Idle question
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Joined: Sep 2004
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From: Boston MA
Idle question
I have a question about idle speeds.
I have a 1968 corvette with a 327 engine in it. The engine has been heavily modified.
The block was bored .030 and aluminum heads and intake were installed.
I also installed a new camshaft with the following specs:
Duration @ .050" Int./Exh.: 216/228 Lift Int./Exh. .454"/480"
I am currently trying to install a new 575 cfm speed demon and I am having trouble determining the correct speed. I have the stock Muncie manual transmission.
The current idle speed is 700 RPM, the mechanical timing advance comes on at about 900 RPM and the initial timing is set around 18 degrees BTDC bringing the total timing to 36 degrees BTDC when fully advanced. The engines don’t seem to ping or detonate, but with the new carb I do not know. Any suggestions about a good idle speed?
On a separate but important note, What is a good high idle speed for the choke. When I bought the carb, I wanted an electric choke because I was having problems with the car stalling when cold. What is a good high idle speed for the choke? Is there a rule of thumb? For example, add 1200 RPMs to the idle speed or something like that??
Thanks,
Joe
I have a 1968 corvette with a 327 engine in it. The engine has been heavily modified.
The block was bored .030 and aluminum heads and intake were installed.
I also installed a new camshaft with the following specs:
Duration @ .050" Int./Exh.: 216/228 Lift Int./Exh. .454"/480"
I am currently trying to install a new 575 cfm speed demon and I am having trouble determining the correct speed. I have the stock Muncie manual transmission.
The current idle speed is 700 RPM, the mechanical timing advance comes on at about 900 RPM and the initial timing is set around 18 degrees BTDC bringing the total timing to 36 degrees BTDC when fully advanced. The engines don’t seem to ping or detonate, but with the new carb I do not know. Any suggestions about a good idle speed?
On a separate but important note, What is a good high idle speed for the choke. When I bought the carb, I wanted an electric choke because I was having problems with the car stalling when cold. What is a good high idle speed for the choke? Is there a rule of thumb? For example, add 1200 RPMs to the idle speed or something like that??
Thanks,
Joe
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Joined: Sep 2004
Posts: 595
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From: Boston MA
Demon recommendations
I just got off the phone with Barry Grant Incorporated the manufacturer of Demon Carburetors. They recommended that set the idle speed between 850 RPM and 1000 RPM.
He said the rest really depends on the engine.
Do you think I should be on the higher or lower side of that?
Also, should I bring down the initial timing and re curve the distributor so there is more mechanical timing?? For Example, idle at 900 RPM and have 15 degrees of initial timing, then change the mechanical timing to 21 degrees bringing the total timing to 36. Or should I just adjust the idle to 900 and leave the timing, but change the springs to come on slower? Or don’t touch the timing and just adjust the idle.
Please help!
Thanks,
Joe
He said the rest really depends on the engine.
Do you think I should be on the higher or lower side of that?
Also, should I bring down the initial timing and re curve the distributor so there is more mechanical timing?? For Example, idle at 900 RPM and have 15 degrees of initial timing, then change the mechanical timing to 21 degrees bringing the total timing to 36. Or should I just adjust the idle to 900 and leave the timing, but change the springs to come on slower? Or don’t touch the timing and just adjust the idle.
Please help!
Thanks,
Joe
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Joined: Sep 2004
Posts: 595
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From: Boston MA
Thanks for the reply PC.
When cold, the car seems to want to stall.
When warmed up, it seems to idle smooth. when I raise the idle it sounds like it is really pushing the car, almost like the car sounds when accelerating and that is why I am thinking about reducing the timing, but I am not completely sure.
Thanks,
Joe
When cold, the car seems to want to stall.
When warmed up, it seems to idle smooth. when I raise the idle it sounds like it is really pushing the car, almost like the car sounds when accelerating and that is why I am thinking about reducing the timing, but I am not completely sure.
Thanks,
Joe
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Joined: Sep 2004
Posts: 595
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From: Boston MA
MSD recommendations - Is this right???
Called MSD.
They said to leave the initial timing at 18 as long as the engine does not ping. Re curve the distributor by removing one light silver spring and replacing it with a blue spring. SO now the distributor will have one silver spring and one blue spring. Then bring the idle up to the new range, 850 to 1000. And the centrifugal advance will come on around 1300 to 1400 RPM and be fully advanced around 2000 to 2100 RPM. The old setup started at 800 RPM and was fully advanced at 1700 RPM.
Do you guys think I will see any decrease or increase in performance or drivability?
I am still not sure about the proper idle speed. I am thinking middle of the range, 900RPM? Anyone think lower or higher? what are the pros of a higher idle vs. a lower idle? Please help, I am very confused!!!
All suggestions welcome!!!
Thanks,
Joe
They said to leave the initial timing at 18 as long as the engine does not ping. Re curve the distributor by removing one light silver spring and replacing it with a blue spring. SO now the distributor will have one silver spring and one blue spring. Then bring the idle up to the new range, 850 to 1000. And the centrifugal advance will come on around 1300 to 1400 RPM and be fully advanced around 2000 to 2100 RPM. The old setup started at 800 RPM and was fully advanced at 1700 RPM.
Do you guys think I will see any decrease or increase in performance or drivability?
I am still not sure about the proper idle speed. I am thinking middle of the range, 900RPM? Anyone think lower or higher? what are the pros of a higher idle vs. a lower idle? Please help, I am very confused!!!
All suggestions welcome!!!
Thanks,
Joe
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Pro
Joined: Sep 2004
Posts: 595
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From: Boston MA
Rob,
Is that with a stock carb?
Is it an automatic??
Do you think it would be different for an aftermarket carb and a manual tranny??
Thanks,
Joe
Is that with a stock carb?
Is it an automatic??
Do you think it would be different for an aftermarket carb and a manual tranny??
Thanks,
Joe
Last edited by 68coupe; Sep 30, 2004 at 09:36 PM.
