Effects of PVC Vacuum And Idle Speed - Team Camaro Tech
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post #1 of 7 (permalink) Old Feb 20th, 07, 05:15 PM Thread Starter
 
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Effects of PVC Vacuum And Idle Speed

Thanks for looking.

Im having a problem with my fairly stock zz4 crate engine. I cant get the carb to idle low enough. The car also runs VERY rich. Ive tried 3 different carbs, checked for vaccum leaks, float lvls, idle mixture, timing, cam timing, brake booster etc. For what ever reason I cannot get the car to idle below 1000rpm (in park). This is with the idle speed screw turned all the way out. When I bought the car it came without a PVC valve and no vacuum lines connect to carb (advance was but thats minor). So in my testing when I purposefully create a vacuum leak I notice that the idle speed is significantly lower.
So my question is:

By creating the 'artificial vacuum leak' buy installing a PVC system will that allow the car to run at a better idle and reduce the car from running so rich?

THanks again!
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post #2 of 7 (permalink) Old Feb 20th, 07, 05:39 PM
DjD
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Dennis
 
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Re: Effects of PVC Vacuum And Idle Speed

Yes you should run the PCV valve but with or without it unless you have a very big cam you should be able to get that thing to idle down. What carb are you running? What is your timing set at? Go back to the basic's of carb tuning, start with making sure you have at least 10 degrees of initial timing. Then close your sec throttle blades all the way and just take some tension off the screw so the blades don't bind. Set your Idle mixture screws to about 1 turn out. Now use the idle adjustment screw to close the pri throttle down or up with the engine running. You did say your floats are adjusted properly didn't you?

That should work if you have a Holley or maybe a Q-jet but if you have a Barry Grant it won't work... Mind you all this is doing is getting your idle back on the idle circuit of the carb. You may have other performance problems to tune out later. If you can't dial in your idle this way you may have an internal vacuum leak or some other issue. Once you have it idled down check your timing. At 1000 rpm you may have been on the timing curve, if so adjust the timing bac up to where it was. Idle should go up but this time just close the pri throttle blades a bit to bring the idle back down.

Let's assume this works and your idle is at 800 rpm. Try to lean out the idle mixture a touch Shoot for highest vacuum reading or best idle. Turn in one idle mixture screw just until the engine falters and then back it out until it's steady. Do the same with the other then adjust the idle if needed. Now shut off the engine and make both mixture screws the same. Most holleys I have tuned on a small block seem to like 3/4 turn but what ever you set your at split teh diff to ge them the same. Example is 1 is at 1 full turn and the other is 1/2 turn. make them both 3/4 turn...

...Dennis

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post #3 of 7 (permalink) Old Feb 20th, 07, 08:21 PM
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Re: Effects of PVC Vacuum And Idle Speed

what is your initial timing, and where is the vacuum advance hooked up?
i've had engines that i couldn't get to idle down unless i either put the vacuum advance on a ported source or backed the timing way off. so the vacuum advance isn't a "minor" thing, as you stated.

you don't plan sincerity.
you have to make it up on the spot.

wanna hear about 20 years ago when i was too smart to know any better?
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post #4 of 7 (permalink) Old Feb 21st, 07, 05:22 AM
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Re: Effects of PVC Vacuum And Idle Speed

Check your throttle linkage- you may have to wind it out a few turns to close the throttle. Sometimes, the simplest things are overlooked.
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post #5 of 7 (permalink) Old Feb 21st, 07, 04:50 PM Thread Starter
 
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Re: Effects of PVC Vacuum And Idle Speed

BAH!

I finally got it figured out!!! (after 1 year of fighting this)

I moved to Colorado from Califonia 2 years ago. The first year here it got REALLY hot and fuel was boiling in the carb. I went out an got a performer rpm air gap manifold and a 1 inch phenolic spacer to try to disipate some of the heat. Ive been having this issue for awhile now (while I was in CA) and didnt notice any recognizable difference in the idle speed quality. I just kept on driving it with the high idle speed and it was bugging me more and more. So after Ive checked EVERYTHING I went out and bought a new carb (Holley Street Avenger 670cfm) and still had the same freaking problem. After much cursing and swearing at my car I noticed that the one thing that I havent checked was that phenolic spacer. Well, sure enough after taking that off I can now get the idle to act normally. HURAY!!!

Thanks guys! I really appreciate the support! I just got my car (68 SS Convertible, #matching engine on the stand zz4 installed now) painted Viper Red with the stock stripes.. It looks awesome I'll post some pictures soon in the 'welcome' section.

Thanks again guys!
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post #6 of 7 (permalink) Old Feb 21st, 07, 07:53 PM
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Re: Effects of PVC Vacuum And Idle Speed

Quote:
Originally Posted by jsharmer View Post
BAH!

I finally got it figured out!!! (after 1 year of fighting this)

I moved to Colorado from Califonia 2 years ago. The first year here it got REALLY hot and fuel was boiling in the carb. I went out an got a performer rpm air gap manifold and a 1 inch phenolic spacer to try to disipate some of the heat. Ive been having this issue for awhile now (while I was in CA) and didnt notice any recognizable difference in the idle speed quality. I just kept on driving it with the high idle speed and it was bugging me more and more. So after Ive checked EVERYTHING I went out and bought a new carb (Holley Street Avenger 670cfm) and still had the same freaking problem. After much cursing and swearing at my car I noticed that the one thing that I havent checked was that phenolic spacer. Well, sure enough after taking that off I can now get the idle to act normally. HURAY!!!

Thanks guys! I really appreciate the support! I just got my car (68 SS Convertible, #matching engine on the stand zz4 installed now) painted Viper Red with the stock stripes.. It looks awesome I'll post some pictures soon in the 'welcome' section.

Thanks again guys!
HUMMM...by moving the carb closer to the manifold...was it a linkage/throttle cable length issue?Removing the phenolic spacer may have changed the geometry.I think the resin spacers are vacuum leak proof unless damaged.Just wondering?
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post #7 of 7 (permalink) Old Feb 21st, 07, 09:55 PM Thread Starter
 
Join Date: Dec 2000
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Re: Effects of PVC Vacuum And Idle Speed

The linkage was fine.. I verified that about 2 dozen times

I have no idea why is makes a difference, but it really did..
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