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#84045 10/10/2007 9:54 PM | Joined: Oct 2006 Posts: 13 'Bolter | | 'Bolter Joined: Oct 2006 Posts: 13 | I've been working on getting my 65 C20 and it's 327 running. I've gone through the ignition (new cap, rotor, wires, plugs) and replaced the vacuum advance. At idle, pulling 17" of vacuum, the advance is all the way to ~31 deg. My experience with later model SBC engines tells me this is bad, as well as the fact the engine idles rougher and hesitates off idle with the vac hooked up. I'm using the vacuum port on the side of the Roch 4GC carb, which is direct to manifold.
What should I be looking for here? I don't really have anything to compare to, since the vac I replaced was simply a giant leak.
The 327 replaced a 283, it's a 275hp from a 66-67 Impala originally with a Holley. The PO kinda cobbled it together, so I'm not sure what came from what.
Thanks in advance!
To old to change, to young to quit.
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#84046 10/10/2007 10:34 PM | Joined: Feb 2004 Posts: 29,845 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 29,845 | There should be a ported vacuum nipple that has little or no vacuum at idle, and gradually increases as the throttle opens. The initial timing usually works best at 8 to 12 degrees, with the centrifugal advance coming up to 35-37 degrees (including initial) at around 2500 RPM. At light-throttle cruise with ported vacuum on the advance, 40 to 45 degrees (initial, centrifugal, and vacuum combined) isn't excessive. The vacuum advance will drop off very quickly when the throttle is opened during acceleration.
On the centrifugal-only race engines, best horsepower usually happens around 36-37 degrees. The extra advance a vacuum unit provides is strictly for light-throttle cruise fuel economy.
Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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#84047 10/10/2007 11:20 PM | Joined: Oct 2006 Posts: 13 'Bolter | | 'Bolter Joined: Oct 2006 Posts: 13 | Thanks, that all fits in with my understanding of the advance system, just needed some reassurance. There is another port on the airhorn right above the secondaries, I'll give that one a try. Though that's it as far as ports from the carb go.
To old to change, to young to quit.
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#84048 10/10/2007 11:36 PM | Joined: Jun 2004 Posts: 8,597 Riding in the Passing Lane | | Riding in the Passing Lane Joined: Jun 2004 Posts: 8,597 | There are adjustable vacuum units available from performance stores that you can taylor it to your engine.
They say money can't buy happiness. It can buy old Chevy trucks though. Same thing. 1972 Chevy c10 Cheyenne SuperIn the Gallery Forum | | |
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