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Old 04-12-2012, 08:15:17 PM   #1
TheJuggernaut
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High idle in Park/Neutral, low idle in Drive

Hi Everyone! I am a long time reader and first time poster! After using the info I learned off this site and a good number of techbooks I took it upon myself to rebuild my '76 350 small block. Everything since break in has been going decent, but I have a small issue I am concerned with.

In Park or Neutral it idles up at 1500. But when I drop it into drive it goes all the way down to around 600. This seems like an excessive drop to me. I had to set the curb idle high so that when it went into gear it wouldn't stall out. That being said, it runs fairly nice, I just hate when i go to park and the thing revs way up!

Here are the specs:
355, .030 over, Holley 670 Street Avenger, TH350, Stock heads, Edelbrock Performer Intake, Slightly larger cam than stock,

It gets around 11 in/Hg at idle, vacuum gauge doesnt seem to jump around much, just slight twitches around the 11 mark.
Timing is set at 13 degrees.

Thanks in advance for any advice!!
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Old 04-12-2012, 08:26:56 PM   #2
73Camaro383
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Adjust your mixture screws and lower the fast idle for the choke.
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Old 04-12-2012, 09:27:46 PM   #3
Damon
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Check the timing both in and out of gear. If the timing is falling way back in gear, that's most of your problem. Usual culprit is a centrifugal and/or vacuum advance kicking in at idle. As soon as you throw it in gear it drops back, making the RPM drop worse than it would ordinarily be.

Also check for vacuum leaks. They play hell with getting a reliable, stable idle.

And, as was said above, make sure your idle mixture screws are set for best idle.
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Old 04-13-2012, 06:04:10 PM   #4
TheJuggernaut
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Thanks for the input. I'll check the advance springs and time it again. As for the fast idle, is that adjustable? I thought that it was either on or off.
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Old 04-13-2012, 07:20:31 PM   #5
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...Also check to see if your vac advance is connected to full vac. port or timed port, sometimes connecting to full vac at baseplate will cause those symptoms, I usually run timed port on side of primary block, hope that helps, DC
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Old 04-13-2012, 09:05:46 PM   #6
73Camaro383
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Quote:
Originally Posted by TheJuggernaut
Thanks for the input. I'll check the advance springs and time it again. As for the fast idle, is that adjustable? I thought that it was either on or off.


There should be a fast idle screw somewhere. My eddy has one, I'm sure your holley does as well. Its only for the choke portion. Once shes warmed up it shouldnt be a factor though.. Might not be your issue but something to check at least
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Old 04-14-2012, 03:00:13 AM   #7
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I had the same issue - exessive idle speed just to keep it running in gear.
culprit was the centrifugal advance and some carb issues.

first check that your distributor is working correctly and the timing isn't changing for the fast idle (ok maybe fast idle can have slight advance) and idle in P and D.
then one step at a time, read your vacuum gauge check the timing and set your best idle.
it took me about 2 weeks to get it.

your looking for no ignition chance between P and D
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Old 04-14-2012, 09:33:29 AM   #8
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Vacume leaks will cause that too
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Old 04-14-2012, 04:38:54 PM   #9
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Ok, just took a look at the timing. In Park or Neutral it is at 16 degrees. When I shift to Drive, down to 2 degrees! The fast idle screw on my carb doesnt seem to engage unless the choke is on full, so I don't think that is affecting it. I'm going to look at the advance springs, see what kind of shape they are in.
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Old 04-14-2012, 04:52:04 PM   #10
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I would set timing at 16 idle(at least to start). With vacuum advance unhooked and plugged off. WIth idle set as low and you can and not die. THat way you shouldnt be getting centrifical advance yet. Then hook back up the vacuum adcance and turn idle to around 900-1k.

Then look for vacumn leaks around the carb.

Run the vacuum advance off the manifold vacuum port on the carb. not the ported. I know eddy carb has a manifold vacuum port on the carb base. Not sure on the holley though. been awhile since i ran one sorry.

Now the above is after you make sure the distributer weights and springs are in good shape (not sticking or flying open to soon).

Really does sound like a vacuum leak but i would check all the above while your at it. It wont hurt to get it dialed in.
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Old 04-14-2012, 06:20:23 PM   #11
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Good plan and I also agree with Hoot on the base timing procedure. You want to check you base timing with the vacuum advance disconnected and the engine idling as low as you can get it. If the centrifugal is kicking in a little at idle you're not getting a true base timing reading. (I.e. you're going to be chasing your tail)

BTW- I think you'll find yoru advance springs are too light or one might even be broken. If you're getting 2* in gear and 16* at ~1500 RPMs in neutral that's 14* of advance kicking in- about 2/3 of what most centrifugal advance systems are capable os supplying (~20-21* max centrifugal advance before they limit out). It should take until about 3000 RPMs to kick it all in. If you're gettinmg 2/3 of that at only 1500 it's probably htting the lid at only about 2000 RPMs. Which is rough finger-math for springs that are too light.

Oh, and I should mention that you are going to like this engine's power SO MUCH MORE when you fix this issue!! It's going to feel like you strapped a second engine under the hood when you lay through on it with proper initial timing and centrifugal advance curve.
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1990 454SS pickup. Original "tow truck motor" 454 upgraded to something a little more "aggressive." Seems pretty stout but the fuel bill is nuts.

78 Malibu. Inherited with only 35K on the odo! Mild 383 with Weiand 142 blower on top. Mid 12s appears to be the best it's gonna do on street rubber. Runs so good I may never drive it again.

"Last remaining QJet tuner on planet Earth!"

Last edited by Damon : 04-14-2012 at 06:27:42 PM.
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Old 04-14-2012, 09:54:01 PM   #12
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on another note, welcome to the board fellow Mainer, these are a great bunch of guys
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Old 04-14-2012, 11:03:43 PM   #13
TheJuggernaut
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Thanks Badazz70, I'm glad to be a part of it. There is an absolute wealth of information on this board, and I really appreciate everyone's willingness to help out all of the members. It makes the whole process that much more enjoyable.

I checked my vacuum readings again with the gauge, and get 11* with a slight, consistent twitch around 11. From what I read this is normal for an engine with a slightly oversized cam. If there was a vacuum leak I should be able to detect it with the gauge, right? Either more erratic behavior of the needle or a lower reading? I'll try spraying some starting fluid or carb cleaner around the intake manifold tomorrow and see if I can't find a leak in there somewhere.

The weights on the distributor don't stick, but they do feel very light.
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Old 04-15-2012, 12:02:56 AM   #14
bigblock665
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dial back timing lean out idle mixture.
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Old 04-15-2012, 12:10:48 PM   #15
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I just went all around the carb base and the intake manifold base with some starting fluid and the idle didnt change at all, ruling out a vacuum leak. I've read a bit on this forum that people have said to hook the Street Avengers up to FULL manifold vacuum and not the timed vacuum port (which is what I am currently hooked up to). What is the reasoning behind that?
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