I've been going through all of my bookmarks, anything I can find on timing and I just read that one again a little while ago that says they are one and the same except at low throttle positions where the ported is actually higher. So currently thinking the manifold is reduced somewhat by the check valve. Manifold brings up the idle speed(feedback shouldn't be a problem if the vacuum advance is all in 2" before the highest manifold reading at idle). Ported comes in way late. Here's what mine looks like. Should be noted that I have 20" of manifold at idle. I haven't mapped the manifold vacuum in relation to throttle yet, but from all the times I've measured it and increased the throttle in the driveway I want to say it increases for a bit.
Ported vaccum rpm
1" 950
2" 1100
3" 1200
4" 1300
5" 1400
6" 1550
7" 1650
8" 1850
9" 2000
10" 2100
11" 2250
12" 2300
13" 2450

It keeps going, but that's where I stopped. Also, it's not 100% accurate because I was watching the gauge and increasing throttle and when it was on a whole increment I checked rpm, they may come in a bit earlier. But I think it's a pretty good Idea. My emissions port was about the same only it came in much slower. Only other port I have on my carb I never got a vacuum signal out of and I went up to around 3k or better, think it's a WOT port and I don't know that I've hit that point yet. Can't imagine what rpm that is sitting in the driveway, but I've been up to 3800 so far.


That kind of derails what I was going to say. That is manifold brings the advance in, it's the same as ported at some point, then ported increases for a bit, then they are the same and it tapers back down to just manifold or nothing at all. Looking at it now it seems like maybe the ported could increase the resistance on the check valve so that manifold doesn't pull on the vacuum advance much off idle, but I won't be able to test that for a few days.

 

I may have missed it, but I didn't see how much timing your vacuum advance is giving you? If they have manifold vacuum in the mix, they may not be giving you that much extra timing anyway. In my experience the smogged engines with the ported vacuum only setup have around 20 degrees advance in the timing. When I start monkeying around and disconnecting the emissions, I find this is way too much for my purposes. Your timing will go up and down constantly depending on throttle position, and you can "feel it" in how the engine responds. I like bumping the initial timing up as high as it will stand and still start hot, and then bring the vacuum advance in with just a little bit of degrees of advance, or not even use it at all. You get a much more solid feel from the engine and much better throttle response from it doing it this way from what I have found.

 

My vacuum can will pull in 24* and I can have it all in at 8" of vacuum.

 

That sounds like the ones I have messed with. That is a lot of advance to be adding and subtracting. I can never run manifold vacuum with one of those stock cans. When I plug manifold vacuum into it, the idle speed shoots way up. So I turn it down, and that messes up my idle mixture. I finally get it idling nice with the hose on manifold vacuum, and then when I put it drive, the engine stalls. So I end up turning the idle back up higher and higher till it will not stall when I put it in gear. But that requires the idle in park to be very high and when I put it in drive it slams in gear. Not good. That's why I like ported vacuum on the distributor on these trucks, especially with a automatic. I bet if you ran it on a manual like that you would end up slipping the clutch a lot to keep it from stalling.

Keep playing with it. The only way to figure it out is keep trying different things.

 

Yeah, I have a lot more playing around to do with it. The spring I made for the mechanical advance seems to be doing alright, may tweak it just a little bit. It's all in later than I want it. Plan to run a hose though the firewall so I can measure vacuum while driving. Will start with ported with the can unhooked, then check manifold the same way to see if that changes my mind.

 

Keeping the timing as advanced as you can without pinging is the key to power and fuel mileage. That's how the modern vehicles work. They advance the timing till the knock sensor hears the engine complaining, and then drop it back a little and then advance it again till it hears the engine again. It does this constantly. So it automatically compensates for fuel quality and other conditions and keeps the timing on the ragged edge.

 

Ok, so I have another thread for timing going in the 300 six forums. This post was also posted there. I'm keeping this thread going as a discussion on that odd vacuum advance set up.

So, the firewall insulation retainer right about the transmission tunnel is the perfect spot to run a silicone vacuum hose through. I did some test drives with a vacuum gauge hooked up. Ran the truck up to 4000rpm in the driveway and measured manifold vacuum. Drove it to see what manifold vacuum did while cruising at different rpms. Did note that vacuum 'kicks' down to about where it was at the last rpm measure when I accelerate. Didn't see a higher drop than that. Also, couldn't get up to 2500 rpms, I hit 10mph over before I got there. Would have to try and measure that high on a run to town, but I don't know that I need to. After I got my manifold numbers I swapped the hose to the spark port and measured it on the way back. Ported doesn't come in until 800 rpm. Also, manifold vacuum is low and the needle bounces quite a bit around idle speed. I think it doesn't like that the vacuum advance isn't hooked up. After I graph these numbers and get a visual I will adjust the vacuum advance and hook it back up to ported vacuum. Then I'll run the same test on manifold vacuum and put a t into the ported line so I can measure it again as well. I guess I should have noted that I checked the vacuum without the vacuum advance hooked up.

RPM; MV in park; MV at cruise; PV at cruise
600 - 18 - (17-18) - 0
1000 - 18 - 16 - 6
1500 - 18.5 - 11 - 10
2000 - 19.5 - 9 - 12
2500 - 19.5
3000 - 19
3500 - 18
4000 - 16

I will map my mechanical advance again before I do anything else. I feel like those numbers will reflect my timing curve to a point.

Now, if you'll notice that manifold vacuum drops off pretty quick and yet ported vacuum continues to increase. I forgot to test the output of both put together, but looking at these numbers it looks like there will always be a vacuum signal on the advance can. I will test them together next. I think I have just enough time before school lets out. I'm curious if ported pulling with manifold will add the signals together somewhat. Possibly pulling a little harder than manifold alone. The two vacuum signals seem to be independent of one another.

 

Started out with the check valve backwards, here's what I got after I swapped it.

Rpm; park; cruise;
700 - 0 - 0
1000 - 0 - 4
1500 - 4 - 10
2000 - 8 - 10(note I did go passed 2k but didn't hit 2500 and the gauge was still going up)
2500 - 12
3000 - 17
3500 - 18
4000 - 17

Will map mechanical advance curve after I pick up the kiddo and graph these points as well as the mechanical curve.

 

Here's a chart of the vacuum readings.

 

If the ported goes higher and higher, while the manifold doesn't, I suspect the port you are using for ported vacuum is actually venturi vacuum. Venturi vacuum will continually rise as more air flows through the carb. Regular ported vacuum will act like manifold vacuum, and flatten out and actually go down as you open the throttle farther and farther. The only thing that generates manifold or regular ported vacuum is the difference between the pressure on top of the throttle blade versus the pressure on the bottom of the throttle blade. When the throttle blade is wide open, there is no difference, so there is no reading, unless your air cleaner is restrictive (most factory ones are a little restrictive).

 

Sounds right. Port at the base of the carb.

Here it is with the mechanical timing curve added. I added two lines, one for advance and one for total mechanical. For future reference those two lines are degrees not vacuum, but scale is the same. Really informative looking at them this way. Looks like vacuum is directly related to timing. So if my mechanical curve changes, so would my vacuum. I feel like the relationship between spark and load is here between the lines somewhere. One more thing I should mention. I didn't go any higher to read the advance because everything got weird. 3500 seems to be where everything comes to a head. I should have two more degrees of mechanical advance but if I go passed 3500 the pointer bounces like crazy and comes back down, just like my vacuum signals did. Also got all kinds of symbols on my timing light. Then the numbers turned into gibberish and the screen went blank and I had to reset it. Need to see what those symbols mean.

 

Just an update, timing advance laws broke down when I tried to dial it in. Numbers were all over the place. Pulled the Dizzy and the gear has a lot of wear. Have been too afraid to check the cam gear. I have done a lot of reading on it and it maybe I have a high pressure oil pump, or the wrong gear was used, or it has a million miles on it. Then I was too busy. Will inspect the gear first chance I get. I'm thinking the wear was causing the timing to get crazy after 3500 RPM. Not sure what to do about the distributor. It's the original distributor best I can tell, still has the factory tag. Distributor and advance can have motorcraft on them. Not sure if I should buy a cheap one, or try to rebuild this one. I have it sitting out on the porch so I can stare at it when I go out to smoke. Haven't looked really hard to see if I can get parts, initial search says no. Guess I could buy the cheap one and swap the parts.

 

If I'm not mistaken the dizzy gear is just held in with a roll pin. I think you can buy softer/harder gears for them...Then you can keep your old one, or just swap the shafts?