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Discussion Starter #1
1974 Spider converted to dual dcoe 40's by previous owner.
When I close the idle adjust screws on the rear carb the car almost stalls as should be. When I close (or open) the front carb screws nothing happens.
Any thoughts?
 

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The front cylinders aren't firing for some reason and/or the idle circuit on that carb isn't functioning at idle. Does it run on all four when you drive?
Is it running on two or four at idle, if you can tell?
Andrew
 

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Is it running on two or four at idle, if you can tell?
How much does the engine rock at idle? An Alfa engine running on 2 cylinders will dance around quite a bit.
 

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Put a synchroniser on the front and rear carburetters and get them working in unison with the adjuster screw on the linkage.
 

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You can synchronize Webers by removing the brass screws that cover the 3 progression holes. The screws are directly over the butterflies. Shine a flashlight in the hole (open one for each carb) while you open the throttle slightly (You can use the idle speed adjustment). You should be able to see the butterfly move across the progression holes. Use the center linkage screw (the one with the 2 springs) to adjust the linkage so that the butterflies are in the same position in both carbs.

This is probably one of my favorite tips from Pat Braden's last book, "Alfa Romeo All-Alloy Twin Cam Companion: 1954-1994".

Looks like it's still available at: Pat Braden  -  Bentley Publishers - Repair Manuals and Automotive Books

And yes, you will get a much better idle on all 4 cylinders!
Mike
 

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1974 Spider converted to dual dcoe 40's by previous owner.
When I close the idle adjust screws on the rear carb the car almost stalls as should be. When I close (or open) the front carb screws nothing happens.
Any thoughts?
Misaligned throttles
 

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It's not just the synching between the carbs; it's also the mixture. You need to get the fuel screws dialed in too; I would so more important, from my experience.
Also, plenty of carbs by now have gummed-up idle circuits that may not be passing any fuel, especially if they sat for a long time, got dirty, etc.
Andrew
 

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Discussion Starter #8
These carbs don't have screws but plugs over the progression holes. Not sure how to pop those out. I'm a little reluctant to pry them out.
 

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Discussion Starter #9
And yes it does seem like it is running on two cylinders at idle and low rpm. If I pull off a plug wire (either #1 or 2) it doesn't make a difference. Runs ok above 3000 rpm.
 

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71 Berlina 74 GTV 17 Giulia Q4
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how about the throttle shafts, are they worn and sucking air? The butterflys could have been contacting the throttle bodies and dug into them allowing air to get past. Are they bronze or steel throttle shafts? The bronze shafts tend to bend over the years. Float levels are very important. Then again a good cleaning and setup might be all that's needed.
 

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I agree with your reluctance to pop out the plugs on the progression holes.

Based on personal experience, there is a very good chance that your carbs are way out of sync. Yes, you may also have plugged up jets and sticking butterflies, but if you can remove 2 spark plug wires and nothing changes, I'm guessing the front butterflies are closed.
The screw in the middle of the two carbs with the two springs is what adjusts one carb to the other. Make a note of where it is now (the angle of the slot) and try one full turn in. I'm guessing that your engine will idle faster. If nothing changes, try 1 turn the other way. If nothing happens either way, you probably have some other problem. You can slow the idle back down by loosening the idle speed screw that is just to the left of the sync screw. Then try fine tuning the sync screw in or out until you get the fastest idle. That should get you in the ballpark of synchronizing. Once that is good, then fine tune the 4 mixture screws. Changing the idle speed effects the mixture settings, so close them then turn all 4 1.5 turns out. Adjust each .5 turn either way to get the fastest idle, then .25 turns using the idle speed screw to slow the engine back down to 900 - 1,000 rpms.

And, once your car idles well enough you may want to track down the synchronizer that Stuart has recommended...

Good luck!
Mike
 

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71 Berlina 74 GTV 17 Giulia Q4
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might want to start with clean plugs to have a known baseline. And double 10-4 on the idle screws affecting the mixture. Years ago I had set my 59 Veloce for the best idle then when I hit 6 grand it sounded like someone was under my hood with a shotgun the fireing back was so loud. It had worn shafts and I just had to put up with a little rough idle and enjoy the top end.
 
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