Weber Idle Adjustment Problem - Alfa Romeo Bulletin Board & Forums
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post #1 of 76 (permalink) Old 06-07-2012, 06:50 PM Thread Starter
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Weber Idle Adjustment Problem

I've been searching the 'Board for help with my problem and I've found some advice but I'd like to get some confirmation I'm headed in the right direction.

My problem is that I can't get any idle mixture adjustment screw response on cyl #2. The other three cylinders adjust normally and the car actually idles and runs pretty well

The car is a Giulia Super. The carbs are 40DCOE27. Thery have been rebuilt as has the 1600 cc engine. There is good spark, the compression on all four cylinders is good. I've checked for manifold air leakage - none that I can detect. The carbs are properly synched and the idle mixture adjusters of the other three cylinders react as they should, i.e., they are set for peak rpm and adjusting 1/4 turn either way degrades the idle. The car idles at about 800-900 rpm with the idle speed adjustment screw backed out and the butterflies closed. Both butterflies in the offenmding carb open and close in unison. I've had the idle jets out and blown them out.

Removing the inspection cover over the progression ports on cyl #1 with the engine running upsets the idle. Removing the cyl #2 inspection cover has no effect.

I'm guessing there is a blockage somewhere in the idle circuit. I'm prepared to remove the carb once more and disassemble it as blowing air through the ports while the carb was on the car did no good.

1. Before I do that, is there anything else I should be looking at?
2. Assuming I do remove and disassemble the carb, is there a particular area I should be payng the most attention to?

Bob A.
Kenosha, WI
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post #2 of 76 (permalink) Old 06-07-2012, 09:05 PM
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Hi Bob, Send me a PM with your e-mail, and I'll send you a step by step rework of this system. I had a customer with a similar complaint, and reworked the circuit for him to find nothing wrong. He did, however, discover a manifold leak issue after the "bad" Weber was reinstalled. This e-mail contains many photos and commentary and video's.


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post #3 of 76 (permalink) Old 06-18-2018, 12:43 PM
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Hi Raymond, is it still possible to get a hold of that walk through? Going through some similar weber issues with my GTV. Thanks.

-Spencer
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post #4 of 76 (permalink) Old 06-18-2018, 03:25 PM
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Hi Spencer,
I no longer have a file of my e-mails from 2012. If you send me a PM with your e-mail address, we could discuss the issue and possible solutions.


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post #5 of 76 (permalink) Old 06-19-2018, 09:14 AM
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Same here.
Car runs fine otherwise but number 2 cylinder drops out at idle no matter what I try.
I've threaded tiny guitar wire through the idle jet, cleaned the idle screw and blown carb cleaner in openings, etc.
Carbs are synched too. 1, 3 & 4 are all pulling the same amount of air.

'67 Duetto, '74 GTV
Past '78 Alfetta w/ TwinSpark
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post #6 of 76 (permalink) Old 06-20-2018, 03:36 PM
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Gordon, thanks will PM
@1978alfa - does the problematic barrel pull nothing at idle? Mine pulls about 2 psi below the others after balancing.
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post #7 of 76 (permalink) Old 06-20-2018, 04:24 PM
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Idle issues

Check for leaks at the bottom & top of the head/manifold connection and on the rubber mounts. A spray of carb cleaner will change the miss if there`s an air leak.
Also swap the idle jets between #1 & 2. If the miss doesn`t follow the #2 jet into #1 then carb is stopped up.

Soakin in a soultion of 50% white vinegar and 50% water for a few hours should desolve any aluminum corrosion caused by water but blowing high pressure air through the sight bowl might clear the passage after a good soaking...

The Vinegar is the safe way but not as effective as a stronger solution of AlumaBright wheel cleaner that I commonly use..
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post #8 of 76 (permalink) Old 06-20-2018, 09:44 PM
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@zuker81: Yes, mine does the same.
Slightly below the other three cylinders.
I swapped the idle jets around and nothing changed so it is something in the #2 idle circuit.

'67 Duetto, '74 GTV
Past '78 Alfetta w/ TwinSpark
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post #9 of 76 (permalink) Old 06-21-2018, 03:28 AM
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Hi all,
To check rubber mountings, we used to tilt the carb up or down gently while the engine was iddling and listened if the revs changed.
For determining if the failure is in the carb, couldn't you change carbs from position and check again? This is a bigger job obviously.
This is not an advice from a carb specialist but it could work I think.
Rgds,
Thierry
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post #10 of 76 (permalink) Old 06-21-2018, 11:32 AM
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@1978alfa - great to know we have a similar issue. This is my first experience with carbs so it helps me feel sane... I'm going to take some steps this weekend and I'll let you know what results. Let me know if you have any breakthroughs.
@alfasprinty - in my case i am nearly certain the carb mounts were sealed but am going to test further when i reinstall the carb after adding a new intake manifold gasket and thoroughly inspecting my mount o-rings and bolt torques. Switching the carbs proves rather difficult as the linkage is one sided - i'm also not sure on a carb converted previously SPICA set up that the carb mounts will allow it. I am going to look at this though because i might be a decent last resort if after cleaning both carbs the issue remains.
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post #11 of 76 (permalink) Old 08-26-2018, 01:39 PM
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I finally got back to this.
Cooler weather is on the way and I want my duetto to be ready.
Compression check. All 4 cylinders provide readings of 180 psi. Good there.
I blew carb cleaner through and made sure the passage is cleaned out.
I put everything back together and the idle adjustment screw is responding now.
Thank you to Raymond Gordon for the guidance!

My new & improved issue is is my synchrometer is showing slightly uneven amounts of air being pulled through the 1 & 2 throats at idle.
It isn't terrible and the car idles much better now since that passage is cleared.

'67 Duetto, '74 GTV
Past '78 Alfetta w/ TwinSpark

Last edited by 1978alfa; 08-26-2018 at 03:53 PM.
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post #12 of 76 (permalink) Old 09-13-2018, 07:03 AM
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I drove my duetto around this weekend and the same problem presented itself again.
I realized my float levels were incorrectly set by the previous owner's mechanic.
Long story but I figured out he had made a hash of things.
The original push rod operated fuel pump was not keeping the bowls full.
Thus, the engine displayed symptoms of a lean mixture even though the original jets were correct.
This mechanic then installed richer (wrong) idle and main jets and bent the tab on the floats to raise the fuel level in the bowls.
I put a 3 psi electric fuel pump on, put the factory spec main, idle, air corrector and accelerator jets back in.
The car ran better but still ran rich and fouled the plugs sometimes.
Too much fuel in the bowls was flooding the circuits.
I had an unused float in my parts bin so I compared the new one to the ones in my carbs and could see the tabs were bent.
When the engine is cold, it runs pretty well because it's running as if the chokes are open.
Once it warms up, things go south. It idles poorly and you can smell that it's running rich.
Hopefully, the float adjustment will do the trick.
More after I do a road test.

'67 Duetto, '74 GTV
Past '78 Alfetta w/ TwinSpark
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post #13 of 76 (permalink) Old 09-13-2018, 07:37 AM
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The level of fuel in the well must be close to the correct level for best operation. If you remove a main jet stack and shine a light down the hole then you can see the fuel level. You can then make a dipstick from a large nail and lower it until it just touches the fuel. There have been a few posts that give the correct distance to the top of the carburetor. It is 1.80" for most 40DCOE's
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post #14 of 76 (permalink) Old 09-14-2018, 04:18 PM
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I noticed the needle valves are 200s.
Spec is 150 for my 1600.
They were adding fuel to the fire, no pun intended!
To my parts bin!
My weber rebuild kit has rubber tipped needle valves.
Anyone have experience with these?
Personally, I prefer the original brass ones if they're still in good condition.

'67 Duetto, '74 GTV
Past '78 Alfetta w/ TwinSpark
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post #15 of 76 (permalink) Old 09-14-2018, 04:28 PM
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With 26 gram brass floats, using #200 (2.0mm) needles and seats can sometimes make high fuel pressure critical. Stay with 175's with 1600's reserving 200's for race 2L or for newer Webers designed for use with plastic floats. These can allow a little higher fuel pressure without problems.
If the N&S has the spring loaded ball touching the float paddle tip, there is no need for rubber tips on the needle in my experience.


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