Timing Question - help =) - Vintage Mustang Forums
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post #1 of 28 (permalink) Old 07-18-2019, 02:54 PM Thread Starter
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Timing Question - help =)

Good afternoon,

Last week I noticed something with my engine timing. 302. Vacuum advance MSD Distributor/6AL spark box. Just decided to validate my initial and total timing. Idle was at 12. Rev'd the motor up to 2500 and saw it hit 38'ish......kept reving to and it kept climbing! VA was plugged in the whole time. Well past an timing marks. Something is way off. Figured it had something possibly to due with stop bushing in the distributor.

So today I put a new bushing in. Old bushing was the big black one w/Blue and Silver (light) spring. I replaced the black with a silver bushing (25) and replaced the old springs with same/same except new.

I went out and disconnected the VA from the MSD. Plugged up the vacuum line with a golf tee and a nipple on the MSD. Intitial coming in at 13 and total looked to be 38. Reved the motor and it stopped. With that combo I would expect 38 total timing reached at 2800RPM. I figured i'm good to go and it was a bad stop bushing or something like that *shrug*

Plugged back in vacuum advance onto MSD and checked timing again. Idle is still 13 but this time where ran to 2500 (by sound) - that total timing kept climbing again. Hit 40 and and kept on going as I gave it throttle.

Got a vacuum gauge hooked up to it and it idles @12 and climbs under to acceleration until WOT and then drops off (as expected)

Oh yeah - car starts like a top and idles great. No pinging or hesitation when I give it throttle.

What gives? This can't be normal behavior.

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Last edited by John_in_SC; 07-18-2019 at 02:56 PM.
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post #2 of 28 (permalink) Old 07-18-2019, 03:01 PM
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Your vacuum advance might be plugged into the wrong port on the carburetor.
But if it’s not pinging... 🙂

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post #3 of 28 (permalink) Old 07-18-2019, 03:12 PM Thread Starter
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Quote:
Originally Posted by MrFlash View Post
Your vacuum advance might be plugged into the wrong port on the carburetor.
But if its not pinging... 🙂
Only two on the carb - both manifold vac.

No pinging - just lots of power and torque. I'm confused. I've done a bit of reading and it appears that it is normal behavior to see it go past what you set your total timing to. I'm just confused as to the science and if my source(s) - google - are giving me sound advise.

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post #4 of 28 (permalink) Old 07-18-2019, 03:59 PM
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I think you are describing normal behavior with your vac. advance in play. It will advance you beyond your total mechanical timing (upwards of 50* in many cases), and that is what it is supposed to do. It will behave differently in gear and under load, but that's when it's effects are most beneficial. You are pulling some vacuum until you are truly wide open, which I would be hesitant to do without worrying about overrevving. You can find out exactly how much vacuum advance you are adding by using a timing light with an advance feature, or by simply running the car hooked up to manifold vac. source and noting how much additional timing you are getting at idle vs. your base timing.
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post #5 of 28 (permalink) Old 07-18-2019, 04:12 PM Thread Starter
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Quote:
Originally Posted by kellysedars View Post
I think you are describing normal behavior with your vac. advance in play. It will advance you beyond your total mechanical timing (upwards of 50* in many cases), and that is what it is supposed to do. It will behave differently in gear and under load, but that's when it's effects are most beneficial. You are pulling some vacuum until you are truly wide open, which I would be hesitant to do without worrying about overrevving. You can find out exactly how much vacuum advance you are adding by using a timing light with an advance feature, or by simply running the car hooked up to manifold vac. source and noting how much additional timing you are getting at idle vs. your base timing.
Yeah - after further reading - its starting to make sense to me.

So - as long as my engine is generating vacuum (acceleration) - the vacuum will continue to advance the timing until WOT. At WOT - engine stops producing vacuum and then the the advance drops off.

Regarding the method to find the vacuum advance...I've got a boring old timing light with no advanced features. I did run the car hooked to a manifold vacuum source and did not notice a difference in idle timing. Was 13 with an without vacuum.

**EDIT**
Sorry - initial was 12 with and without Vacuum

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Last edited by John_in_SC; 07-18-2019 at 04:15 PM.
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post #6 of 28 (permalink) Old 07-18-2019, 04:52 PM
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John, that's because you are using ported vacuum off the carb. Your vac advance is '0' at idle, because the port you're using is above the throttle plates. At part throttle, your timing will gain about 12-14 degrees or so

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post #7 of 28 (permalink) Old 07-18-2019, 05:06 PM Thread Starter
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Quote:
Originally Posted by Grimbrand View Post
John, that's because you are using ported vacuum off the carb. Your vac advance is '0' at idle, because the port you're using is above the throttle plates. At part throttle, your timing will gain about 12-14 degrees or so
I think I'm in a manfold vac port (but I could be wrong). See attached pic. Skinny black line goes to the distributor. Fat one goes to my VAC gauge.

Let me know if I'm mistaken.
Attached Images
File Type: jpg IMG_0772 (1).jpg (475.0 KB, 10 views)

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post #8 of 28 (permalink) Old 07-18-2019, 05:16 PM Thread Starter
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@Grimbrand - you maybe right.

I found some documentation that the "outside" port is metered and "inside" port is manifold

I don't have the instructions so I'm on hold with Holley now to confirm.

Thanks for calling that out. I may go swap them anyhow just to see.

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post #9 of 28 (permalink) Old 07-18-2019, 06:04 PM Thread Starter
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@Grimbrand

You were correct. On the SQ750 - outside port is metered and inside port is vacuum

Will fire it up later and see what the change does. Wonder if this is the cause of my AFR quest.
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post #10 of 28 (permalink) Old 07-18-2019, 06:19 PM
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It seems ok to me. From the description, 12* BTDC at idle both with and w/o vac adv hooked up, you seem to be using ported vacuum.

IMO, the best read explaining vacuum advance (from a GM engineer, no less) is...
http://www.camaros.org/pdf/timing101.pdf

This article may raise in your mind whether you should have manifold or ported vacuum going to the vac advance canister. It really depends on your engine (eg how radical the cam is, etc). But if you ask the question on the thread, prepare for a major "religious debate" ;-) :-)
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Last edited by P-51; 07-18-2019 at 06:33 PM.
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post #11 of 28 (permalink) Old 07-18-2019, 06:57 PM Thread Starter
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@P-51 - great article! I’m more “woke” about timing than I was this morning 😁
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post #12 of 28 (permalink) Old 07-18-2019, 08:52 PM Thread Starter
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So that did it. Intial timing 12* at idle w/o VAC. With "manifold" vac - 32*. 20* too much for vacuum advance?

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post #13 of 28 (permalink) Old 07-18-2019, 09:35 PM
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From that article: "Ported Vacuum” was strictly an early pre-converter crude emissions strategy and nothing more. Don’t believe anyone who tells you that ported vacuum is a good thing for performance and drivability – it’s not. Anyone with a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don’t understand what vacuum advance is,how it works, and what it’s for. There are lots of long-time experienced mechanics who don’t understand the principles and operation of vacuum advance either, so they’re not alone."


And (with vac advance + base mechanical timing) you should probably wind up around 20-ish at idle. Your engine's vacuum will tell you where it's happiest, when you tune it. Highest manifold vacuum = where it should be.
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post #14 of 28 (permalink) Old 07-18-2019, 10:03 PM
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Id probably adjust the vacuum advance at the distributor to get it to about 36 total and see how it runs there. Id probably also try initial at 14 and see what that does.
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post #15 of 28 (permalink) Old 07-18-2019, 10:45 PM Thread Starter
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Quote:
Originally Posted by Grimbrand View Post
And (with vac advance + base mechanical timing) you should probably wind up around 20-ish at idle. Your engine's vacuum will tell you where it's happiest, when you tune it. Highest manifold vacuum = where it should be.
Same principle hold true for idle tuning. Happier, higher and more stable engine vacuum trumps AFR? I'm at 20 vacuum advance now.

Quote:
Originally Posted by patrickstapler View Post
Id probably adjust the vacuum advance at the distributor to get it to about 36 total and see how it runs there. Id probably also try initial at 14 and see what that does.
To get there I think I need to change the stop bushing to the Green one (23). Car seems happiest at 12*..maybe 13* that would give me 36* total at 13* initial. To get 36* on the Red bushing - I need to retard the timing to 11*
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