I have a question regarding the vacuum advance setup on my 1977 ford f100 with a 302. Currently, I have it set up like this: Vacuum line from the T at the back of the intake manifold to a ported vacuum switch, and then a vacuum line from the ported vacuum switch directly to the distributor. There are no diaphragms or valves of any sort in line of the vacuum line.

To my understanding, the job of the ported vacuum switch is to open up when warm and allow vacuum to flow through the switch. I also know that I should NOT have vacuum to the vacuum advance at idle, but with my current setup, once the engine is warm I DO get vacuum to the vacuum advance at idle. I have always had a problem of this engine idling too fast once it gets warmed up, and I think that my current vacuum advance setup is to blame. Today I removed the vacuum line from the vacuum advance at idle and engine RPM's dropped a considerable amount.

My question is how do I change my setup so that I will only get vacuum upon acceleration? On my other pickup with a 4 barrel holley, I have the vacuum line attached to an upper port on the carburetor, which only applies vacuum upon acceleration. However, this pickup has a 2 barrel motorcraft 2150 and as far as I know none of the vacuum nipples on the carburetor are ported (don't give vacuum at idle). If anybody has any input on this issue it would be greatly appreciated.

 

may this help you....
I got this from my smog shop after I rebuilt my motor and had to smog it for Ca reg,





let me know if you want pictures instead
(forgive the blue spot I painted a part not realizing it was on the back of this diagram)

 

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OK, first of all, you need to understand what "ported" means. It means that the vacuum is taken from above the throttle plates at idle. This would cause the vacuum to be "turned off" at idle, but come on just off idle as the vacuum "port" in the venturi is exposed to manifold vacuum. "Manifold" vacuum is taken from the manifold (hence the name) or from the carb *below* the throttle plates. It has nothing to do with your vacuum switch.

It sounds to me like you have your vacuum switch hooked up to manifold vacuum, not ported. If you truly don't want any vacuum advance at idle, you will need to switch the vacuum source from manifold to ported.

I should add that the setups on Fords of that era that I have seen have 2 vacuum sources, manifold vacuum when the engine is cold, then switch to ported when the engine is warm. More vacuum advance helps the engine run better when cold. That's how my '78 Ranchero is setup.

 

Thank you for the picture dads1977, my pickup currently doesn't have any of those inline valves other than the PVS valve. That might be what is required to fix my problem. I did some reading and heard some say that the carb nipples on the passenger side of the pickup are ported. I will have to look at my pickup later to see if this is the case.

 

You have a ported vacuum port on that carb, that's what the dizzy needs to be hooked to.

 

I understand this. However, if you look in any parts magazine that vacuum switch is referred to as a ported vacuum switch, which is why I referred to it as ported. It is located on top of the intake manifold in a water outlet which causes the switch to open once the water/antifreeze reaches a certain temperature. It is ported in a different sense based off of temperature rather than ported based off of throttle.

 

Thank you jackburtonme, I hope you are right!! I will look at my pickup later and search for it. If there is one then I'm betting this will solve my problem.

 

Engine vacuum actually goes away (momentarily) on acceleration, the timing gets retarded at that point. Once the speed levels off the vacuum comes back up, and so does the vacuum advance. I only mention this to help understand what goes on.

The "ported" vacuum isn't supposed to have any vacuum at idle, but this depends entirely on what you have your idle RPM set at! If it is set higher than 550 or 600 or so, then the throttle plates start to crack open and guess what - you have vacuum. If you loosen the hold-down and rotate the distributor body back and forth you'll notice what advancing the timing does to idle RPM - it speeds up.

So what can happen is a kind of feedback loop, too high idle will bring in more vacuum advance, and that will bring idle up even more.

It really helps to have a tach and a vacuum gauge when making adjustments

 

Like Tedster9 just said, your carb needs to be correctly adjusted so at idle your not drawing any vacuum out your ported vac nipple on the carb. Make sure your idle screw isn't holding the throttle plates open at all at idle. This goes for your fast idle screw too for the choke. I've seen chokes improperly set up, and the fast idle screw holding the throttle open a little to where you cant adjust your engine idle correctly.

 

^^^^ This is why a gauge works really well, at least till ya get the hang of it. The carb needs to have the throttle plates nearly closed at idle. Just a tiny bit open, and ONLY the idle circuit providing fuel. Very common mistake. The following is a picture from a good page on carburetor tuning.

http://members.tccoa.com/392bird/tuning.htm

 

this pic should help a lot of people understand how the ported vacuum port draws/pulls/creates it's vacuum...Thanks Tedster9

 

These are a couple of excellent points that should help get a carb tuned up better.

Ported vs manifold is only relevant at idle, period. It has no effect on the timing at any other point of operation.

A vacuum gauge is highly useful in determining what's going on with your engine and I think all tuning should be done with at least the knowledge of where you are at for manifold vacuum levels. You will often find other problems you didn't realize you had! Also, if you have a Holley carb, the manifold vacuum level has a direct effect on how you will jet the carb, which power valve you will select, etc.

A tach is also highly useful. I like to set my timing first, then check the timing (and vacuum levels) at various RPM intervals. Having a tach under the hood right next to a vacuum gauge is my preferred setup for tuning.

Also, I've read quite a lot about ported vs. manifold vacuum and what I've come away with is that either can be done with equal success (barring local emissions control regulations). The key is that you have to have an end goal in mind of how you want the engine to operate. There isn't really a right way, there are just different solutions for different target results. I'd start with committing how you want the engine to perform to paper first, then determine the best combination of fueling and ignition adjustments to provide those specific traits.

 

Manifold vacuum will make yer engine run cooler, more fuel efficient and will "tame" an engine will a "big" cam. Back in the day (Tedster 9 might remember....hahaha) all we had was manifold vacuum....hence the need to disconnect and plug the vacuum line to the carb when timing it. Cooler, more fuel efficient running is really not that big of a deal to want to have manifold vacuum and the time and patience involved in getting the engine to run right, in my opinion.

Ported or timed vacuum is probably the best way to go, just hook it up directly from the carb to the distributor.

On your 2150, the ported, or timed vacuum is on the right side (passenger) of the carb.

Invest in a vacuum gauge and a good multimeter. Some of them have a tach function. Or you still pick up a tach/dwell meter. It's a necessity to have at least those two tools in your toolbox when playing with these vehicles.

Good stuff, all of the above.

 

IMO manifold vacuum would be best if you can get away with it. More timing means more efficiency, as FB pointed out. The only reason to use ported, IMO, would be if it is idling too fast with manifold vacuum and you already have the throttle plates closed.