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Discussion Starter · #1 ·
I am tuning my carb on my 73 C20. It is a replacement carb- Edelbrock 1406. I tuned it to the previous engine 2 years ago. This year I thought I'd get around to resetting the idle mixture screws with a vacuum gauge (I only drive it 3 times a year- I know pisspoor excuse). I started at 850 rpm and was showing initial manifold vacuum of 16lbs. I brought the screws out by matching half turns until there were no more gains in vacuum. I ended up with 18.5 lbs- maintaining 850rpm all the while. Now I go to give the truck a tune up which calls for 600rpm 4 degrees BTC. Problem is I can't bring the idle down to 600 rpm by backing out the idle screw- the one below the carb linkage on the driver's side I run out of screw; it backs off the cam. How do I get the rpm down?
 

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Disconnect the vacuum advance, plug it. Advance the spark timing to 12 degree`s before top dead center, lock distributor, reconnect vacuum advance, make sure it`s connected to a full time manifold vacuum source (one that pulls vacuum even at idle) This will effectively raise the idle speed, you can then bring it down to the proper idle RPM.
 

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Okay I misunderstood your before post, my apology. However, I have to ask is there a adapter plate in use? Or does the engine have a vacuum leak? If the engine is stock and 16 inches of vacuum is what you read then it suggests there`s a vacuum leak.
 

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Discussion Starter · #5 ·
Double Vision NAILED IT!

I checked to see about the base gasket leaking under the carb and found it was mashed in the front from over-tightening it. A spray bottle of water got the engine stumbling so I cut a new gasket, tightened it sequentially and no more leak. The RPM went up, as did the vacuum. I was able to bring the RPM down to the recommended 600 and the timing in @4� BTDC.

Thanks for the help.
ED
 
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