Vacuum Advance Question - Team Camaro Tech
Troubleshooting Diagnosing problems done here.

 
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post #1 of 8 (permalink) Old Dec 9th, 00, 05:50 PM Thread Starter
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Have a 68 Camaro with a '78 Vette 350SB Motor(don't know the compression) as the motor was done by a previous owner. Running a GM HEI distributor with an ACCEL Coil along with a MSD 6200 Ignition. My question or problem is that I just switched out the carb today to a Edlebrock 1405 to match the manifold. The vacuum advance on the old Holley was capped along with the vacuum advance on the distributor. Determined that the carb has a full vacuum present at idle. However, when I hook a hose from the vacuum advance on the new or old carb to the distributor the car backfires and boogs down. My question is why would the car be set up like this? Wouldn't I be losing any power by not having it hooked up? The Car runs fine with the new carb set up but still have a slight detonation at the higher end of the power band or under load(for instance under acceleration in 3rd or 4th gear).
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post #2 of 8 (permalink) Old Dec 9th, 00, 07:19 PM
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It sounds to me like the base timing is to much timing. If that is the case then when you plug in the adv it is so much timing it cuts out. Check the timing with a light, advance unhooked, at idle, try about 10 degrees. If you have a degree timing light put about 36 total timing in it.

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post #3 of 8 (permalink) Old Dec 10th, 00, 05:22 AM
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Are you hooking the line to the right vacuum port on the carb? The dist needs ported vacuum, not manifold vacuum. Find a vacuum port that has no vacuum at idle and about 15 inches of vacuum with the RPMs at around 2000. This is the port you want your dist hooked to. Manifold vacuum is high at idle and drops off as RPMs increase. You don't want the dist hooked to this. Are you setting the timing with the vacuum unhooked? You should unhook it to set the timing. Check the total timing with the engine at about 3000 RPMs. It should be around 40-42 degrees with the vacuum hooked up and about 36 with it unhooked. This is just my opinion, but it should work.
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[This message has been edited by big gear head (edited 12-10-2000).]
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post #4 of 8 (permalink) Old Dec 10th, 00, 11:08 AM
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There are many opposing theories on ported-vs.-manifold vacuum for advance. Ported vacuum was invented as an early approach to emissions control, with debatable results; the idea was that with retarded spark (no vacuum advance) at idle, more unburned hydrocarbons would be present in the exhaust gases, which would support fuller combustion of all exhaust gases in the exhaust manifolds with the Air Injection Reactor (A.I.R.) pump systems. With full manifold vacuum on the advance unit, your engine will idle smoother, will run cooler at idle (reduced exhaust gas temperature in the exhaust ports, which are surrounded by coolant passages), and will be more responsive off-idle, as the advance doesn't have to "catch up" with the increase in rpm - it's already there. That's why, in the late 60's and early 70's, when A.I.R. pumps were on almost everything, the first thing that was done to "de-smog" an engine was to plug the "ported" vacuum source and connect the advance diaphragm to full manifold vacuum (followed by busting the plastic caps off the idle mixture screws, re-setting idle mixture a bit richer, and re-setting the base timing).

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post #5 of 8 (permalink) Old Dec 10th, 00, 05:36 PM
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It may be that your pickup coil inside the distributor is bad. The wiring to the coil (after being moved back and forth by the vacuum advance many times) will develop an internal break which only shows itself when the advance moves the coil. That is why it backfires and bogs when you hook the advance back up. Just my two cents. Hope it helps.

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post #6 of 8 (permalink) Old Dec 11th, 00, 08:17 AM
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mtidwell

I intentionally hooked mine up to the unported vacuum just to see what would happen. As you described it bogged and coughed but seemed to idle okay. If you search the Net you'll find all kinds of articles on this subject with about a 50/50 mix of ported versus unported (confused me). I'd take big gear heads advice for starters.
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post #7 of 8 (permalink) Old Dec 13th, 00, 08:11 AM Thread Starter
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Thanks Big Gear Head. Your advice worked. Sorry, don't know a Sue Tidwell.

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post #8 of 8 (permalink) Old Dec 15th, 00, 12:40 PM
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Glad to help. My wife's natural mother's name use to be Sue Tidwell. I've been trying to find her for years.
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