1981 straight six manual 4x4 project

[Gary Lewis]

Good point Pete! I should be more specific instead of anthropomorphizing. When I hooked my distributor up to manifold vacuum it caused a very rough idle, and greatly reduced power while driving. The idle was much faster (higher rpm) than when hooked to carb vacuum, yet so rough it would barely stay running without some throttle. I am not familiar with the differences between initial timing and mechanical advance and vacuum advance. I've usually just disconnected and plugged the distributor vacuum and set the timing with the light at idle. Obviously I need to educate myself on timing and carb tuning. It seems to run well where I have it, but that is just a subjective feel, I am sure there are gains to be had by using better methods.

Gary, I guess that is one drawback to a large single tank. If you get bad fuel you get a lot of it. However I think one would be equally likely to purchase the same amount of bad fuel in two 19 gallon tanks if so equipped. I do know that it hurt to buy gas at $4.09 per gallon inside the park when Beaty and Amergosa Springs had it for $2.45!

If a 300 six is set up to run well with carb vacuum it won't like manifold vacuum. That's because the carb vacuum is limited in total vacuum as well as being ported. But manifold will be there all the time and have a lot more vacuum. And, if you are running it where it wants to be with carb vacuum then manifold will be way too much.

As for the big tank, it is well worth the risk of getting bad gas to have that much capacity.

[Ford F834]

If a 300 six is set up to run well with carb vacuum it won't like manifold vacuum. That's because the carb vacuum is limited in total vacuum as well as being ported. But manifold will be there all the time and have a lot more vacuum. And, if you are running it where it wants to be with carb vacuum then manifold will be way too much.

So to set it up to run manifold vacuum you set the timing much further back (less advanced) using the light, then let the vacuum advance it?

[Gary Lewis]

If a 300 six is set up to run well with carb vacuum it won't like manifold vacuum. That's because the carb vacuum is limited in total vacuum as well as being ported. But manifold will be there all the time and have a lot more vacuum. And, if you are running it where it wants to be with carb vacuum then manifold will be way too much.

So to set it up to run manifold vacuum you set the timing much further back (less advanced) using the light, then let the vacuum advance it?

Maybe, or maybe not. To answer that we'd need to know what your carb vacuum does. Maybe it is the same as David's, but maybe not as both of you are running non-stock carbs.

Here's some reading on the subject, which I will also do to refresh my memory:

• David's Ported or Manifold Vacuum - Personal Observation

• David's Gas Mileage Recipe - 4.9L/300 (I think he did it on this forum as well?)

[Gary Lewis]

Maybe, or maybe not. To answer that we'd need to know what your carb vacuum does. Maybe it is the same as David's, but maybe not as both of you are running non-stock carbs.

Here's some reading on the subject, which I will also do to refresh my memory:

• David's Ported or Manifold Vacuum - Personal Observation

• David's Gas Mileage Recipe - 4.9L/300 (I think he did it on this forum as well?)

Ok, according to this post in David's Gas Mileage thread he's running manifold vacuum. And, in the other thread, in the above link, he was having rough running while coasting, so turned the idle mix 1/2 turn - I think. And, he's running the vacuum advance pretty much all the way to the right, meaning all the advance he can get at high vacuum.

So, given that I think you could go to manifold vacuum. And if it doesn't run well then maybe you have too much advance? If so, you might try backing off of the initial just to prove it. If that proves it, put the initial back and then turn the vacuum screw a couple of turns counter-clockwise to back off of some of the vacuum advance. Just keep records.

[Gary Lewis]

Ok, according to this post in David's Gas Mileage thread he's running manifold vacuum. And, in the other thread, in the above link, he was having rough running while coasting, so turned the idle mix 1/2 turn - I think. And, he's running the vacuum advance pretty much all the way to the right, meaning all the advance he can get at high vacuum.

So, given that I think you could go to manifold vacuum. And if it doesn't run well then maybe you have too much advance? If so, you might try backing off of the initial just to prove it. If that proves it, put the initial back and then turn the vacuum screw a couple of turns counter-clockwise to back off of some of the vacuum advance. Just keep records.

David - What about you creating a "project" about your truck, what's been done, and the results? I had to do a lot of reading to get to what I think the status is. But, if you had a project you could lock it so there aren't any comments and post the status right up front, and update it as things change. You could have links in it to other threads, both here and on FTE, but it would consolidate a lot of info and work.

Thoughts?

[Ford F834]

Maybe, or maybe not. To answer that we'd need to know what your carb vacuum does. Maybe it is the same as David's, but maybe not as both of you are running non-stock carbs.

Here's some reading on the subject, which I will also do to refresh my memory:

• David's Ported or Manifold Vacuum - Personal Observation

• David's Gas Mileage Recipe - 4.9L/300 (I think he did it on this forum as well?)

Thanks Gary, I remember reading at least the gas mileage recipe thread now. I understand it to a point, but I have no knowledge of how to tune centrifuge advance or vacuum advance on the distributor. I don't know anything about measuring AFR or using the information. David sent me a long list of YF carb numbers but mine was not on the list, so I have no idea what my junkyard mutt originally came from. Before I go wasting too much turd polish on this thing, I should at least give it a few good highway runs and see what my gas mileage is under decent conditions. Being a tall stance heavier 4x4 with no overdrive I certainly don't expect to match David's mpg numbers, but I do hope I can beat my 4.11 geared trucks that were pretty consistently hitting 15mpg.

[Gary Lewis]

Thanks Gary, I remember reading at least the gas mileage recipe thread now. I understand it to a point, but I have no knowledge of how to tune centrifuge advance or vacuum advance on the distributor. I don't know anything about measuring AFR or using the information. David sent me a long list of YF carb numbers but mine was not on the list, so I have no idea what my junkyard mutt originally came from. Before I go wasting too much turd polish on this thing, I should at least give it a few good highway runs and see what my gas mileage is under decent conditions. Being a tall stance heavier 4x4 with no overdrive I certainly don't expect to match David's mpg numbers, but I do hope I can beat my 4.11 geared trucks that were pretty consistently hitting 15mpg.

I agree you should give it a few tanks of highway runs to establish a baseline. That's very important. Then we can work from there and know if we've helped or hurt.

The centrifugal advance is probably all-in by 3000 RPM, which you can check with your timing light. With the vacuum disconnected you can map RPM vs advance over the initial advance. But changing it on a Ford dizzy isn't all that easy as things are buried. The curve is determined by two springs that allow weights to swing out and advance the rotor vs the shaft. There will be a light spring and a heavy spring, and you can change out the heavy for a lighter one to bring things in earlier. And you want all the timing you can get w/o pinging at WOT at any RPM.

Once the initial and the centrifugal are set you start dialing in more vacuum advance by turning an allen wrench inserted into the vacuum port on the canister clockwise. Each turn moves the window of advance up in the vacuum range, and frequently it is 1" per turn. Keep turning until you get some pinging at light throttle in a tall gear and then back off so you never have any pinging.

As for AFR, that's something we can determine by installing my wide-band AFR meter.

[PetesPonies]

If a 300 six is set up to run well with carb vacuum it won't like manifold vacuum. That's because the carb vacuum is limited in total vacuum as well as being ported. But manifold will be there all the time and have a lot more vacuum. And, if you are running it where it wants to be with carb vacuum then manifold will be way too much.

So to set it up to run manifold vacuum you set the timing much further back (less advanced) using the light, then let the vacuum advance it?

No, here is how it works. With manifold vacuum you will get a lot of advance ( limited by the advance cannister ) at cruising and idle. These are times when the manifold vacuum is highest. At WOT you will not have an vacuum advance. Therefore, you need a mechanical advance set up as if it was a distributor with vacuum advance. Like, for example, a 289 HP. So you want an aggressive timing curve directed by the mechanical weights. But what manifold vacuum will give you, generically, better idle quality and better fuel mileage. When engines have a "large" cam, meaning a lot of overlap, running manifold vacuum will allow them to idle better. This isn't going to be the issue with you. But you can see great MPG with the full advance at cruising speeds. I also like the fact that you remove vacuum advance and it's inaccuracies, when at WOT. But as stated, it means you should have the distributor set up correctly to take advantage.

Also, with your problem . . did you check the timing when at idle and you said it was rough? Bottom line with any advance is . . how much BTDC is the spark happening? On some of mine, I have seen 20- 25 degrees at idle, with vacuum connected.

[85lebaront2]

No, here is how it works. With manifold vacuum you will get a lot of advance ( limited by the advance cannister ) at cruising and idle. These are times when the manifold vacuum is highest. At WOT you will not have an vacuum advance. Therefore, you need a mechanical advance set up as if it was a distributor with vacuum advance. Like, for example, a 289 HP. So you want an aggressive timing curve directed by the mechanical weights. But what manifold vacuum will give you, generically, better idle quality and better fuel mileage. When engines have a "large" cam, meaning a lot of overlap, running manifold vacuum will allow them to idle better. This isn't going to be the issue with you. But you can see great MPG with the full advance at cruising speeds. I also like the fact that you remove vacuum advance and it's inaccuracies, when at WOT. But as stated, it means you should have the distributor set up correctly to take advantage.

Also, with your problem . . did you check the timing when at idle and you said it was rough? Bottom line with any advance is . . how much BTDC is the spark happening? On some of mine, I have seen 20- 25 degrees at idle, with vacuum connected.

Here is what I posted in another area on here a while back concerning spark advance:

On distributor curves, the mechanical advance is based on the best power under load without detonation, this basic idea goes way back as far as the moveable advance system whether it was a level on the steering column or a centrifugal advance mechanism. The Windsor V8s can take 36-38° total mechanical advance at around 5000 rpm, mechanical advance is 2 stage, a quick initial, up to around 3000 rpm for a street engine advance of 28-32°, the remainder coming in slowly to max rpm.

The vacuum advance came later as an economy measure and it makes up the difference between under load and light load conditions. There are two main styles, a vacuum brake system used by Ford on the flathead V8s and 6s with the front of the cam distributor and used by Mallory on their street distributors for years. The second is a vacuum can that has a diaphragm attached to a rod which moves the breaker plate (except on the old Chevy in-line 6, on those the whole distributor moves) this system has either an adjustment screw in side the can, or on older Fords a removable cap under which was a spring, shims and a hollow stop tube inside the spring. These are pretty straight forward, at X inches of vacuum you get Y degrees of advance.

What happens with the EGR is not that you can get more advance, it dilutes the mixture enough that the vacuum advance does not cause detonation. A specific example, the mid 80s Oldsmobile built 307 ci V8, if the EGR wasn't opening, you would get a light throttle spark knock at roughly 35-40 mph in 4th gear lockup, a bit more throttle and it would vanish when the computer added fuel by decreasing the down time on the metering rods on the E4ME primaries.

Now, to further confuse everyone, for a long time, Ford used Holley distributors, with no mechanical advance, only vacuum, called a Loadamatic distributor These had a 2 stage set of springs on the breaker plate, one very light one for the venturii signal, the other a heavier one for the throttle port signal. Depending on the year and application many had a spark control valve on the side of the distributor. This blocked the throttle port so that the very weak venturii signal didn't leak, when the throttle signal was there the valve moved in under the higher vacuum to allow it to the distributor diaphragm to receive the full vacuum. Some carburetors simply used a small check ball in the throttle passage. Because this system ran at maximum advance at cruise conditions, Ford found that it was necessary on some applications to use a second diaphragm on the back of the main one connected directly to manifold vacuum to quickly retard the main system under sudden WOT conditions. A quick way to recognize these systems, the diaphragm "can" is very flat on these as there are no springs inside it.

The next piece came with the emission systems, 1966 in California, and 1968 nationwide. One of the problems with many engines was idle emissions, unburned hydrocarbons and carbon monoxide (CO) also oxides of nitrogen. It was found that if the carburetor idle circuits were at the transfer slot that the idle could be set lean enough in the jetting that the engine could run cleaner. In order to do this, the initial timing needed to be reduced. The other solution was air injection. Each of the big three used a different approach, Ford used two systems, IMCO for IMproved COmbustion and Thermactor or air injection. Both systems used a lazier distributor curve and in some applications a lower initial (static) timing. It also was the death knell for the Loadamatic distributor as it's advance would vary with altitude and to some degree barometric pressure differences. The leaner low speed mixtures also would cause the Loadamatic equipped engines to misfire at low speed cruise due to the advance being fully in.

I hope this isn't too confusing, but hopefully it will help understand how the spark advance systems relate to other systems. All have to work together in harmony for best power and economy.

As for a choice of manifold vs. ported vacuum, the port in most carburetors and non computer controlled fuel injection systems is very close to the bottom or back side of the throttle plate (only one is used on multiple barrels) most Ford and Chrysler engines and some GM used a ported spark. One of the problems with direct manifold vacuum is the timing can retard if the vacuum goes below a certain level. This causes the engine rpm to drop, further reducing the vacuum. The manufacturer can use a light spring in the vacuum can so it has to fall way off in order to be a problem. It is more of a problem with automatic transmissions due to the engine loading in gear. This can also be a problem if the centrifugal advance comes in at too low an rpm. I used to see the older small block Chevrolets with a Powerglide come into my shop and the complaint was "I just got a curve kit and installed it, now I can't keep it running in gear and it races in neutral." Stall speed on the old Powerglide was real low and even a mild engine would drop 3-400 rpm from neutral to drive. As far as total vacuum advance, I never saw any difference between manifold and ported vacuum, the difference was what it did at idle. Whatever system, you still need to disconnect the vacuum when setting the timing, just be glad you aren't trying to set timing on a pre computer Oldsmobile V8, it was set at 1100 rpm, no vacuum because they wanted to be sure the initial centrifugal advance was in. Royal PITA, move the distributor, reset the rpm, check the timing, it would take 10-15 mins to set one correctly, but boy did it make a difference!

[Gary Lewis]

Here is what I posted in another area on here a while back concerning spark advance:

On distributor curves, the mechanical advance is based on the best power under load without detonation, this basic idea goes way back as far as the moveable advance system whether it was a level on the steering column or a centrifugal advance mechanism. The Windsor V8s can take 36-38° total mechanical advance at around 5000 rpm, mechanical advance is 2 stage, a quick initial, up to around 3000 rpm for a street engine advance of 28-32°, the remainder coming in slowly to max rpm.

The vacuum advance came later as an economy measure and it makes up the difference between under load and light load conditions. There are two main styles, a vacuum brake system used by Ford on the flathead V8s and 6s with the front of the cam distributor and used by Mallory on their street distributors for years. The second is a vacuum can that has a diaphragm attached to a rod which moves the breaker plate (except on the old Chevy in-line 6, on those the whole distributor moves) this system has either an adjustment screw in side the can, or on older Fords a removable cap under which was a spring, shims and a hollow stop tube inside the spring. These are pretty straight forward, at X inches of vacuum you get Y degrees of advance.

What happens with the EGR is not that you can get more advance, it dilutes the mixture enough that the vacuum advance does not cause detonation. A specific example, the mid 80s Oldsmobile built 307 ci V8, if the EGR wasn't opening, you would get a light throttle spark knock at roughly 35-40 mph in 4th gear lockup, a bit more throttle and it would vanish when the computer added fuel by decreasing the down time on the metering rods on the E4ME primaries.

Now, to further confuse everyone, for a long time, Ford used Holley distributors, with no mechanical advance, only vacuum, called a Loadamatic distributor These had a 2 stage set of springs on the breaker plate, one very light one for the venturii signal, the other a heavier one for the throttle port signal. Depending on the year and application many had a spark control valve on the side of the distributor. This blocked the throttle port so that the very weak venturii signal didn't leak, when the throttle signal was there the valve moved in under the higher vacuum to allow it to the distributor diaphragm to receive the full vacuum. Some carburetors simply used a small check ball in the throttle passage. Because this system ran at maximum advance at cruise conditions, Ford found that it was necessary on some applications to use a second diaphragm on the back of the main one connected directly to manifold vacuum to quickly retard the main system under sudden WOT conditions. A quick way to recognize these systems, the diaphragm "can" is very flat on these as there are no springs inside it.

The next piece came with the emission systems, 1966 in California, and 1968 nationwide. One of the problems with many engines was idle emissions, unburned hydrocarbons and carbon monoxide (CO) also oxides of nitrogen. It was found that if the carburetor idle circuits were at the transfer slot that the idle could be set lean enough in the jetting that the engine could run cleaner. In order to do this, the initial timing needed to be reduced. The other solution was air injection. Each of the big three used a different approach, Ford used two systems, IMCO for IMproved COmbustion and Thermactor or air injection. Both systems used a lazier distributor curve and in some applications a lower initial (static) timing. It also was the death knell for the Loadamatic distributor as it's advance would vary with altitude and to some degree barometric pressure differences. The leaner low speed mixtures also would cause the Loadamatic equipped engines to misfire at low speed cruise due to the advance being fully in.

I hope this isn't too confusing, but hopefully it will help understand how the spark advance systems relate to other systems. All have to work together in harmony for best power and economy.

As for a choice of manifold vs. ported vacuum, the port in most carburetors and non computer controlled fuel injection systems is very close to the bottom or back side of the throttle plate (only one is used on multiple barrels) most Ford and Chrysler engines and some GM used a ported spark. One of the problems with direct manifold vacuum is the timing can retard if the vacuum goes below a certain level. This causes the engine rpm to drop, further reducing the vacuum. The manufacturer can use a light spring in the vacuum can so it has to fall way off in order to be a problem. It is more of a problem with automatic transmissions due to the engine loading in gear. This can also be a problem if the centrifugal advance comes in at too low an rpm. I used to see the older small block Chevrolets with a Powerglide come into my shop and the complaint was "I just got a curve kit and installed it, now I can't keep it running in gear and it races in neutral." Stall speed on the old Powerglide was real low and even a mild engine would drop 3-400 rpm from neutral to drive. As far as total vacuum advance, I never saw any difference between manifold and ported vacuum, the difference was what it did at idle. Whatever system, you still need to disconnect the vacuum when setting the timing, just be glad you aren't trying to set timing on a pre computer Oldsmobile V8, it was set at 1100 rpm, no vacuum because they wanted to be sure the initial centrifugal advance was in. Royal PITA, move the distributor, reset the rpm, check the timing, it would take 10-15 mins to set one correctly, but boy did it make a difference!

Bill - How much centrifugal/mechanical can a 300 six take? And when should it be all in, and how fast should it come in?

Ditto the vacuum? The problem here is that the Carter doesn't have true ported vacuum, and still gives ~5" at idle and only something like a max of ~12". So, in order to get a vacuum advance that is responsive to the load we changed David's to manifold.

Last, what process should one use in order to get the initial, centrifugal, and vacuum set up properly?