Vacuum advance tuning

[Anonymous]

If you have 68 distributor with the original vacuum advance canister, take a wrench and remove the end where the vacuum hose fits. Unscrew the fitting & expose the spring & spacer. you can add washers to limit the vacuum advance travel. You will have to experment.
If the canister is sealed you can try to adjust the advance with an allen wrench. Adjust from there, your choice is direct manifold vacuum or use ported vacuum as explained by jack. Any questions just give a shout. William

Aha! This is where I was not understanding correctly- that the spacers are inside the cannister- from the article I thought they were in the dist. itself. I have to go look again, but it seem's to me the the cannister is two halves, and they are pressed together with an outer lip over the inner lip where the two halves meet- the half that the vac. line goes into has what look's like a nut built into it- if you turn that the cannister turns. I know the there is not an allen adjuster in there as I've tried all my wrenches.

So are you saying that I separate the two halves to get at the spring? Either way, I'm going to have to wait. The YF that I returned last sunday was the only one in stock- the next one is back ordered from Autoline in Canada and word is I might have it next Wednesday. In the meantime, I'll have to run the 1100 and fill the tank every 180 or so odd miles- mileage is definately worse than what the 170 got!

Joe,

I had way too much advance with manifiold vacuum at idle (40 deg and very poor idle) so my thought is that if I go with manifold hookup, I have more "correction" to make within the advance system than before.
Playing with mixture screw adjustment did not resolve the poor idle I got with manifold vac.

I 'spose I could adjust the vac. advance now with the manifold vacuum since the 1100 is not getting port vac. at all, and like you, my manilfold vac. readings were only slightly higher than at the carb (YF). However, won't adding more shims to reduce adance at idle give me less advance when I want it ?(just above idle is where I want it to kick in, right?)

When I do finally get the carb, and if I can figure out how to adjust this vac. advance cannister, about how much advance DO I want from the cannister at various engine speeds?

Thor

 

[Lazy JW]

15 degrees of vacuum advance really should be plenty, 20 degrees is great gobs (I think it's too much). You want max vac advance at part throttle to increase cylinder pressure and no vac advance at full throttle (lots of cylinder pressure already). I wouldn't expect 40 degrees advance at idle to work well but 25 probably will be tuneable. I've seen engines with big cams that NEEDED 20 degrees of initial (not vacuum) advance to idle well. As far as your concern about not getting enough vacuum advance after recurving the canister, just remember Initial+Mechanical = 35 degrees, then add 15 for vacuum and you have 50 degrees Total. That should be adequate for most anything.

Sorry I can't help with specifics on recurving your distributor. My Motorhead days back in the 70's were pretty much squandered on the Dark Side so never had any Enlightening Experiences with Ford until marrying into an Enlightened family of Fordoholics
Joe

 

[Anonymous]

just remember Initial+Mechanical = 35 degrees, then add 15 for vacuum and you have 50 degrees Total. That should be adequate for most anything.

Thanks, Joe,

This is going along with what Mustang Six said, but it still confuses me. I thought total advance (int+mech+vac) was suppossed to be no more than 36-42 deg. If I get detonation after I've added washers in the cannister to get a more reasonable amount of advance, and hooked up my vacuum line again, how do I know if I should reduce vacuum advance, or intitial?

As for Joe's comment about wanting full vac. advance at "part throtte", can anybody be more rpm-specific? I'm assuming this would be under 3000 rpm somewhere, but anything below full throttle is part throttle

I'm assuming advancing my initial timing until the idle gets rough would be an accurate way to know how much advance the engine can handle at idle from the vacuum canister, right?

 

[Lazy JW]

This may be easier to understand if you think of the vacuum advance as being an extra freebie, to wit; The engine needs to be able to start, run, idle, drive, accelerate, cruise, drag race, skid logs, whatever, with only initial+centrifugal advance and no vacuum advance. It shouldn't stumble, ping, backfire or cause gas pains When all of this functions correctly, then you can add the vacuum advance for the express purpose of increasing fuel economy at part throttle cruise. If it then starts to stumble, ping, backfire, or cause gas pains, then the vacuum advance curve needs to be modified (probably limited). Defining "part throttle" is a bit difficult. With a ported vacuum such as my YF, when the throttle is slightly opened, just enough to uncover the port, it immediately pulls in max advance. As the throttle is opened further, the vacuum drops and so does the advance, until at some point it is simply running on initial+centrifugal. Remember, the vacuum advance is LOAD DEPENDANT and NOT rpm dependant. Thus, when I'm climbing up out of Mica Flats with a load of hay at 2000 rpm standing on the throttle, I probably only have about 25 degrees advance (just guessing) but when I go down the other side in third gear at 3500 rpm and open the throttle just enough to open the port it will be about 50 degrees advanced ( TOTAL) . So we need to think about vacuum advance in terms of how much LOAD is on the engine. Lightly loaded, lots of vacuum advance, heavy load, no advance.
Joe

 

[66 Fastback]

Joe has given you some good guidelines. Just remember that if the vac advance kicks up the idle speed, adjust the idle back down. You want to keep the idle speed down in order to minimize the contribution from the centrifugal advance when setting the timing and idle speed. Centrifugal advance may kick in at as low as 1000 rpm and should be fully advanced by 3000 rpm. As Joe has mentioned, total advance at WOT should be in the range of 35 deg. You might want to plug the vac advance and take timing readings at 500 rpm increments up to about 3000 rpm. Keep an eye on the temp guage, because a car can overheat if left idling with the vac advance disconnected. Then repeat the same drill with the vac advance connected, recording all the readings. This will show you when and how much centrifugal advance is kicking in at a particular rpm. If you are not getting enough centrifugal advance or it is coming on too slow, you may need to play with the weights or springs. Try some road tests with the vac advance disconnected. If if you get pinging with no vac advance you need to reduce the initial timing or change the springs or weights on the centrifugal advance mechanism to reduce the centrifugal advance at that particular rpm range. Road tests will tell you where you need to make changes.
The reason I went thru the above explanation is because if you get pinging with vac advance connected, it may not necessarily be the vac advances fault. You need to make sure it is not pinging with the initial and centrifugal advance because if it is, all the modifications to the vac advance may be in vain. Then you may end up backing off initial timing and decreasing throttle response when it may be the centrifugal that needs the adjustment.
Something you might want to check on your vacuum advance is to hook up a vacuum pump to the distributor. Let the car idle and take advance readings at various increments of vacuum. You will see that there is a minimum vacuum required to start moving the advance and another reading at which 100% of the vacuum advance is kicked in. If the vacuum is fully advanced at low levels of engine vacuum, it will likely give you problems with pinging if your engine has relatively high vacuum. Conversely if you have a lumpy idle & low engine vacuum and the vacuum cannister does not fully advance with your engine's normal vacuum, a different cannister or may be required. Ideally the vac advance should be fully advanced at a vacuum level 1 to 2" below the engines idle vacuum.
Doug

 

[JOHN G]

having same problem with mine (3 webers). I'm using port vac and its pinning the advance at idle in gear! Try retarding it and see what happens-too much advance will cause high vac-this doesn't seem to remedy my problem, but maybe yours. A master tech friend of mine is going to help wire up a vac solenoid that has an electrical trigger on it that may be able to hold off vac to the canister until the needed time-this will also require a vac ball that i can get at the boneyard. I have been told by several sources including Clifford that 32-36 degrees total timing is what is best-they also suggest a different dizzy, but i'm not buying into that. Mine pings at wot, but i reduced some of that by running a larger 12v lead to the coil-it was starving under high rpm for amps and causing detonation problems.

 

[Anonymous]

If the vacuum is fully advanced at low levels of engine vacuum, it will likely give you problems with pinging if your engine has relatively high vacuum.

The above quote is indeed my situation with manifold vac.,and taking a closer look, not only does it not have the allen adjustment, the fitting does not unscrew to expose the inner spring and shims. However, I the old dual advance cannis ter that came with the engine does come apart and have about 3 shims in it. It's an ugly looking canister with the extra line out the side, but I'm going to install it for the moment to see if I can adjust things enough to improve the situation. If it does, I will look for a more "attractive" cannister when I get around to detailing things

Thanks, John, Joe, and Doug- those last three posts clarified things much better as far as what I'm trying to accomplish. Being that I have no pinging at the moment, I should be able to play with the cannister and if I get pinging I'll know that's the problem. The lack of ported vacuum with the autolite I"m running now is probably (and hopefully!) the reason for my very lousy gas mileage- about 40 miles per 1/4 tank!

Thor

 

[Anonymous]

Okay, I spent some time on this last night. I hooked up to manifold vacuum, and with about 12deg. of intitial timing, when I hooked up the vac. line to the cannister I got 40 deg. advance at idle!

With no vac, I had 30-32 deg, total timing at higher engine speeds. With the advance connected, I got over 50 total.

I then crammed as many small washers as I could inside the cannister (even a lockwasher :roll: ) . With initial advance backed off to 10 deg., I had 20 deg. total at idle (or 10 deg vac.), and 40 deg. total. I did not have my tach hooked up, but once it got up to 40 it crept up to about 42 total.

I can't get any more washers/spacers in the cannister or I won't be able to get the threaded end cap back on. I'm not getting pinging as far as I can tell (still have noisy exhaust leak) but isn't 42 still too much total advance since total is supposed to be no more than about 35? I drove about 40 miles today on about 1/4 tank of gas- so there's no mileage improvement

I thought somebody said vacuum advance only works under load- if so, why the extra 10 deg. of advance (with the vacuum hooked up )at 3000 rpm when there is no load?

Thor

 

[Anonymous]

I thought somebody said vacuum advance only works under load- if so, why the extra 10 deg. of advance (with the vacuum hooked up )at 3000 rpm when there is no load?

It is just the opposite. Under load there is little to no vacuum advance.
At wide open throttle under driving conditions, the contribution from the vacuum advance drops to nothing because the engine vacuum is low. At these conditions, you want in the ball park of 35 deg of total advance which is from the initial and the centrifugal.

There are times when you can be driving along at higher rpm's, and the engine load is light as when Joe described running downhill. The throttle plates could even be nearly closed, and there will be sufficient vacuum to advance the vacuum advance. Total timing readings of 50 deg or higher are possible under these conditions are not destructive.

 

[JOHN G]

tinkered with mine yesterday-the vac diapraghm requires 4 lbs less to open than what my engine is putting out-wouldn't i have a stumble in accel right about the time i would engage the secondaries. the wot pinging stopped and surfaced at midrange throttle. are you guys setting the base timing at 12 with or without the engine in gear and what rpm?

 

[Anonymous]

There are times when you can be driving along at higher rpm's, and the engine load is light as when Joe described running downhill. The throttle plates could even be nearly closed, and there will be sufficient vacuum to advance the vacuum advance. Total timing readings of 50 deg or higher are possible under these conditions are not destructive.

At 3000 rpm, how could the throttle plates be almost closed? I'd think they'd be at least 1/2 of the way open since our engines only rev to about what- 6,000 rpm. I've never gone there, and don't plan to

If this is the case, then why the heck has everybody (or so it seems) been telling me for the past two weeks (see my other thread on "major tuning problems) 50 deg. is too much and I shoud disconnect the vacuum until I find a way to reduce the advance? I'm surprised that in 20 years of working on cars I've never heard that this amount of total advance is OK

If this is okay, should I then remove the extra shims I just put in until I either get pinging or idle quuality starts to go bad? Can too much advance hurt fuel economy? I'm probably getting about 14 mpg- not good!

John,

I set my idle at about 800 rpm idle speed in park with the vac. disconnected.

 

[Lazy JW]

...... Remember, the vacuum advance is LOAD DEPENDANT and NOT rpm dependant. Thus, when I'm climbing up out of Mica Flats with a load of hay at 2000 rpm standing on the throttle, I probably only have about 25 degrees advance (just guessing) but when I go down the other side in third gear at 3500 rpm and open the throttle just enough to open the port it will be about 50 degrees advanced ( TOTAL) . So we need to think about vacuum advance in terms of how much LOAD is on the engine. Lightly loaded, lots of vacuum advance, heavy load, no advance.
Joe

The conditions that I referred to will normally only happen when COASTING. Like when I'm coming down Cougar Gulch Hill into Coeur d' Alene, Idaho with a load of hay and I'm in third gear letting the engine hold me back at 3500 rpm, the throttle is closed, the mechanical advance is maxed out, and if I was using manifold vacuum it would be maxed out also. And even with using ported vacuum I only have to open the throttle slightly to get max vacuum (and advance). I promise, the TOTAL advance can, and will reach 50 degrees or more under those conditions with no harmful results (there is really no load on the engine). Now, when I get to the bottom of the hill and shift up to fourth gear and mash on it to keep my speed up, the vacuum immediately goes to zero and the vacuum advance drops off, leaving only initial+mechanical. If I were to lug it down to 1500 rpm at full throttle and somehow crank in 50 degrees advance it would merely knock big holes in the pistons.
Joe

 

[Anonymous]

Everyone, (myself included) has been saying that 50 deg of total ignition advance at idle is excessive. We have been recommending something in the range of 25 deg total advance at idle (10 initial + 15 vacuum). At idle the contribution from centrifugal advance should be very small, so all you are left with is initial + vacuum advance. At higher rpms, the centrifugal advance it fully advanced perhaps accounting for a total of 25 deg on its own. Adding the initial advance of 10 deg gives you the proper advance for wide open throttle conditions of 35 deg.

Higher Engine speed does not necessarily imply that the engine load is high. Now depending on engine speed, engine load and throttle plate position, the vacuum advance will contribute some portion or even all of its 15 degrees of advance and could go as high as 50 deg. (Actual numbers may vary, your mileage may be different, add'l fine print and disclaimers etc, etc.)

I can't tell you how much impact vacuum advance has on fuel economy. If it affected it by 10% that would seem pretty significant. I think in order to make bigger gains in the fuel economy, you will need to address the carb jetting.
Doug

 

[addo]

Well, as to economy - replaced the Holley distributor (yes, really a Holley diz) in the Irrational with a Bosch electronic unit last December. It gained almost 50% in economy and with mechanical advance only! Engine is too worn to idle down and sort the vaccum advance, so it's disconnected.

 

[Anonymous]

Adding the initial advance of 10 deg gives you the proper advance for wide open throttle conditions of 35 deg.

Okay, I think this is where I've been getting confused (I've been confused about how I'm confused ) I keep reading about the 50 deg. of advance with vacuum at lower speeds being okay. If 50 deg. of advance at 3000 rpm is okay, as is unanimously agreed to, how do I check for the 35 deg. at wide open throttle that '66 is speaking of? Wide open throttle in park = thrown rods, and oil all over the driveway following in eviction from my landlady (no wry comments, Addo )

I'll pop a few washers back out of the cannister to see if it will idle smoothly at 25 deg- I don't think it did the other night when I tried it but that could be due the the carb I have on there right now.

As for carb jetting, the current 1100 seem's to be jetted about right although it's got the poor idle I get with all the 1100's I've tried. I just picked up the 2nd YF today, but it was missing the choke fitting so I have to deal with that tomorrow before I get to try it.

 

[Anonymous]

Sounds like your distro is fubar'ed.....why not replace it to when you install your new carby :

Just FYI...A V 8 adjustable vacuum advance will not fit on a 6 banger distro....or at least it wont interchange onto a Duraspark distro. I have one of each sitting in my shop...

Doug

 

[Lazy JW]

.....I keep reading about the 50 deg. of advance with vacuum at lower speeds being okay.....

NO! NO! NO! 50 degrees is okay at low LOAD :!: Please do not confuse low RPM with low LOAD :!: The only way that 50 degrees can happen is for the engine speed to be high enough to max out the centrifugal advance and then have the throttle shut enough to pull in full vacuum advance.

Now, to check your max centrifugal advance, it is NOT necessary to give it wide open throttle. First, mark the timing pulley in degrees up to at least 40 degrees BTDC. Every five degrees will suffice for this purpose. Then, plug the vacuum advance. Now, using the timing light, check the advance while carefully revving the engine up until it doesn't advance any more. It should be all done advancing by about 3000 rpm, and shouldn't go over about 35 degrees. The centrifugal advance is dependant on engine SPEED, not throttle position or load.
Joe

 

[Anonymous]

Sedanman;
Boy, you've sure run the gamut in this post!
Vacuum advance is used to improve engine efficiency. It does this by increasing the engine's RPM for a given throttle setting. For example, you already know that applying vacuum to the distributor at idle makes the engine run faster, because it advances the timing.
When the engine load increases, the manifold's vacuum decreases proportionately. The throttle plate vacuum is artificially created in the venturi of the carb: it comes in 2 flavors - delayed and non-delayed. The delayed version is typically used on trucks, where the vacuum does not appear until 1/8 throttle or more, and then only under acceleration. The non-delayed version used in light (i.e., passenger) vehicles does not exist at normal idle speeds, but appears during light acceleration and up to about 1/4 throttle. This non-delayed vacuum is often used at the distributor because it improves throttle response and efficiency if the driver is easy on the throttle, and it disappears under heavy throttle to prevent ping and reduce engine temperatures.

Normally, there should be no vacuum in the "spark port" at idle. If vacuum is happening at idle, then the idle is either too fast or the initial spark advance setting is too high. My 200 starts to create slight spark port advance when the timing is +12 degrees and the idle is 800 RPM. Dropping the idle to 600 RPM then makes the spark port vacuum just barely disappear. At this time, I have 12+26(dist)+22(vacuum) degrees for a total of 60 degrees, with 9.2:1 compression, and I get 24 MPG hiway/20 city. (This improved my MPG from 14/16 without the vacuum advance.) There is no ping, even on the cheapest gas. Throttle response is very crisp, too!

The "total advance" numbers of 32-36 degrees refer only to the (mechanical advance + initial advance) in the distributor. The vacuum advance can be VERY high if the diaphragm has a weaker spring: this will improve gas mileage under easy driving conditions. If the diaphragm spring is stronger, more vacuum is needed to create the advance, so less advance happens. For better mileage, a weak diaphragm spring with lots of advance capability is desirable. If you were to modify the engine and make higher compression, the diaphragm spring must become stronger to make the advance less for a given throttle setting (to prevent ping).

 

[Anonymous]

Okay, after spending many hours of trial and error on my timing this weekend, here's my conclusions,

1)Manfold vacuum is just too much- even with my dist. cannister crammed full of washers to retard advance, I get about 25 deg (14 in/11 vac) at idle. When I hooked back up to ported vacuum with no vac. advance at idle, it made a huge differance off the line and the car felt much quicker.

2)The carter YF pulls way more vacuum than the 1100 did. I put the canister back in that I had been running with the 170 and 1100 for two years with no problems (non-loadamatic); It gave 55 degrees of advance at only 1500 rpm and ran terrible!

3) With no vac. advance hooked up, 14 deg. initial did not ping at all, but the car idles better (sounds smoother at the tail pipe) at 12 deg so I left it there.

4) With no vac advance, I'm getting 22 deg/1500 rpm, 31/2000rpm, 33/2500, 35/3000 and stay's the same in higher rpms.

5) With the ported vacuum and a bunch of washers in the cannister, it has 22deg/1500, 35/2000, 43/2500, 50/3000, and 57/3500 rpm.

6) Since the cannister had fewer washers in it before I started chaning things, I took the extras back out. This got me 24 deg/1500 rpm, 46/2000, 54/2500, 60/3000, and at 3500 rpm it went to about 62 deg.

I can't say that I feel a real seat of pants differance, but the carb is not working properly so it's hard to tell it performance is better/worse after removing the washers. The manifold vac. experience showed you can indeed have too much, so I may be better off with puting more shims back but I'm really not sure. The differance of advance between 2000 and 3000 rpm is 10 deg. after removing shims from the cannister. But since those figures are when the engine is not under load, I'm not sure what that differance really means.

Once I get the carb right, I think I'll have to have a professionl re-curve it to get the "sweet spot" since I really can't seem to tell by seat of pants.

S-man

PS- the article in Mustang and Fords this month about distributor tuning says total advance as read by a timing light, with vacuum hooked up, should be more than 36-40deg. Are they ignorant on the subject 50-60 deg. being okay if there is no load?

 

[Lazy JW]

Your mechanical advance numbers look pretty good, certainly close enough to tune the carb. Now, PLEASE unhook the vacuum advance until everything else is tuned correctly. It just makes things so much more difficult trying to sort things out with a known problem in the mix. Vacuum advance simply is not necessary to make this engine perform well. It will help your economy later on though.
Joe