Engine hates vacuum advance

[Broncojunkie]

For a couple years, I've been running a Pertronix 510 hp digital ignition box and MSD distributor with mechanical advance only (no vacuum advance). I seemed to always be chasing a misfire here and there. Sometimes just a slight miss at cruise or acceleration, other times bucking and backfiring. I've worked through several issues, including a defective air filter, bad plugs/wires, charging system failures, and most recently, complete loss of spark due to a bad Pertronix ignition box (although I found the MSD distributor also had issues with internals rusting, which I understand has been an issue in recent years).
Being fed up with quality of expensive aftermarket "performance" parts, I decided to swap back to a Duraspark 2 setup. I already had everything...all parts either new or slightly used, having installed it in the past, chasing down other ignition gremlins.
Upon installation, the car ran 100% better. Even a long-running low-rpm "hiccup" (slight misfire) was cured. I had previously chalked that one up to a carb issue, but I guess I was wrong.
As it is now, car runs great. It cruises up and down the road with zero issues and seems to have plenty of power. Initial timing was set to 12 degrees btdc and total mechanical timing is around 35. Decent throttle response. No bogging off the line.
The only issue is that my vacuum advance is unhooked. When I attach it, idle goes to crap. Any blip of the throttle causes backfiring through the exhaust. Timing is erratic. Basically, the engine just hates it. I tried Adjusting the vacuum advance, retarding/advancing the timing, recurving (springs only), adjusting the carb, and even swapping out the vacuum advance can. I've tried manifold and port vacuum.
What am I missing?

[qikgts]

Jeff Cooke was a huge proponent of using ported vacuum. His tuning tips were always spot in my opinion. Start there and maybe richen up the mixture a tad at idle. Also check to make sure you are getting a good pump shot.

[Broncojunkie]

Jeff Cooke was a huge proponent of using ported vacuum. His tuning tips were always spot in my opinion. Start there and maybe richen up the mixture a tad at idle. Also check to make sure you are getting a good pump shot.

All the above were tried yesterday. Ported does fix the idle, but as soon as the throttle plates start to open, same timing issues appear. I did also play with the fuel mix, but that didn't help at all. I didn't mention this, but I do have a wideband installed as well. I can get afr dialed in nicely. It seems to be reading spot-on. That is, when I make changes such as running on the rich side, the results/symptoms will match what the gauge is showing.

[Broncojunkie]

I guess I didn't add much about the engine itself. It's a 331 (stroked 302) with some cheap aluminum heads (Flotek). Good valve springs that match the x303 cam. Steel 1.6 Comp rockers. Carb is a new (last summer) Proform 750 mechanical 2dry. I get the same results with a Brawler 650 (also mechanical 2dry) and an older rebuilt Holley 4150 650cfm. Intake is an Edelbrock RPM Airgap. New battery and 130a alternator (chasing down a recent wiring issue) and putting out good power. I also just upgraded my battery cables and terminals, but that was just because I didn't like the old parts-store style terminals and one of them was developing a crack.

[bwguardian]

Seems to me it's fuel/carburetor related. The three carburetors you have tried are all mechanical secondaries...aka double pumpers. Have you tried a vacuum secondaries carburetor like the original 4180c?

[Broncojunkie]

Seems to me it's fuel/carburetor related. The three carburetors you have tried are all mechanical secondaries...aka double pumpers. Have you tried a vacuum secondaries carburetor like the original 4180c?

I haven't tried that. I do have a vacuum secondary carb lying around, but it will need a rebuild. I've thought about this and my conclusion is that these issues are happening all within the scope of the idle, transition, and primaries. The secondaries don't really even come into play. If they're open, then I should be getting no vacuum and timing advance is solely dependent on mechanical advance.

I know there are a lot of variables to consider, so maybe I'm missing something there.

[Broncojunkie]

The vacuum advance seems to be adding a ton of timing. It is adjustable and I've adjusted it all the way back and even tried swapping to a different vacuum can. If I set initial timing at around 12 degrees, vacuum unhooked, the timing will advance to over 40 (hard to get exact reading because at that point, timing light/mark is bouncing around and idle is rough).
It doesn't seem to me that it should be adding so much timing. I can dial the timing back only so far, until the vacuum can hits the intake. I'll have to pull the distributor again or reposition the plug wires. But at that point, I think the mechanical advance doesn't come in fast enough to make up the difference as the vacuum advance drops out. So maybe my entire issue is just dancing around this fine line between vacuum advance and mechanical advance.

More info on this:
I did install lighter springs, heavier, one light/one heavy, etc in a few different combinations. No luck so far, but I haven't tried dialing back the distributor to get the idle advance/timing to a reasonable number. I wanted to exhaust other measures before going this route, as I feel it's like putting a band-aid over the real problem. In my mind, I should have enough adjustment between mechanical advance (springs) and adjustable vacuum can to get this thing close. Should vacuum advance add 30 degrees at its lowest setting? If not, do I have 2 bad vacuum cans?

[qikgts]

If you want to start over with an OE can, here is a link to where there are a few places that have the OE can for an '85 Mustang. Aftermarket number is VC-221. OE number is D3AZ-12370-C.

https://www.rearcounter.com/D3AZ-123...rts250572.html

Something tells me they won't be cheap but you won't know until you ask.

[85MUSTANGTGT]

The vacuum advance can should only pull about 10 degrees of timing. It is limited by a tab inside the housing and a slot in the arm. The last can I got from the parts store had the tab, but no slot stamped in the arm. After installing it I was having bucking and surging during steady cruise. Once I put a timing light on it I realized what was going on, that it was pulling 30-40 degrees of timing. Instead of getting a replacement like a normal sane person, I made an adjustable limiter to install in the distributor. Hopefully the next replacement can I get is made correctly.

[Broncojunkie]

Qikgts & Thomas,
Thx for the info! This is the stuff I was hoping to find!

I've been busy today, but found a spare minute here and there to do some more testing. Unfortunately, the results have me questioning my initial findings.

With the distributor I'm using, I took my hand pump and drew a vacuum. Inside the distributor housing, I picked a spot and made a mark. Then, I released vacuum and let the magnetic pickup return. I made another mark. The resulting width between the two marks were around 1/2" (maybe 5/8"). I found another old, used distributor in my stash. No idea what it came from, but it is magnetic pickup and looks like it may even have a steel gear (not cast iron). Vacuum advance diaphragm appears to be working, so I performed the same test. The result was the same. No noticeable difference in the amount of movement between the two when pulling full vacuum. This would lead me to believe that, if installed, this distributor would also add a ton of vacuum advance. Total mechanical advance appeared to be spot on with the distributor I'm using. So I'm running under the assumption that it's not the problem. I have 3 distributors that appear to be advancing the pickup the same distance (vacuum advance). So maybe I'm missing something else? I may have to sleep on this one. It's not adding up.

[qikgts]

I think I understand what the issue may be. The can is coming in too early and as Thomas noted, it's advancing too much. Because the x cam pulls far less vacuum at idle than a stock engine, the can is engaging too early. Think of it like a power valve.

Maybe Performance Distributors will sell you a known good and adjustable can. www.performancedistributors.com

Crane used to sell one but I think they are like a needle in a haystack now. Part number was "99607-1".

[85MUSTANGTGT]

The pickup should only move about 1/8" to advance 10 degrees. This is the adjustable limiter I mentioned in my previous post.

[Broncojunkie]

I think I understand what the issue may be. The can is coming in too early and as Thomas noted, it's advancing too much. Because the x cam pulls far less vacuum at idle than a stock engine, the can is engaging too early. Think of it like a power valve.

Maybe Performance Distributors will sell you a known good and adjustable can. www.performancedistributors.com

Crane used to sell one but I think they are like a needle in a haystack now. Part number was "99607-1".

Some good points here (no pun intended lol)! The x cam, I'm sure, has less vacuum than a stock cam. I honestly couldn't tell you what kind of vacuum a stock engine makes. It's been many years since I've seen one. I do recall doing a little reading on the alphabet cams and the x303, in particular. I think Ford was pretty cautious when it came to duration, overlap, and just general "rowdiness" of these cams because they wanted them to function within the somewhat limited range of air/fuel flow while still being manageable by the emissions system. So the vacuum is better than what most folks would expect. I think it makes about 18 inches at idle with everything adjusted well. I'd have to double check that, because it's been a while since I've had the gauge on it AND everything tuned properly.

Also, either I'm misunderstanding something here concerning vacuum advance or you are. I admit it took me a while to comprehend the idea that power valves add fuel with drop in vacuum. With vacuum advance, timing is added as vacuum increases (as I understand it). So a low vacuum situation shouldn't add more timing advance UNLESS I'm overlooking something here!
One thing about engine tuning that they don't tell you when you're learning, is just how many different things can affect it. I used to think timing advance was only needed because the engine rpm was higher, requiring a spark to start the ignition process sooner...just so the flame front has time to produce power before the piston was too far back down in the compression stroke. It wasn't until later that I learned about how different air/fuel ratios burn at different speeds, also requiring altered spark timing events. Another aspect is density of the air/fuel mix in the cylinder.
After trying to figure all this out and when spark was needed at what position in the rotation, I realized why engineers went through all the trouble of developing computerized systems that could monitor, process, and adjust this spark (and air/fuel) requirement in real time. Throw in some more variables that I still can't wrap my head around, such as barometric pressure, temperature, humidity... and it really gets hairy!
Sorry to vent but this is the stuff I try to avoid for the above reasons lol! To your next point... different vacuum advance cans on the market. I've read about these having markings that tell you what they are rated for, or even a part# that you can use to look up the specs. I have 5 or 6 and can't find a single mark on any of them. As I'm replying to this, I've already seen Thomas's post after yours, which is relatable... so I'll continue in the next post.

[Broncojunkie]

The pickup should only move about 1/8" to advance 10 degrees. This is the adjustable limiter I mentioned in my previous post.

If 1/8" equals ~10°, then that would definitely explain my issue! Looking at my newer/aftermarket distributors, the arm coming from the vacuum can is straight while the older ones are bent in a sorta "s" shape. The bend in the arm is right up against the plate to where it looks like it could be what limits the movement, but I manually moved it across is range and it didn't touch. I'm not seeing any other studs or notches or anything that would limit movement. But then again, all of my distributors have vacuum advance that moves well in excess of 1/8". They're at least 1/2" (hard to measure exactly due to their position and being located along a curved edge).
My buddy next door is a retired mechanic. He's a chevy guy, but knows his way around all makes and models. He opened up a cabinet in his garage a couple weeks ago and showed me a collection of at least 5 old Ford distributors. I may take the mighty vac hand pump over there and go through a few of his distributors just to see how much movement the vacuum can allows and whether or not any type of "stop" is in the designs.
Your diy setup is pure genius. I'd have to think that we're not the first with this issue, though. Does anyone produce a similar modification made for these distributors, you think?

[Broncojunkie]

Double post

[Broncojunkie]

Another update.
I went next door and checked my buddy's old distributor collection. None of his distributors have vacuum advance with as broad of a sweep as mine. I came back and started looking at mine again. Vacuum cans on both new distributors are adjustable. With the adjustment all the way counterclockwise, I still get full sweep of the arm, but the arm is much stiffer. Removed from the distributor, I can grab the arm and move it in and out. When I tighten the adjustment screw all the way in, the arm is much easier to move in and out (still full sweep). My assumption, since both new distributors appear to work the same, is that adjustment only changes the sensitivity (or how quickly the advance comes in). This makes perfect sense, although, I found that this was either a design flaw OR a defective vacuum advance (or in my case, 2 defective units).

I couldn't really test adjustment on my old distributors because one diaphragm was bad (leaking) and the other, adjustment screw was too deep for my allen wrench. I dug out my T-handle allen wrench set and, as you probably gathered by now, I tested adjustment on the old unit. To my surprise, adjusting the screw does, in fact, change the total sweep of the arm! If I loosen the screw all the way, I get zero advance with vacuum (and can't move by hand). A few turns in, and I get 1/8" movement or so. If I crank it all the way in, the arm will move significantly (amounting to 3/8 to 1/2 inch movement in the magnetic pickup plate assembly).
So my original hunch that the advance can was bad was actually incorrect. BOTH cans were bad! I've been burned this way before. Never presume just because you compare one new "suspect" part to another new part, that the other new part isn't also defective. With today's quality control, I'm sometimes surprised when something actually works out of the box.

[Broncojunkie]

That was definitely it. I've been out driving it around. Good as new!

[qikgts]

Also, either I'm misunderstanding something here concerning vacuum advance or you are.

You did an excellent job explaining what's happening with the vacuum advance. Thanks for pointing out what I got wrong. That got me to do some reading and I found excellent info here on the site.

http://vb.foureyedpride.com/showthre...m-advance-long
http://vb.foureyedpride.com/showthre...vacuum-advance

I understood that the advance was added when vacuum is high and when answering I totally discounted that in the stock configuration manifold vacuum was acting on the can at idle (when cold) too. Another thing I had wrong was I thought that vacuum had to be at a certain point on the "high" side before it would come in, like when you put your foot down at cruise and vacuum starts to drop. I thought it had to hit a certain point, down from it's max, before kicking in.

It's pretty hard to troubleshoot when each part you're using is defective, that's for sure!

[Broncojunkie]

Oh, trust me....I had to do a lot of reading and testing on the car to reacquaint myself with how vacuum and timing work lol! If you would have asked me last week, I wouldn't have all of it stored in my dwindling memory bank! It's funny how just discussing these things can help point you in the right direction, though. Certain things just trigger ideas as the discussion goes along.

This is also a good example of why it often takes so long to fix these old cars up. For example, several yrs ago, I told the parts store guy that the tfi module they sold me was bad. They bench tested it, it showed good. So they handed it back to me with a smug "I told ya so" attitude. I continued testing, swapping parts, cursing and fighting with the car trying to figure out why I was losing spark. I finally talked the parts store guy into swapping it out. Still, no spark. I was beat down, at this point. After literally pinning out the harness and figuring out how the electronics worked on the system, I decided I couldn't be wrong. Everything I did led me to believe the module was bad. The same guy sees me walking into the store, tfi module in-hand (not this guy, again lol). He bench tests it, and of course, it shows good. I asked him to test it again. He laughed and hit the button again. FAIL! Every test after the initial was a fail. It just needed a little heat. They had 2 other modules in stock and both of those failed, as well. He apologized several times and gave me a refund (which was against their policy for electrical parts). I went to NAPA and picked one up, slapped it on, and it's still running fine to this day.

So this is definitely not my first rodeo with bad aftermarket parts lol!

[85MUSTANGTGT]

Glad to hear you're getting things figured out.

The screw adjustment in the vacuum nipple of the can has nothing to do with how much timing the can pulls. It changes the spring pressure on the diaphragm to adjust how much vacuum it takes to move, or when the vacuum advance changes.

If you look at picture #4 of the advance can in this rockauto link, you'll see the tab and slotted arm I mentioned before. This is the limiting mechanism. If you look closely, the arm is stamped "10" for 10 degrees.

https://www.rockauto.com/en/moreinfo...VzxjgE6YIkci9Z

I expect you'll find your over-advancing cans do not have this key feature.

[Broncojunkie]

Thomas,
I see what you're saying now! And you are correct. The adjustment screw on both of my new vacuum cans do not limit the length of travel. On the other hand, the old can I used for replacement does. With the can removed and having it in my hand, I can adjust the screw and manually move the arm in and out to check travel. So I guess Ford redesigned this at some point?
Another interesting bit... the arms on both new vacuum modules are made exactly like the pic you reference from Rockauto, but neither of mine have a number stamped on them. Possibly an oversight by the manufacturer who didn't fully comprehend the original engineers' intentions?

[85MUSTANGTGT]

I'm sure it's just the result of todays crap-quality cost-cutting manufacturing process.

[WilliamCapri]

For whatever it’s worth, my stock 302 with the 4180c makes about 20-21” Hg at 800 rpm. I’m at 14 deg. initial timing and run mid-grade gas. I’m pretty sure I could run regular but it starts and gets to idle a lot better on mid.