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rpm too high

856 Views 4 Replies 2 Participants Last post by  jjj
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.
Doublevision, does this mean I have to keep my timing at the new 12 degrees BTC after I get my rpms down? I'm not following :confused:

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.
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.
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