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6/29/14 7:22pm - Original Message: 'Idle drops slightly when plugging vacuum leak? UPDATE'
dads2vette Lifetime Member
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Ash Fork, AZ - USA

Vette(s):
1981 Red Interior White exterior

Joined: 4/18/2007
Posts: 1038
Headlight doors don't work.  Started when I fixed the lack of vacuum to the tranny modulator.  So I was doing a preliminary check by clamping hoses off the intake and carb.  First one was for the headlights...doh.  Sure enough, that must be where the leak is from.  I'm guessing from the switch or dash someplace since I don't want to take the dash out again.  Anyway, I clamped the hose between the check valve and the filter and the idle got rougher.  I thought it would have been rough pre clamping.  Maybe the idle is set to fast with the vacuum leak?  Anyone have ideas?

On a positive note.  With the phone and texting help of sarge81 I was able to get over my fear of exploding the engine when installing the new roller rockers.  Seems to run "smoother", I'm sure that's in my head but that's where I like to keep all the good stuff even if it's hooey!  Also bumped the mpg from 13.5 on the first tank to 14.1 the second.  At that rate I'll just have to drive another 31,000 miles to make up for the cost.  Of course, it would have been much more expensive to use the car as a flower pot instead.





|UPDATED|6/29/2014 5:22:01 PM (AZT)|/UPDATED|


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Dave C..........My mantra: I can not be bought!!      Long and short term leases available.
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6/24/14 10:18am - Reply: 'Re: Idle drops slightly when plugging vacuum leak?'
Case75
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, - Canada

Vette(s):
1975 Corvette 383 stroker w/Patriot 190 heads, Eagle stroker kit, Comp 279TH7 cam, DUI distributor, Edelbrock 7116 intake, Edelbrock 1806 carb, Hedman Hedders, TH400 w/TCI Breakaway 2400 stall, Transgo 1-2 shift kit.

Joined: 6/19/2012
Posts: 230
That is a bit strange. I would expect the RPM to rise a bit as well. How many inches of vacuum do you have at idle? Are you running a vacuum advance on the distributor? If so, is it hooked up to ported or manifold vacuum? If it's manifold, is the vacuum advance fully in at idle? Or is the idle dithering a bit? I'd try plugging the vacuum accessories and get the idle right and then start diagnosing leaks from there.  

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Build Date: May 7, 1975. 383 w/267 RWHP/310 RWTQ
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6/24/14 10:57am - Reply: 'Re: Idle drops slightly when plugging vacuum leak?'
dads2vette Lifetime Member
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Ash Fork, AZ - USA

Vette(s):
1981 Red Interior White exterior

Joined: 4/18/2007
Posts: 1038
Greg, 1st thing, not a stock engine.  Yes running an advance. Not sure of vacuum readings, I'm picking up a vacuum tester today.  That is a good idea about blocking all but necessary vacuum ports then setting idle.
Thanks, 


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Dave C..........My mantra: I can not be bought!!      Long and short term leases available.
In the words of Zora Arkus-Duntov "Is your seat belt fastened? Alright, Let's go"


Yeah, I know, Shut up Dave.
Lifetime Member #192


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6/24/14 12:34pm - Reply: 'Re: Idle drops slightly when plugging vacuum leak?'
Adams' Apple Lifetime Member
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Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

Joined: 11/8/2003
Posts: 19457
Normally, a vacuum leak will raise the engine rpm. If you have adjusted the carb, and timing while it had a vacuum leak, fixing, or eliminating that leak will cause the idle rpm to go down.
I agree with eliminating ALL of the external vacuum devices from the engine, and make the carb/timing adjustments. Then add one vacuum connection at a time to find which system is creating the leak, and how bad that leak is compared to other systems. Headlights, brakes, hvac, smog, distributor, even the carb itself....any of these systems can, and usually DO have small leaks which by themselves are not enough to create a major problem....but....add them all together, and you get a different story. Gotta start with the one that leaks the most, and go from there, one system at a time. Chasing vac leaks sux....pun intended....Tongue


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6/29/14 7:23pm - Reply: 'Re: Idle drops slightly when plugging vacuum leak? UPDATE'
dads2vette Lifetime Member
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Ash Fork, AZ - USA

Vette(s):
1981 Red Interior White exterior

Joined: 4/18/2007
Posts: 1038
OK Vacuum leak was at the headlight switch.  Now I just have to reset the idle.  Easy shmeazy.Thumbs Up

____________________________________
Dave C..........My mantra: I can not be bought!!      Long and short term leases available.
In the words of Zora Arkus-Duntov "Is your seat belt fastened? Alright, Let's go"


Yeah, I know, Shut up Dave.
Lifetime Member #192


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