Idle drops too low
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Joined: Aug 2012
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Idle drops too low
'78 L82 w/auto. trans.;Elgin 1607 cam, Edelbrock 1406 carb.. Set idle w/vac. ga. (12-13" high as I can get reading vacuum off front port of carb), curb idle set @900 rpm. Problem is idle drops to 500 rpm when I put it in gear and hold the brakes or when I come to stop sign. Too low --almost dies out. Checked for vacuum leaks; unhooked all vacuum line with exception of brake and vacuum advance and made no difference. On one site I read to set the air/fuel idle screws with someone sitting in the driver's seat, car in drive and holding the brakes. Any advice would be appreciated.
Melting Slicks
Joined: Jan 2003
Posts: 2,713
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From: Shreveport LA
LA Events Coordinator
Had the very same problem about 2 months ago. My RPMs also dropped whenever I put it in Drive. I checked everything from timing (36*@ 2500rpm), carb adjustments, & vacuum leaks. The timing was spot on.... The carbs were adjusted and working perfectly and I could not find ANY vacuum leak UNTIL I accidently bumped the vacuum line to the distributor advance and the engine sped up. Found out that vacuum line had a cut in it (underneath so I was unable to find it initially) therefore it was not advancing the distributor when I put it in DRIVE. So I would check the vacuum line to the distributor. Hope this helps.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
sounds to me like your distributor vaccum cannister needs to be changed to a vacuum cannister that pulls full vaccum at a lower vacuum reading.
first check the vaaccum cannister with a vaccum pump and see where it pulls in full vaccum. you are looking for the inches of mercury reading.
then compare this is the vacuum you are getting from the port you are using for the cannister. there must be a difference of several inches of vacuum OR when the car gets put into gear the idle drops down and the vacuum drops down cutting out the vacuum advance which further reduce engine rpm.....
Lars has articles on this and he explains it way better then i can and gives you the reference numbers for the different vaccum cannisters. the vacuunm cannisters cost $15 at NAPA
first check the vaaccum cannister with a vaccum pump and see where it pulls in full vaccum. you are looking for the inches of mercury reading.
then compare this is the vacuum you are getting from the port you are using for the cannister. there must be a difference of several inches of vacuum OR when the car gets put into gear the idle drops down and the vacuum drops down cutting out the vacuum advance which further reduce engine rpm.....
Lars has articles on this and he explains it way better then i can and gives you the reference numbers for the different vaccum cannisters. the vacuunm cannisters cost $15 at NAPA
Thread Starter
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Joined: Aug 2012
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Thanks for all the responses!
First, my timing is set at 34 degrees @ 3000 rpm and the vacuum advance is manifold vacuum. However, the 12-13" I mentioned in my first post was taken at the carb.
Bob....By the vacuum canister, I assume you're referring to the advance diaphram on the distributor -- is this correct? I never considered checking that; good advice.
I guess I never gave a complete run down on the car; it's not stock.
All of the emission control system has pretty well been removed and it has been converted to straight dual exhaust. It has a Weiland (sp) aluminum intake manifold and, as mentioned, an Edelbrock 600 cfm.
One more question "how do I contact Lars"?
Thanks again.
First, my timing is set at 34 degrees @ 3000 rpm and the vacuum advance is manifold vacuum. However, the 12-13" I mentioned in my first post was taken at the carb.
Bob....By the vacuum canister, I assume you're referring to the advance diaphram on the distributor -- is this correct? I never considered checking that; good advice.
I guess I never gave a complete run down on the car; it's not stock.
All of the emission control system has pretty well been removed and it has been converted to straight dual exhaust. It has a Weiland (sp) aluminum intake manifold and, as mentioned, an Edelbrock 600 cfm.
One more question "how do I contact Lars"?
Thanks again.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
yes the vacuum advance cannister on the distributor. i had this same problem and i think the new vacuum cannister i put on pulls full vacuum at 8". this means that even when you put the car in gear you are still pulling full advance.
heres Lars email address ask for his vacuum advance cannister paper and be sure to tell him if its for HEI or points distributor
mailto:V8FastCars@msn.com
heres Lars email address ask for his vacuum advance cannister paper and be sure to tell him if its for HEI or points distributor
mailto:V8FastCars@msn.com
