Topic: Idle drops slightly when plugging vacuum leak? UPDATE
in Forum: C3 Engines
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Lifetime MemberAsh Fork, AZ - USA
Joined: 4/18/2007
Posts: 1058
Vette(s): 1981 Red Interior White exterior
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?
|UPDATED|6/29/2014 5:22:01 PM (AZT)|/UPDATED|
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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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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Lifetime MemberAsh Fork, AZ - USA
Joined: 4/18/2007
Posts: 1058
Vette(s): 1981 Red Interior White exterior
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,
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.
Yeah, I know, Shut up Dave.
Lifetime Member #192
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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....
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....
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Lifetime MemberAsh Fork, AZ - USA
Joined: 4/18/2007
Posts: 1058
Vette(s): 1981 Red Interior White exterior
OK Vacuum leak was at the headlight switch. Now I just have to reset the idle. Easy shmeazy.
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.
Yeah, I know, Shut up Dave.
Lifetime Member #192
in Forum: C3 Engines
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