I'm really getting sick of this idle problem. I recently changed intakes, and I've checked for any vacuum leaks, and I can't find ANY.

What is happening is this: When I THINK I have my idle set at 800 RPMs at idle, I'll take it around the block, and when I stop at a light, the idle will change to 1000. Then if I give the throttle a little "blip", the idle may decrease back to 800 again- sometimes not.

I popped open the hood to check the linkage, and it appears that the primaries are not closing fully. I tightened the return spring, but no dice - same deal. If I push the carb linkage towards closed with my finger, It seems to move maybe an additional 1/16 of and inch, and it will return to 800 again.

I looked closer today, and my idle adjustment screw isn't even touching the stop on the throttle linkage, and I know that's not normal. Is it right to assume the primaries should be "cracked" open slightly in order to get enough air through the carb to maintain an idle?? Do I just need to increase the idle screw till it contacts the linkage and retard the timing till I can maintain proper idle at 800?? I'm stumped on this one... I'm positive that the idle setscrew contacted the carb linkage before. Maybe the reason it's sticking is because I'm now trying to close it down further than before??

Any suggestions appreciated.

BTW it's a CCC Q-jet.

 

It seems that you have a vacuum leak.
You shouldn't be able to idle @800 with zero throttle opening.

 

Very common problem with carbs, but you pretty much know the problem.

You might need to clean the throttle shaft(s).

Some times you need to take a fine grit sanding the bores where the throttle blade seat. Over time a ridge can form.

You might verify that the throttle cable isn't the culprit.

I'm having the same issues with my NEW Holley, but haven’t gotten around to correcting it yet. But it's suppose to be a beautiful day tomorrow

BTW: mine is also about 1/16, and worth 200rpm. All it takes is a little blip, so I know where your coming from. Need to get rid of the little bit of risistance on the closing. The funny thing is I even for the twin springs that suppose to help with this.

Ron

 

Yea those guys hit it for a qjet. Note though for roterry on a Holley the fast idle cam causes this and the adjusting screw behind the choke needs to be turrned out a bit.

 

Thanks for the replies...

305sbc - The first thing I thought about was a vacuum leak since I just changed the intake, but I was seriously careful when installing the intake gaskets and made sure the "china wall" gaskets stayed in place and were sealed w/silicone. I also checked it by spraying carb cleaner around all seams and it seems fine. Plus the engine runs smoothly w/out surging.

Ron - Yeah, maybe I do prettty much know what the problem is...I just don't know why I suddenly need to close the throttle blades more than they used to be. I thought about lightly sanding the carb bores as you mention, and I may still do that! Aside from the idle problem, off idle it runs awesome for a 305. Good call about checking the throttle cable because I had to bend the cable bracket slightly since the carb sits about an inch higher on this intake. But that's OK. I've disconnected the cable while idling and it did'nt make a difference. I have twin return springs also - and I even shortened them a little - but it didn't help.

I'm going to tinker again with the timing/idle to see if I can tune this out....if not, I may be pulling the carb. I'll post the results - just hope it works.

 

Intake gasket is not so likely.. check the lines.
Best way I know to do this is simply plug all vacuum lines at the source.... the brake-line port in the rear of the carb, the PCV port on the front of the carb, the two small ports beside the PCV, and any other ports on your intake. Start it up and run it simulate cruise and driving and see if it still happens.
If it's cured then it sure looks like a vacuum leak and you can begin the elimination process of vacuum lines.... sometimes turns out to be dry-rotted rubber diaphrams in brake-boosters, vacuum-advance canisters, EGR-valves, and sometimes just a sticky PCV valve.
If the problem is still there, then it points to the throttle shaft.
Well this is how I'd test it anyway.
good luck

 

'SSC' I don't have a choke on this Holley carb #9381...
It just doesn't close aaallll the way, if I let off the throttle slowly.
It's probably still a little tight, since it's new.

Hey 'Confuzed1' you might shot the throttle shaft with some gumout to get rid of any grime. Don't lube from the outside though.
I'm assuming it's not closing all the way, and not needing to close more cause of a vacuum leak since you have to manualy close it that final 1/16 or so to the throttle stop.

ummm - sometimes bolting & unbolting the carb can cause a slight warpage to the base??? this could cause a slight hangup in the throttle shaft. That's something to think about also. Make sure it's not over torqued.

Ron

 

I'm embarassed to post this, but it was something really ....

I was just getting ready to get the timing light out, get the distibuter wrench and check the timing - along with checking dwell on the carb.
I then looked real closely at the length of the throttle cable and noticed that it definitley wasn't binding , cause there was a little play left in it when I forced the throttle plates full closed w/my fingers. Hmmmmmm......So I went ahead and unclipped start to pull the cables off the carb, and I disconnected the cruise cable first. Guess what?? I noticed how the arm on the carb suddenly closed itself that last 1/16 of an inch!!! I was the friggen criuse control cable that was just BARELY too tight.

I just knew it was running too good to be a vacuum leak....My A/F ratio guage wasn't showing lean either, but it's accuracy is questionable. I only have two vacuum hoses that aren't new on the car.

Now running smoothly at 750 RPM's w/out having to "Blip" the throttle anymore to get it to idle! :hail:


BTW...305sbc - I've located quite a few leaks doing just what you described. I'd have a hard time diagnosing it if it turned out to be a leaky brake booster. Maybe use a hand held vacuum pump maybe???

 

Guess what mine isn't doing it anymore? I think it's the temperature for me. It's been in the 60's & 70's lately, and the last time I tested her it was in the 30's
Again It only has 5 miles on the carb ...

Good to see that you fixed the problem...and I would thank a vacuum pump should work as a testing tool for the brake booster.

Ron