355 idle issues

[78longbed4x4]

Hey guys I have a 78 Chevy 4x4 with a 355, victor jr intake, Holley 670 street avenger, accel distributor headers and all that good stuff. Now I can't seem to get it to idle correctly, it's too high. If I back the timing off a little it will idle down where it normally would but then it won't run right, it stumbles and acts like it's too rich. As soon as I advance it a little to where it will run good again it idles high. I've checked for vacuum leaks and I've tried to adjust the air and fuel mixture and it just won't idle where it's supposed to. Should I maybe rotate my plug wires on the distributor one way or the other? I'm usually pretty good about keeping the ole girl runnin good but this had me baffled.

[FlatBlack77]

did you adjust the idle adjustment screw?

[78longbed4x4]

Yep...it's backed all the way out and it still wants to idle about 1200 or so

[bd]

How 'high' is 'too high?'  Is the cold fast idle screw clearing the fast idle cam when the choke is fully open?  Has it ever idled correctly since you've owned it?

[78longbed4x4]

When I had a 750 edelbrock on i never had any problems. And I never had any problems with this Holley either until recently. And no I don't think I've messed with the fast idle screw

[bd]

So, what is the idle RPM?  Electric or manual choke?  Inspect the fast idle cam and verify that it isn't sticking on the low step.  How did you go about checking for vacuum leaks?  Have you made any recent changes that coincide with the high idle?

[78longbed4x4]

It's an electric choke and it idles about 1300 or so. I used carb cleaned and sprayed around all the vacuum hoses and around where the carb and intake fit together and I never heard any changes in idle so I don't think it's a vacuum leak. When I changed the fuel filter a few weeks ago is when I noticed it idled high, even after I adjusted the carb.

[bd]

What happens if you disconnect the throttle cable from the carb?

[78longbed4x4]

I never thought of that. I'll try that tomorrow after work and see what happens. It could be just a linkage problem

[bd]

While you're there, point your eyeball at the choke cam as well to make sure the choke linkage isn't involved.  Let us know what you find and we'll go from there.

[rich weyand]

Did you mess with the distributor lately?  Did you clamp it down tight when you were done?  What is your base timing set at?  Have you checked it to see that it is still where you set it?  Is your vacuum advance correctly plugged into manifold vacuum or do you have it plugged into timed (ported) vacuum?

[DnStClr]

What are you thinkin Rich, that maybe the throttle plates aren't closing?

[bd]

What are you thinkin Rich, that maybe the throttle plates aren't closing?

The simplest things are the easiest to overlook.  An engine needs air to run.  1200 - 1300 RPM is a significant amount of air for an idle speed screw that is backed all the way out.  That air is either pushing past the throttle plates or entering through a vacuum leak.  Since the OP has already sprayed for vacuum leaks the next fundamental is determining whether the throttle plates are in fact being held open.  Issues with the fast idle cam and throttle control are so basic they are easily missed.  There are other problems that can affect throttle closing, too, but why not start with the easiest to check? 

So, right or wrong, Don, that's what I'm thinkin....

[78longbed4x4]

Ok guys so I disconnected the throttle cable and it didn't make any difference but I got to messin with it and I found the throttle return spring was a little stretched so I went to Oreillys and got a new spring and it idles pretty nice now, about 800 or so.

[DnStClr]

Awesome. Another square back on the road!