By the time I get to the first stop light it idles fine. I have a small flat spot right off idle so perhaps there is a bit more work to do - but for now I'm satisfied.

Richard, it is quite possible my cam timing especially for these cams might not be perfect, but like my using the same adjustment for each of the idle screws, I don't know if I could tell - first which way to make the incremental adjustments - then if it has helped at all. I do have Ricambi's 'cam timing kit' though.

Thank you all for your comments.

Biba

[Biba69]

Sorry forgot. To visit TeriAnn (a gurl): Weber DCOE for Triumphs

[ciscotex]

Congratulations. I ran the 10.5 Autodelta cam on the street eons ago, 2 liter, 40 DCOEs, 36mm venturis and had a sub-1000 rpm idle. FWIW, I believe you might get rid of that flat spot by going up one jet size in the idle jets.

Good work and thanks for posting the link.

Paul