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I have a 67 Duetto with the Twin Weber's. I purchased a new set several years ago, so now they are "Emission". I have never screwed with the Emission screws. I thought I had them balanced last year but this year it is even better. The idle comes down quickly, no slightly hi then slowly going to idle. My Air/Fuel screws are at 2 turns. No sputtering or back firing when letting off the gas. Is 2 turns too much, the spark plugs show a little rich but no buildup, a small grey at the very tips. I have not done a spark plug test othen than at idle but looking for input.

I will do a spark plug test going about 4,000 RPM and shutting the key off. I have not found an uphill hill and a place to pull over once the key is shut off.

Al
 

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Hi Al,
I can't comment directly on your question about how many turns, but if your plugs are looking good after running at idle, and the behaviour at idle, and in small transitions coming off closed throttle, are good, then you're probably close. You can use tools like the transparent spark plug (Colortune) to set the idle mixture (only), or you can plumb in an O2 sensor and LED readout, which can be very useful for the entire range. And just to point it out, that 4000 rpm going uphill is definitely in the main circuit - your idle mixture settings are irrelevant there. If you want to do spark plug cut tests in that sort of condition, you can also hold the car back with your left foot on the brake. I know, it sounds "interesting", but I've done it.
Cheers
Neil
 

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A plug chop at 4000 will tell you more about your main jets than anything else. The idle adjustment screws only influence the mixture at low rpm. Emissions carbs require more turns out than classic Webers. I would not be concerned about 2 turns out if your car idles well and pulls cleanly off idle.
 
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