Total advance--limiting the vacuum advance.

dennis112 · Feb 7, 2006

I installed my Duraspark Distributor/MSD 6AL combo during the winter and noticed I had a high speed miss when the motor was revved over 4000 RPM--in neutral. I changed plugs and added MSD wires and the problem persisted.

While playing with a vacuum gauge and timing light today I discovered the cause of the miss--too much total advance caused by the Crane vacuum advance canister. If vacuum is unplugged from the unit, I get a clean rev to 6000+. Once hooked up, the engine will again miss at higher speed. The advance is hooked to ported vacuum.

I set timing to 36 degrees total (initial+mechanical advance) which is all in a 3500rpm (vacuum unhooked.) With the vacuum advance hooked up, the total timing appears to hit 65-70 total degrees (although with the miss it will jump around alot.) I thought that total (initial, mechanical, vacuum) should be a max of about 50 degrees.

My first question is will this be a problem when I put it back on the road? Will vacuum be so high when cruising that I will have the miss?

Should I limit the amount of travel that the vacuum advance can add?

If so, how is this done in a Duraspark? Anyone have a picture?

I wish to retain my vacuum advance for street driveability since the strip is about an hour away.

mustangdave · Feb 7, 2006

What is your initial timing set at/vaccuum unhooked and plugged? Should be between ten and fourteen for performance. Is your vac. advance adjustable?

dennis112 · Feb 7, 2006

mustangdave said:
What is your initial timing set at/vaccuum unhooked and plugged? Should be between ten and fourteen for performance. Is your vac. advance adjustable?

Initial is 12-14 degrees.

Yes the vacuum advance is adjustable and with it fully adjusted to be at its stiffest (fully CCW) the timing still over advances (but probably at a higher RPM????)

The adjustment is working because at full CW I can easily turn the advance mechanism in the distributor. When full CCW, I am not strong enough to over come it.

mustangdave · Feb 7, 2006

Sounds like you need a lighter spring in the mech. advance but my advice to you is the same I give all with these problems...CALL MSD FOR ADVICE. They have seen everything and most likely have atoll free line for such problems.

ForceFed70 · Feb 7, 2006

Hold on here.

Just because you disconnect your vacum advance and the miss goes away doesn't mean there is too much advance.

Timing too far advanced will generally result in pinging way before missing. If the engine isn't pinging I'd do a little more diagnosis 1st.

Start by trurning down your initial advance. If the miss stays... then your problem isn't from too much timing.

brianj5600 · Feb 7, 2006

At 4k with the throttle blades cracked in neutral, vacuum is higher than 4k at wot. I'd drive it before I started messing with anything. What was vacuum at 4k in neutral?

dennis112 · Feb 8, 2006

70_Nitrous_Eater said:
Hold on here.

Just because you disconnect your vacum advance and the miss goes away doesn't mean there is too much advance.

Timing too far advanced will generally result in pinging way before missing. If the engine isn't pinging I'd do a little more diagnosis 1st.

Start by trurning down your initial advance. If the miss stays... then your problem isn't from too much timing.

No pinging at all--just starts missing.

Hmmm, I'll retard the timing today and get back to you.

dennis112 · Feb 8, 2006

brianj5600 said:
At 4k with the throttle blades cracked in neutral, vacuum is higher than 4k at wot. I'd drive it before I started messing with anything.

Sort of what I was thinking. Yes it was 4K in neutral. I plan on getting the car out and running it some early next week (weather permitting.)

It just that 30-35 degrees advance from the vacuum canister seems excessive.

brianj5600 said:
What was vacuum at 4k in neutral?

I was too consumed with the advanced timing to notice. I will warm up the engine and post the vacuum later today.

302 coupe · Feb 8, 2006

why us ethe vaccum advance at all? If you have 12 or so initial on just mech, and 36 or so total on just mech, you have no need for vaccum advance. You might want to swap to a lighter spring to allow full advance to come in quicker, but you don't need more than 36 degrees total. My duraspark was the same way, it would go way off the scale with the vacuum hooked up, I set it ti run on full mechanical and have no complaints.

brianj5600 · Feb 8, 2006

302 coupe said:
why us ethe vaccum advance at all?

MPG!

dennis112 · Feb 8, 2006

I followed the advice here and did more research on the car today and feel I am chasing a problem that probably doesn't exist when driving.

When I drop the initial timing from 12 degrees to 6 degrees, I no longer have the miss at high RPM with ported vacuum. Timing light verifies this on my marked balancer. Seems to support my thoughts of timing advancing too far.

Measured vacuum at 4100rpm (when it is just starting to miss) I have 23lbs of vacuum on my ported distributor line. Obviously I will never see that kind of vacuum when driving the car. By the way, vacuum is fairly steady when held at high RPMs.

why us ethe vaccum advance at all? If you have 12 or so initial on just mech, and 36 or so total on just mech, you have no need for vaccum advance. You might want to swap to a lighter spring to allow full advance to come in quicker, but you don't need more than 36 degrees total. My duraspark was the same way, it would go way off the scale with the vacuum hooked up, I set it ti run on full mechanical and have no complaints.

After thinking about it, my intended use would allow me to use only mechanical advance. This is only a toy and is not needed for daily driving. It is only an hour to the drags, which is about as far as I plan to go on any good day. I live fairly rural, so normal local driving would be limited to short duration jaunts of 10-30 miles. No cross country trips are needed.

If I don't have an issue when the car is driven again, I'll retain the vacuum advance. I suspect it will work just fine under load since the only way I would get 23lbs of vacuum would probably be going down a steep grade. Otherwise I'll either limit the amount of advance (like I can on my Chevy truck's HEI) with a mechanical stop OR run strictly initial and mechanical advance.

Will work more on the mechanical advance though to get it in my 3000. I added a spring kit before I installed the Duraspark, but used only one of the aftermarket light springs which resulted in all being in at 3500. Obviously I plan on visiting that area again.

dennis112 · Feb 9, 2006

Did a little Google research tonight and discovered that vacuum "over-advancing" is very common with the Duraspark when used in a performance application.

Here is how it was dealt with by various people:

1. Bought an aftermarket distibutor instead.

2. Disconnected the vacuum advance. Driveability seemed OK--confirms 302 coupes post.

3. Drilled and tapped a hole in the equivalent of the "points plate" and install a screw to limit the total amount of vacuum advance.

4. Get an older type of vacuum advance canister that had square nuts on it--it is capable of disassemby. You add spacer washers to limit vacuum advance, if needed.

My Duraspark distributor is like new--it has a nice tight shaft and a nice low movement mechanical cam that limits total mechanical advance. I have a aftermarket spring kit for it, the adjustable canister, and I've already replaced the reluctor. Seems like a waste of 2 bills to replace it with an aftermarket one.

Figure I'll first try something on the line of #3 and create a positive stop that will limit total travel, but not affect the vacuum adjustment. Trying to use the old Ford advance that I have is starting to look good too and for the fun of it, I might consider just disconnecting the vacuum advance and living with. Since its my nature, I gotta tinker with it first--got plenty of time since winter keeps it in the garage.

Glad to know I am not the only one to ever have this problem and that I have options.

Thanks for your help.

302 coupe · Feb 10, 2006

brianj5600 said:
MPG!

have you read the guys sig? A 393 w/ Vic jr heads, etc, who cares if it mpg drops from 6 to 5.5, lol.

dkrobson · Feb 27, 2006

duraspark adjustment

check out thid sight for adjusting the max centrifical advance
http://www.ford-trucks.com/lc/lc.ph...k_distributor_recurve_instructions_index.html

10secgoal · Feb 27, 2006

302 coupe said:
have you read the guys sig? A 393 w/ Vic jr heads, etc, who cares if it mpg drops from 6 to 5.5, lol.

Hey, I'm having my cake and eating it, too ! I need to get a new tripometer but I'm guessing I'm getting somewhere in the neighborhood of 15mpg. And I've be runnig 87 lately.
Dennis, doesn't that duraspark have a allen adjustment screw in the vacuum port ? Should be able to knock down the vacuum advance.

thehueypilot · Feb 27, 2006

When I took mine to the chassis dyno I had a high speed miss that started above 4k. It turned out to be the spark plug gap was too wide and being blown out like a candle at high speed. Try reducing the plug gaps to .025 and try again before you spend more money. Just a thought....

dennis112 · Feb 27, 2006

10secgoal said:
Hey, I'm having my cake and eating it, too ! I need to get a new tripometer but I'm guessing I'm getting somewhere in the neighborhood of 15mpg. And I've be runnig 87 lately.
Dennis, doesn't that duraspark have a allen adjustment screw in the vacuum port ? Should be able to knock down the vacuum advance.

Allenwrench makes the vacuum advance timing come on at a later RPM, but it still will cut out even with maximum turns for maximum stiffness.

dennis112 · Feb 27, 2006

For now I have followed 302 coupe's advice and disconnected the vacuum advance. Great driveability, just as he said.

I will visit this area again when the weather improves and I can get back out on the highway.

Max Power · Feb 27, 2006

It does sound like you have enough advance strictly mechanical.

Also, cars don't ping at idle, regardless of advance. It takes load to ping.

10secgoal · Feb 27, 2006

dennis112 said:
Allenwrench makes the vacuum advance timing come on at a later RPM, but it still will cut out even with maximum turns for maximum stiffness.

It's been a while since I had one with a vacuum advance. But I think it limits the amount it's advanced. Not when it happens.
I had the same problem when I built this motor a few years ago. Advanced to 60 deg also.

Total advance--limiting the vacuum advance.

dennis112

mustangdave

My rearend needs a stud and two nuts.

dennis112

mustangdave

My rearend needs a stud and two nuts.

ForceFed70

That's why they call it "dope"

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