Vacuum Advance - My Mechanic is Wrong?
Thread Starter
Joined: Mar 2018
Posts: 2
From: California
Vacuum Advance - My Mechanic is Wrong?
Hi everyone,
I have '66 with a hot running 347 that has some engine run on / dieseling problems. I noticed that my vacuum advance was on the emission control (timed vacuum) port rather than the manifold vacuum port. As soon as I switched ports my engine dieseling and run-on was (mostly) cured. After taking my car back into my mechanic, I noticed that he put the vacuum advance *back* onto the emission control port -- stating "it needs to be on the metered side". I tried telling him that I have horrible run-on problems while using that port, but he just seemed convinced that it should be on the emission control port. I guess my question is ... is he wrong / what could possibly be his line of thinking?
I have '66 with a hot running 347 that has some engine run on / dieseling problems. I noticed that my vacuum advance was on the emission control (timed vacuum) port rather than the manifold vacuum port. As soon as I switched ports my engine dieseling and run-on was (mostly) cured. After taking my car back into my mechanic, I noticed that he put the vacuum advance *back* onto the emission control port -- stating "it needs to be on the metered side". I tried telling him that I have horrible run-on problems while using that port, but he just seemed convinced that it should be on the emission control port. I guess my question is ... is he wrong / what could possibly be his line of thinking?
6th Gear Member
Joined: Dec 2005
Posts: 5,212
From: Hills of California
Thats actually a tough call. Ive run on both and found that I enjoyed metered vac better because it keeps timing lower at idle when there is no vac signal. I found this to be good because the higher timing at idle caused the exhaust to smell acidic from high Nox.
So running on the timed vac would retard your advance at idle because there would be no vac signal as the throttle plates are blocking that port this should cause idle RPM to be lower than if you ran on the manifold vac SO that means it should be LESS likley to run on when using the timed vac port.
Running on the manifold VAC would INCREASE your base timing by about 10deg and should cause engine RPM to increase this would make run on MORE likley. This is because manifold vac would provide a vac signal at idle and that would advance your timing.
I would buy a timing light so you can observe what is going on and make your own decisions.
I wouldn't necessarily blame the mechanic at this point as timed vac is factory location. I think there are other issues occurring here.
1) when running timed vac is the IDLE RPM lower than when your run manifold vac?
2) does switching between the two cause an IDLE RPM change? you can do this while running just block the open port when you switch
-Gun
So running on the timed vac would retard your advance at idle because there would be no vac signal as the throttle plates are blocking that port this should cause idle RPM to be lower than if you ran on the manifold vac SO that means it should be LESS likley to run on when using the timed vac port.
Running on the manifold VAC would INCREASE your base timing by about 10deg and should cause engine RPM to increase this would make run on MORE likley. This is because manifold vac would provide a vac signal at idle and that would advance your timing.
I would buy a timing light so you can observe what is going on and make your own decisions.
I wouldn't necessarily blame the mechanic at this point as timed vac is factory location. I think there are other issues occurring here.
1) when running timed vac is the IDLE RPM lower than when your run manifold vac?
2) does switching between the two cause an IDLE RPM change? you can do this while running just block the open port when you switch
-Gun
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