'68 289 Vacuum Advance Routing - Ford Mustang Forum
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post #1 of 9 (permalink) Old 01-14-2020 Thread Starter
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'68 289 Vacuum Advance Routing

Hi all,


Been working on renewing the cooling system the last couple of weeks (ten minutes here, ten minutes there and wham you're at it two weeks!) and when the thermostat housing was going back on I broke the ported vacuum advance switch. The original had two ports plus one that was filtered to atmosphere. The new one has three ports and the diagrams I'm finding online don't match what is going on with my engine.


The original had two lines, one to the manifold distribution port, the other went to the ignition advance actuator but outboard of the diaphragm.



Not sure what to do with three lines, any help/direction would be appreciated.


Thanks!
Bart

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1968 Convertible, 289/auto
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post #2 of 9 (permalink) Old 01-14-2020 Thread Starter
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so here's a more specific question, in the diagram above it shows one line goes to the distributor. Is that to the engine side of the distributor diaphragm or the side that is away from the engine?


If it goes to the engine side of the advance diaphragm, where does the line on the other side of the diaphragm go to or come from?


Thanks


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post #3 of 9 (permalink) Old 01-15-2020
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That vacuum switch was part of the early pollution control stuff. It is connected to the distributor's vacuum advance on the 'middle' plug, to the carburetor (ported vacuum) at the top, and straight manifold vacuum at the bottom.

The idea is to run off ported vacuum (zero vacuum advance at idle) which retards the timing. That dumps still-burning fuel into the exhaust, raising temps in the manifolds and theoretically letting the air-injection (thermactor) system work efficiently. When the engine coolant is above a certain temperature (210? Can't remember for sure.) the switch cuts over to feed the distributor manifold vacuum, which raises the engine timing, helping it run cooler and spin the engine-driven fan just a bit faster. I am not certain, but I think perhaps even the non-Thermactor cars got this switch, because the lower cylinder pressure from the retarded timing helps prevent nitrous oxide formation during idle, even though it really raised hydrocarbon pollutants and CO (carbon monoxide) because of the poor burn quality.

While there seems to be endless debate over ported (or timed) vacuum vs. Manifold vacuum, I feel strongly that these cars run cleaner, better, and cooler on straight manifold vacuum anyway. There are a lot of heated discussions in other posts that you can find, but I will refrain from getting long-winded here. =)
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post #4 of 9 (permalink) Old 01-15-2020 Thread Starter
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Another helpful explanation
https://www.7173mustangs.com/thread-...-control-valve



Thanks for the reply. Here's the problem, there are two ports on the vacuum advance unit, one on each side of the diaphragm. The diagrams for cars with AC show three-port vacuum control valves, cars without AC show two-port vacuum advance control valves. The diagram below is for a car without AC. In looking at that diagram, I'm reading that the line on the port closest to the distributor should go to the vacuum advance valve but that's not how our car is plumbed so either someone made a mistake many years ago or I'm reading the diagram wrong.





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post #5 of 9 (permalink) Old 01-15-2020 Thread Starter
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another diagram. i'm thinking my car was plumbed wrong, not a huge deal since it won't stop it from running but it isn't doing what it's supposed to do.



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post #6 of 9 (permalink) Old 01-15-2020
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Unless you are doing a concours restoration or are forced to comply with some California law you are best to eliminate the "smog" switch altogether, plug the rear port on the distributor and the upper port (ported vacuum) on the carburetor. Then connect the front port on the distributor directly to manifold vacuum, or some port on the carburetor that is below the throttle plates.

This has been argued many times in this forum, but for most street engines, full manifold vacuum to the distributor is best. The reason is that your carburetor should be set up for lean at idle, rich at WOT, rich during acceleration, and lean while cruising. Rich fuel mixtures burn rapidly while lean mixtures take a lot longer to burn and build pressure.

Set your initial advance to 8 degrees with the tube plugged off, then reconnect it. This will enable easy starting (rich mixture, no vacuum, advance "retarded" at 8 degrees). At idle vacuum will be high and your advance will be high, giving a smooth idle with a lean air-fuel mixture. Step on the gas and your vacuum goes to zero and your advance goes back to 8 to burn the rich mixture quickly. Get to cruising speed and maintain it- your vacuum will be high along with the advance, allowing extra time to burn the lean fuel mixture.

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post #7 of 9 (permalink) Old 01-15-2020
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I agree with Yadkin completely. And both of the pics above were for later cars, with more vacuum-controlled smog stuff (and in the top example, a dual vacuum advance distributor). Neither of those pics would be helpful for you to hook up your car's vacuum lines, even if your thermal switch is fine. The bottom pic makes me want to weep. I can't believe how convoluted and awful the mid 70s smog motors were.



You are much better off ignoring all of it, and as Yadkin suggests, just running a single line from the manifold (NOT the carb's timed/ported vacuum!) straight to the dizzy, and using his timing numbers. Unless of course you're doing a concourse restoration. It will run much better.

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post #8 of 9 (permalink) Old 01-19-2020
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Grimbrand you should see the '81 Corvette that I am currently working on. I neglected to take a "before" picture but I have a large box of smog stuff that I pulled off that sorry small block. Stuff now in the box includes an air pump, brackets, third engine pulley to run it, massive EGR valve, vacuum switches, and several yards of vacuum hoses, tees and connectors. None of that stuff worked to reduce pollutants- all it did was waste fuel, reduce power, and create a nightmare for owners. The car runs much better and I can actually see the engine. It literally looked like a scene from an Alien movie.

I'm keeping all the pollution control equipment that makes sense and makes the engine run better: PCV, carbon canister and evaporation controls.
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post #9 of 9 (permalink) Old 01-19-2020
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That's the spirit, Yadkin!
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