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High Idle, At wits end!!


StewyN

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So this is semi connected to some of my older posts, i'm just tying it all together and trying to wrap  it up. To restate ( and for those new to my topics ) I recently found out I was running on just one coil, fixed that problem and now it idles pretty high. Ive reset my timing (had a problem with my auto choke, got that figured out [ i think lol] ) reset timing again, but there's still a high idle. Idle screw is backed off as much as possible, timing is 3 btd.... If this helps you any, when its in gear and i press the brake slightly it rev's down about where it should be (sounds like it anyway) but push the clutch in and it rev's back up... Where do i go from here? Your help is greatly appreciated:)

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you dont need a diagramn just make sure all spots that are hoses are plugged or connected. if so also try and use propane around the intake manifold and were the carb attaches if your idle raises from the propane you found your leak. there might be a idle air mix screw on side of carb you might have it backed out to far letting air in

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I find vacuum leaks with a stethoscope.  take the diaphragm off the end, and use the loose hose end.  Get it close to the mounting plate of the carb, and the manifold gasket looking for a sucking sound.  They even sell them at some auto parts stores.  But could this just be too large of an idle jet?  or even a damaged carb?

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Check throttle cable has some slack in it and throttle can freely return to idle.

 

Pull the throttle towards the closed position.... does it move? If so find out why it hangs up and does not close. Possible causes are the throttle shaft is worn and the throttle plate is catching the side of the barrel.

 

Look down the secondary barrel with a flash light. It should be completely closed. Dirt or (something) could have gotten caught the last time the secondary barrel was kicked in and it's jammed. With a pencil gently try to push down on it to close it. Try opening and closing the linkage on the back side of the secondary. You will need to hold the throttle over half way open because of the interlock. My Z24 secondary was simply stuck from sitting and has a slightly high idle. This was the cause. I pushed it closed and it promptly stalled because I had all the adjustments turned down.

 

Check the fast idle cam is completely out of the way of the closing throttle and not holding it open.

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I notice you have an '85 truck.  Is this a Cali truck (6-wire carb) or a federal truck (2-wire carb)?

 

That said, if the idle screw is backed out all the way and it still idles, my guess would be more toward a linkage problem (like DM was looking for), rather than a vacuum leak.  An engine can't run without fuel, and you can't get fuel without air going through the carburetor the right way.

 

Easy way to check: If you have the truck idling, and then you cover both barrels of the carb with a gloved hand, or a towel, or something else that's not going to accidentally get sucked in, the engine should stall.  To follow up on datzenmike's test, if you idle and the engine stalls when you cover the _secondary_ barrel (the one closest to the exhaust side), then your secondary isn't closing properly.  If it only stalls when you cover the primary, then the primary isn't closing as far as it's supposed to.

 

If you can cover both barrels and the engine still runs, I don't know what to say...

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Yes and no.

 

At proper idle, the throttle plate in below the port that sends the vacuum signal to the distributor. So at idle there is no vacuum signal. Once the throttle is opened slightly the throttle plate rises above the port and intake vacuum is sent to the distributor.

 

The fact that you have a vacuum signal tells me that your throttle plate is not fully closed to the idle position.

 

Again... check that the throttle cable is loose enough to allow the carb to idle properly

Check the the fast idle cam is not in the way of the throttle, preventing it from closing to the idle position.

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The fast idle cam should be rotated out of the way when the choke is off and the engine warmed up.

 

 

Hold the throttle open slightly and open and close the choke plate. You will notice the fast idle cam drops down into position as the choke plate is closed, and is rotated up out of the way when the choke is opened. You may have to bend the linkage connecting it to the choke it make it lift higher and out of the way.

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After poking around found out my valve cover was very loose, tightened it down. This helped, some, but still vacuum advance at the distributor. Checked the idle cam, choke wide open, the cam rests on the very last step. To be clear this is NOT right? cam's supposed to be completely clear of the screew?

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I got it to where the cam swings clear of the screw, it defiantly was a major part of the problem. It idles beautifully, but there's still a bit of advance at the distributor, 3 btd without advance, with it it's at about 8-9 btd. Any thoughts on where i should look next? Your help is greatly appreciated. 

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On most carburettors, the vacuum advance is from a port in the venturi, just above the throttle plate at idle.  At an idle, with the port above the throttle, no vacuum at the port, no vacuum advance.  If you open the throttle slightly, that port is now below the throttle plate and the port is exposed to manifold vacuum, and the distributor advances.

 

Everything is interrelated.  If the idle is too fast, mechanical advance can start coming in.  If the throttle is open too far, the distributor gets  vacuum advance, and you end up retarding the timing, to get the "idle" timing correct.  The retarded timing needs more throttle to run the engine.   The open throttle slightly opens the vacuum advance port, ETC, ETC, ETC.

 

Adjust the valves, make sure the timing is set below a speed the mechanical advance comes in,  Make sure the timing is set, taking intio account the vacuum advance.  Some engines require you unplug the vacuum advance from the distributor, and plug the line, and then set the idle timing.   Oh, yea, the engine has to be completely warm to set the timing, and adjust the carb.

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Remember when setting idle speed the engine has to be at normal operating temperature, warmed up.

 

Disconnect the wire going to the automatic choke.  Does it idle slower now.  If so either the automatic choke is adjusted to tight or it is malfunctioning.  Is there power at the automatic choke wire with the ignition on?

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