Timing Advance

After putting in new plugs & wires I thought I would play with the timing a little via the vac gauge method instead of using the timing light. My initial timing produced 15 in Hg. I advanced the distributor between 1/2 to 1 inch and the vac went up to 20 in Hg and it was purring really nice. Had to lower the idle screw some even. I plugged in the manifold vac to the vac advance and started backing out the driveway. Pinging all over the place so I retarded it some and tried another drive. It did better but mid range pinging under acceleration.

Btw, it is a '73 CJ which, of course, has open heads.

For this method I'm after the highest initial timing that gives the most vacuum. I then have to check total timing (mech and initial) and not be more than 36 BTDC. If more than 36 adjust the mechanical to get to 36 (more on that in a later post). Then if midrange pinging still occurs...go with a stiffer light weight spring on the mech advance so the mech advance comes in a little later.

I kept retarding the distributor until the pinging stops like it was before I messed with it. I need to determine optimal engine RPM I want for when the timing is all in. More to come later.

72Hcode is our resident tune up expert. Sounds to me like you'll be changing the springs on your mechanical advance to keep your 20in Hg. As with all things we have to play with it to find the sweet spot. For now, retard until you get the best performance. The road your getting ready to travel will blow your mind!

72Hcode is our resident tune up expert. Sounds to me like you'll be changing the springs on your mechanical advance to keep your 20in Hg. As with all things we have to play with it to find the sweet spot. For now, retard until you get the best performance. The road your getting ready to travel will blow your mind!

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Mechanical advance is one of those topics we talked about a long time ago and now I'm messing (playing) with on another car. I guess I've been blessed all this time to have good mechanical advance on my other cars.

Your going to be really surprised at what you've been missing from a performance standpoint when you get all to come together using what you already have except the advance springs.

In my quest this summer for improved drivability and fuel economy I have been working with timing. I have experienced the same situation. Timing for the highest vacuum reading has caused detonation and drivability issues. I have tried ported, manifold and no vacuum to the distributor. I would like to think this whole issue through. Currently I have backed off the initial timing (to where I don't know) and switched to ported vacuum. What I do know is the tune for my manual trans set-up is WAY different than the tune for the auto.

If the timing is set to give maximum vacuum at "initial" timing wouldn't hooking the vacuum advance to manifold vacuum increase the low throttle timing even more? Would this effectively retard timing under WOT? If so should timing with manifold advance be set with the vacuum advance connected? Would ported vacuum to the distributor give additional advance at higher rpm? Is ideal cruise timing different than for max power?

OK...first, you have to plug the vacuum lines to the distributor. Then, advance the dizzy until you reach maximum vacuum, but this will typically raise your idle speed. You have to keep your idle speed down so you don't use your intermediate circuits on the carb (which throws off your vacuum). Eventually, the vacuum will lower and your idle speed will lower as well. Find the sweet spot of max vacuum at your idle speed. When done, reconnect your vacuum lines to the dizzy, and do a test drive. If you hear pinging, back the dizzy off and try again.

Personally, I hate this method and prefer setting the timing with a timing light. I only use vacuum settings when adjusting the idle screws, and even then, I use a tach, my ears, and my hands to find the smoothest running idle mixture. Then I set the timing.

Help a newbie out!

I bought an andavce timing light so I could see where my car is set.

It's a 1973 351c with Edelbrock Performer intake and Thunder Series Carb. It runs pretty well, and idles great... right about 700RPMs in Drive at a stop. Right about 800-900 in park.

Anyway, I fired up the timing light, and it idle, my car is set close to 30 degree advance.

Does that make sense? Seems quite high to me, but then... I'm ignorant when it comes to tuning motors.

Anybody?

Maybe a tutorial on reading initial and final timing?

All I did was let it idle, then turned the advance knob on the timing light until the TDC mark on the balancer steadied under the little triangle. The knob was turned to 30.

Did you have the trigger lead on the #1 wire? Was your vacuum advance hooked up? If so, what vacuum source is it hooked to?

Yes it was number 1. The dizzy vacuum advance was connected to one of the ports on the carb I believe.

Disconnect the vacuum advance (plug it with golf tee) for the correct readings.

Yes, 30 does seem high.

As stated, disconect the vacuum advance, plug the line to avoid a vacuum leak while testing.

Make sure you are on #1, this is the front passenger wire, the one closest to the front of the car.

Put a really clear mark on the 0TDC mark on the balacer.

Does your fancy gun have a 4/6/8 cyl switch? Make sure it is on 8 cyl

Let it warm up and test. Turn the dial until the light flashes while the tdc mark you made is lined up with pointer. The dial setting should indicate your advance.

It's possible you have, say 16 degrees initial advance and the vacuum is pulling it another 14 or so. This can happen if you are using manifold vacuum for the vacuum advance.

What kind of distributor do you have?

CZ-75, check out the 'Distributor for Ram-Air' thread in the tech forums. Don65Stang points out some good information for timing the 351C (regarding distributor timing advance, and whatnot - with a link to the information he found). http://www.7173mustangs.com/thread-distributor-and-ignition-info-thread

Basically, unhook the vacuum advance (and plug it if you like), advance the timing to 16, then connect the vacuum back up. The vacuum should pull 20, with your final timing advance hovering in the 36 range. That's where life is happy for the 351C - or at least mine anyway, with everything being freshly rebuilt or otherwise brand new. Obviously, this would be a "per-spec" setting. I noticed mine was a little happier at 17-ish... so that's where it lives (for now, until I find out there's something else I should've adjusted differently).

Also make sure your distributor vacuum is plugged into the driver side vacuum port on the carb ('constant,' the other is 'timed') and cap everything else you don't need.

Thanks, guys!

I'll retest this evening.

As mister4x4 pointed out in a round about way, timing will vary from one engine to another. Unless you have a brand new, factory spec engine, and running leaded gas, all factory setting are out the window. I have not used a timing light for 25 years when an old mechanic explained all the variables to me. He told me not all advances, carbs, heads, etc. are created equal and therefor require a personal setting especially as the engine ages and compression falls. He showed me to leave everything hooked up, bring engine to 2500 rpm, and set it just slightly retarded and that's the best it will get. This method has worked for me for years. just my 2 cents

OK, so if I get this correctly:

WIth the vacuum advance on the dizzy unplugged from teh vacuum port (with same port temporarily plugged), set the timeing around 16, give or take a few, depending on how teh engine performs. DO all this at idle with the engine warmed up.

Then, reconnect the vacuum advance, and the timing will automatically advance another approximately 20 degrees, landing around 36 in total.

OK, so if I get this correctly:

WIth the vacuum advance on the dizzy unplugged from teh vacuum port (with same port temporarily plugged), set the timeing around 16, give or take a few, depending on how teh engine performs. DO all this at idle with the engine warmed up.

Then, reconnect the vacuum advance, and the timing will automatically advance another approximately 20 degrees, landing around 36 in total.

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Correct. And take a few minutes to feel around a little (advance a degree, retard a degree, or two, or whatever) to see if things improve when you crack the throttle. Caspianwendell is correct - all engines are not created equal. Especially when you add on the aftermarket parts, such as we have (Edelbrock this and that, Duraspark, et al).

Freaked me out when I saw the 36-ish timing... then I remembered I forgot to remove vacuum advance from the dizzy. Then, it ran like crap at 5-6 BTDC. Then I just timed it by 'feel,' and it felt WAY better. Then I re-read the thread I mentioned, and the next day set it to 16 w/o vacuum, and it was [almost] spot-on.

That's an interesting idea for timing, CW - when I get my tach working, I'm going to give it a shot. ::thumb::

OK, thanks for your help, guys!

The vacuum advance had been connected to the passenger side port. So I disconnected it and plugged it. Timing was right around 30 degrees

Then I played with initial timing until I landed at 18. No pinging, and the engine runs strong and idles well.

Plugged the vacuum advance into the driver side port, and the advance shot up to 40.... still running nicely.

Went for a drive, and wow, what a difference! I don't think I was getting any vacuum advance before. But now she acts much better with good throttle response, and no coughing and sneezing when I stomp on it.

OK, thanks for your help, guys!

The vacuum advance had been connected to the passenger side port. So I disconnected it and plugged it. Timing was right around 30 degrees

Then I played with initial timing until I landed at 18. No pinging, and the engine runs strong and idles well.

Plugged the vacuum advance into the driver side port, and the advance shot up to 40.... still running nicely.

Went for a drive, and wow, what a difference! I don't think I was getting any vacuum advance before. But now she acts much better with good throttle response, and no coughing and sneezing when I stomp on it.

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AWESOME!! ::thumb::

Did you convert over to Duraspark yet?