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I have an In-line 6 cylinder with three carbs with double barrels.
I have tried to pass emissions by trying to adjust them myself and after my adjustments, it started to clank and backfire. But when I tried to put it back, it still is rough and ((((shakey))))), and backfires due to different fuel mixutures and each cylinder. So far no hope on getting it smooth.
 

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What type of procedure are you using??

You can pass emissions with the triples?? Where do you live?

Anyways, you shouldn't have to mess with any screws if you want to lean out your mixture, and thus reduce emissions. Remember, carbs use jets to meter air/fuel. All you have to do to lean out your mixture is reduce the jet size. Say your running 150 Main Jets, to reduce emissions, just run 140. Adjusting Idle is slightly different, since you have an idle mixture screw to mess with. All you have to do is get the engine warm, and start turning in each screw an equal amount. Try about 1/2 turn at a time. Do this until the engine is just about to die, and then barely turn them out. You want the car to barely idle, but idle well enough to be stable for the test.

In regards to your current situation, you need to start from square one.
First, get a book!!
Second, loosen all the throttle linkages from the main linkage rod. Once they are all loose, begin to turn each idle speed screw in until the engine will run. Now back them all off until you get the engine to run @1000rpm. It may run rough, but they should all be synchronized. Use an air flow meter to make sure each carb is pulling the same amount. Once they are all even, begin messing with the idle mixture screws, by letting more gas in (unscrew these), until the idle smoothes out. Then, go back and adjust the idle speed until it's back to an acceptable idle (800-1000rpm). Keep repeating that procedure until you get a steady idle, and all carbs are sychronized. Then, re-attatch your linkages, and you're done. There are alot of steps missing here, just too many to list. So I highly suggest to bust out some cash and buy a book, or pay someone to do it. You can cause engine damage from this poor running condition.
 
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