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Very High Idle


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So i have had a high idle for quite sometime now and just changed my spark plugs and tried to change the idle but now have mad it worse. i can put it in 3rd gear and not give it throttle and cruise down the road. i can work on four barrel carbs but know nothing about these. Any thoughts, only screw i know of is the throttle screw.. here is the carb i have.

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I just fixed a high idle on my 85, by making sure the large tube to the bottom of the air cleaner assy was fully seated. I'd bet that your vaccum lines aren't up to snuff. Spend the afternoon and go thru all of them. You should have a vaccum diagram for your 720 on the inside of the hood.

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When I bought mine my fast idle was all screwed up, I'd start the truck up with this insane idle (didn't have a tach back then so not sure exacts). We pulled the carburetor off twice just to adjust it. The base of mine was also really lose from whoever owned it before me.

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The fast idle adjustment shouldn't need to be touched. It is set at the factory to open the primary a set amount. This will give a fast idle if 1,800 to 2,200 RPMs. Without a tach to tell you the RPMs, simply thinking it seems high is just an opinion.


As mentioned check that when warmed up the choke is fully open or off. If the choke is partly on the fast idle cam may be also.


Hold the throttle half way open and close and open the choke plate. You should see the fast idle cam drop down when the choke is closed and rise up out of the way when the choke is opened.


WD-40 or use carb cleaner on all the linkages around the fast idle cam to clean and lube them.


Once you are sure that the fast idle cam is not causing this check that the throttle cable is not too tight and holding the throttle open slightly.


Next look down the secondary barrel with a flash light. The throttle plate should be fully closed but possibly something is caught in it or it is just stuck.

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