#41  
Old 01-22-2020, 12:33 PM
RWS RWS is offline
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Yes, sorry, I should have been more specific: my Q-Jet is a 1970 RA III carb, and it does not have ported vacuum, but I do appreciate some later carbs that came on engines with EGR systems do.

  #42  
Old 01-22-2020, 12:59 PM
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Interesting, did not know that. I can learn something here every day.

Sent from my moto g(6) play using Tapatalk

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  #43  
Old 01-22-2020, 04:51 PM
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No 1970-74, except 1971 455H.O., Pontiac Quadrajet uses the "ordinary" ported source for the ignition vacuum advance above drivers side mixture screw.
Instead these engines uses the TCS system to control the vacuum advance.
No vacuum advance at idle, only with cold or to hot engine and when final transmission gear is used.

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  #44  
Old 01-27-2020, 05:26 PM
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Default Ported Vacumme

I wanted to give an update on my 70 Goat.I have been doing some tuning on my car when I got back from the engine shop the mechanic had some vac lines going to the wrong places. He had the distributor vac line going to the choke pull off. Once I sorted that out it started much easier with the choke hooked up. Then I had the distributor vac advance hooked to a vac port in the back of the carb. Needless to say still running rough a drinking gas. Today I hooked vac advance line to manifold vac wow made a nice difference. It ran smoother more responsive and better gas mileage. Took a winter 40 mile drive temp never got past 160 oil pressure 60 pounds.car was running sweet. Cant wait to get my new DUI distributor installed next week. Thanks for all the helpful input. Brian

  #45  
Old 01-27-2020, 05:31 PM
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Default Benefits of manifold v ported

That's good news.

Looks like a test case for why I said that manifold (unless a big duration cam) is how these motors were intended to run. Manifold vacuum leads to better throttle response and lower overall temps (especially siting in traffic) v ported. Ported was the engineers response to EPA regulations and not at all a performance enhancement.

I think of it as the timing is already there as you give it part throttle rather than that half second delay if running ported before the dizzy advances. (Obviously on hard/full throttle there's no difference between manifold and ported because vacuum drops, and just get the mechanical advance.). But most folks drive their cars in traffic and there manifold is typically better on a stock-ish setup. That's my experience.

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  #46  
Old 01-28-2020, 11:04 AM
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"Then I had the distributor vac advance hooked to a vac port in the back of the carb."

Any port on the back of the baseplate would have been manifold vacuum. There are no ported sources available on 1970 Pontiac carburetors that I am aware of. The rear of the baseplate has 1/8"NPT threads for various fittings used by the factory, all would be manifold vacuum.......Cliff

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  #47  
Old 01-28-2020, 02:28 PM
455abody 455abody is offline
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The only port I see Cliff is the tree coming out the back for my power brakes other tree port is capped off. I am using the port on the intake manifold seems like the engine likes it. Does the vacumme on the intake manifold port decrease as the rpm goes up? Thanks Cliff

  #48  
Old 01-29-2020, 07:42 AM
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RPM has nothing to do with vacuum production falling off far as manifold or ported. Light engine load produces high vacuum. Vacuum changes or drops off quickly with increased throttle angle/engine load. The harder you load the engine the less vacuum is produced. At very heavy and full throttle the vacuum available above and below the throttle plates is pretty much the same, or zero.

ALL sources below the throttle plates will show basically the same readings and operate the vacuum advance in the same fashion.

This includes the "Tree" in the rear of the baseplate or any other source in the intake manifold.....Cliff

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  #49  
Old 01-29-2020, 01:31 PM
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I would avoid using a source on a single runner for the ignition vacuum advance, like the one on the #1 runner on a Pontiac intake, as the signal pulsates and are somewhat weaker than any plenum source under the carburetor throttle blades.

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  #50  
Old 01-29-2020, 01:56 PM
455abody 455abody is offline
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Do we know how the factory had the vacumme routing going to the intake manifold vac source? My 70 assembly manual does not show---car seems to like the vac advance going to the intake manifold vac source.

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