Alright well the time has come for me to try and tune the vacuum advance on my 351M. Had someone drive the truck around today with me in the passenger seat watching vacuum reading and at highway speeds around 1,800-2,300 the truck seemed to be around 7-9 or 11-13 on the vacuum gauge. I am running manifold vacuum at the ported port on the carb. Reason is I have to have the transfer slot open more so it is reading manifold, it HAS to be like that for the truck to idle, it wants more air. 13 Degrees initial, 23 degrees at 2,000rpm so 10 degrees mechanical. When I have the vac canister adjusted all the way Counterclockwise the truck acts like it has a performance cam at idle but faster up and downs, runrunrunrun, that deal. When I adjust all the way Clockwise the truck backfires / misses and will not run at all. It runs and drives near perfect if not perfect with no vacuum advance, I could drive it all day like that but I am moving very soon long distance and would like the extra MPG from what I heard it makes a giant difference. Maybe the canister is bad, it holds vacuum with a hand pump fine.
Also if yall are wondering all the way Counterclockwise vacuum advance starts at 9 and is all the way in by 15. Clockwise starts at 4 and is all the way in by 10 I believe.

 

You still have something screwy going on.

Your mechanical all in at around 2500 should be at least 10 degrees more than you have now. somewhere 34-38 normally. then add 15-20 vacuum advance to that on a stock engine. if you can't get it there and make it run properly there's no point in trying to tune on it until you get the underlying problems fixed. I don't remember what all you've done so far .

 

Vacume advance canisters do not affect manifold vacume. If the carb needs more air to idle, you adjust idle mixture screws. Having transfer slots open more is not supplying more air to the mixture, it's just setting a higher idle and does nothing for fuel mixture, but it does put a steady vacume on the distributor vacume canister thus advancing the timing a bunch at idle.

Once warmed, mine idles great with timing set total mechanical advance at 35 BTC, I have a short mechanical advance kit with light springs so it's all in before 2,000. I disconnect vacume and plug the hose for setting. I adjust idle as low as it runs with no stumble or to spec, whichever is lowest, the adjust idle mix screws for fastest idle, and readjust idle to keep it low. When no turning of screws raises idle any more, I've reached best. Lock it and reconnect my ported vacume line to the canister.

 

Couple thoughts:

Has engine ever been rebuilt?

11-13 on the vac gauge if cruising at mostly no load (not accelerating) at 2000rpm seems way too low. 18-20 should be closer to what you want. So that's one issue to resolve.
What is the vac reading at 650-750 idle rpm?

Next, need to find root cause of having to open throttle plates too much to get it to idle. If you have vacuum module line on the ported carb outlet you should not be getting the equivalent of manifold vacuum from it. So a little more investigation there. Maybe I am misinterpreting on this.

If the vac can holds vac when tested, normally they are fine. I always confirm a replacement module by checking how much it increases advance at full stroke. You could try plugging the vac line to module, take rpms up to get all the mechanical in, then with timing light increase advance manually by turning distributor 10-15 more degrees and see if it acts the same as with module connected.

But really I think the first order of business is to get your vacuum readings more in line with normal. If you can, compare your gauge to another to make sure its accurate

 

ok so here is the deal, I have spent months and literal days or research trying to get the engine to idle with the transfer slot at a square, it WILL NOT do it, period. Idle mix screws do stuff but nothing to raise the idle oh about 700 rpm. When I open the transfer slot/throttle blades a bit the engine runs amazing but exposes the ported port to vacuum making it the same as manifold. Opening the transfer slot / throttle blades do in fact add more air into the engine because the blades are open more allowing more air into the engine Transfer slot also affects idle air fuel because as more transfer slot gets exposed it goes from idle circuit to adding a run circuit on top compounding. When I rev the engine idling in park it gets up to about 20 vac but at highway speed cruising it seems to be around the readings stated above maybe I miss read. I am not here to figure out why the engine wants more air at idle, oh and it does not want more fuel. I am here to ask how my vacuum advance canister should be set up / tuned running manifold vacuum.
As for mechanical I need to go out today and see if it picks up the other 10 degrees at 2,800.
And the engine has been rebuilt, passes a compression test.
I also could type up a Very lengthily article about all this but people never read them and reply.

 

Is your dizzy a Duraspark ?

If yes, it sounds like it could be a faulty pickup.

What is your altitude ?

 

Yes it is a stock Duraspark 2 distributor. I Have replaced to pickup / hall effect sensor 3 or 4 times and this one seems to work well and is in spec. Altitude is 1300ft

 

If you used Standard Ignition pickups, part number LX204, then suspect the current one as being faulty, even if it passes the ohms tests, and replace it with a Napa MP700SB, and hopefully the Napa one is fault free out of the box.

This might actually be the problem referred to in your ''misfire before and after engine rebuild'' thread.

Your vacuum advance is not at fault.

At your altitude, I would also expect 18-20'' vacuum at an idle of +/- 800 rpms as suggested earlier by Mark8man.

 

Yeah I actually have not got back to that thread yet I have been swamped, the misfire is fixed, A while back I was afraid to drive the truck but my dad really really wanted to take it out. So we went on a little trip and he drove, about 20 miles total and he romped on it and really gave it the beans. After some black smoke from a miss at full throttle up in the rpms it runs great. Yesterday took it for a trip into town and it ran near perfect minus the brake pedal not returning all the way:/. Stood on it and even without vac advance it shifts really fast and hard and moves out for a stock setup.
Also yes I did get the SIP LX204 but I think it works fine.
As for vacuum I will have to have him take it out again with me in the passenger seat and get some more readings.

 

Haha, don't be so sure !!

The symptoms you described tell me it's not fine, but I could be wrong.

No need to drive the truck to get vacuum readings, especially as they fluctuate between 0'' and 25'' during differing driving conditions.

Use the idle reading as your benchmark. That is sufficient.

I'm glad your misfire problem is sorted.

 

You don't adjust idle speed with idle mix screws. You set the idle as low as it will run using the curb idle adjust screw there ot the throttle lever ... then you adjust idle mix screws for best / smoothest idle which is also the fastest, then you adjust idle RPM with that curb idle adjust screw that affects the lever on the side of the carb by setting a limit as to how far the carburetor will shut.

 

The first thing you need to fix is the fuel transfer slot being open to much and yes I know you said you have worked on this and this is where it works.
All you did was a Band-Aid fix and you may have more than 1 thing going on.

Is your cam that large that it will not idle without more air?
What carb do you have? Holley's you can open the secondies to let more air in and close the primaries.
The secondary has a small screw you can get to from the bottom. When off pull the screw out and put it in from the top for adjustment.
Adjust this sdrew so it ever so little opens the secondies just enough so you can adjust the primaries and the mixtures.

Once you get this fixed its time to plot your timing. What is the timing set to at idle?
Mark down the idle speed & timing deg. and every 500 RPM up to 3000 or even 3500 RPM. This is with out vacuum, just centrifugal.
Then hook up the vacuum and do the same plotting. Now you may want to adjust it all the way CW and do the test then say 10 turns CCW and plot again.

You need to get the centrifugal dialed in before anything with the vacuum.
Its a long road that I am going down now with my 81 F100 with a 300 six.
I was getting pinging with no vacuum so I had to change out springs. Pull dist, change springs, road test & plot.
I am on my 3rd time needing to pull and change a spring but I am close now with centrifugal then I can dial in vacuum side.
Dave ----

 

The transfer slot adjustment can not be fixed. The carb i have is a Chinese unit that is smaller than stock. Trust me even though its Chinese its brand new and actually decent. The truck will not run if I close the transfer slot/throttle blades I could chase it for another year and it won't work the carb is to small but it's cheap and I have NO money to spend on a 4bbl and an intake so I HAVE to make due with what I got. I am really trying to be nice because yall have helped me out a lot in the past on here but I need to make it very clear that the transfer slot is not getting fixed band aid or not. The truck runs the best it ever has since probably over a decade ago plus when I was still playing with dirt and sticks, a little kid. What I need help with is simply getting the vacuum advance working. This can be solved regardless, you can run manifold advance on vehicles and while most (I included don't like it, it is what I have to do) don't like it and would prefer ported it is what it is. Idling in park at 2,000 rpm the truck is about 20 inches of vac my issue it if I turn the key inside the vac advance canister all the way to the CCW which is what I would need to have vac advance all in at 20 inches the truck does the medium fast ups and downs. Looking back all the way CCW the advance starts and 9 and in all in at 15 hmmm I wonder if the canister it bad? I need to double check that in the morning. Also, for the springs both looked to be stock, them and the weights. Across the board the truck is stock including cam.

 

Well if the slots cant be fixed then you cant use ported vacuum for advance.
You said you cant adjust the secondaries to stay open just a smige then another trick is to drill a vary small hole is each of the primary blades.
Start really small and go up a little at a time.. I think they say to use number drill bits and not fraction bits you get in a normal drill bit set.

The holes makes a vacuum leak to let in more air that you need to be able to close up the slots.
Thing is to go just large enough to be able to close the slots but not to big that you cant control the idle speed.
Some of Holley carbs I have seen come through with holes on the blades just for this.

Now you said all the way CCW. When I ran mine all the way CCW when it got to the end of adjustment it just clicked like the screw was out of the nut but had pressure trying to push it back in. All the way CW it just stopped.

A hand vacuum pump works great for this next part but can be done with a vacuum gauge teed in to the vacuum advance can.
What you will need to do is check at what the vacuum is when the arm just starts to move with it all the way CCW.
Then you will need to check at what the vacuum is when the arm just starts to move with it all the way CW.
This way you know what each end of the scale the vacuum is with CCW or CW.

I cant read my notes but it all made sense when I wrote it down.
I think with the adjustment CW it took less vacuum (think 9hg) to advance than all the way CCW (think 16hg).
That would kind of make sense as I am 7 turns CCW from stop of CW to stop the pinging.
CW the timing was coming it too soon and I was getting pinging.

Now before you go messing with the vacuum side of advance you need to get the centrifugal dialed in first, no vac when doing this.
It sounds like you have not done the centrifugal side yet unless you have and you found that both factory springs work the best for you.

My centrifugal was coming in too soon and was getting some pinging with my timing at 10* BTDC
But at 10* BTDC ther motor was harder to start both cold & hot, it liked 16* BTDC but I am at 14* BTDC right now for testing.
Once I get the centrifugal dialed in, have 2 more spring change, and if that is good I will bump the timing up to 16* BTDC.
Then I can work on the vacuum side even that I have played with it a little where its at because I think this is where I will end up at.

Its a lot of work to dial in centrifugal and then vacuum but that is the only way it can be done right.
Each time I had to do a spring change the dist. had to be pulled, taken apart, spring changed, put back together, put in the truck, timed to 14* BTDC, plot the timing curve and a road test each time. With out plotting the curve and road test you dont know where you are or went.
I did it from idle up to 3500 RPM in 500 RPM jumps.

BTW this is all on my 300 six motor but a v8 would be the same.
Dave ----

 

Thank ya for commenting back it really does help me and get me thinking. I am running a stock 2150 style 2bbl right now on the truck and have heard of the drilling holes but I really would prefer not to. My intention right now it to run manifold which is why I am wondering what's going on with not running across the vac advance range. The other day I took my distributor apart and was checking stuff over and it for sure has the stock springs in it which should work the engine being stock and all. I have the timing one degree above what the valve cover says at 13 initial. I also was checking over the OEM Motorcraft vacuum advance canister and I stripped hex screw inside out:/ Super bumped because it was stock and "Vintage" but it is also over 40 years old. I am ordering a new distributor off rockauto. Just so happens that the OEM Ford distributor does have some play, more than some. About a 1/32" up and down and some side to side. The whole reason I got on here was to figure out why its running horrible with manifold vacuum source and I am aware ported it better but not attainable right now. With manifold from what I have seen there should be no revving up and down or backfiring and not running at all when adjusting which tells me something is wrong with the distributor vac or the cheap pick coil. I am getting ready to move long distance and would like the extra mpg on an over 500mile trip. If I had to guess the mpg will probably be pretty bad, it does have 245s on it so maybe not to bad, maybe 5? It seems like the truck gets really good gas mileage now but I have take it on a voyage and see.