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It seems like lately there has been a surge in discussions on timing advance, specifically vacuum advance. Because of this and the advice given on the forum, I decided to try and delve into the mechanics behind advance and clear up any misconceptions I have heard. I just so happened to stumble upon this article from the on all cylinders blog from Summitt. The second half describes the purpose of vacuum adance quite well in relation to charget density in the combustion chambers.

http://www.onallcylinders.com/2015/...ing-timing-advance-in-street-vs-race-engines/
 

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Good read. I have been following the post of Matt and others, and trying to take it all in. I'm just getting to the point where I know enough to be dangerous. Was just out playing with the timing light, and adjusting the carb today.
Now I want to learn a little more so I can get everything I can get out of it.

This is the first time I ever wrote down what it was doing.
With vac advance unhooked.
700 rpm 11*
1100 rpm 11*
1150 12*
1200 14*
1250 16*
1500 18*

Advance hooked to manifold vac at 700 rpm.
35*

Carb tuning at the manifold vac, 35* timing.
1 turn out, stall.
1.5 turns out 500 rpm 19 hg vacuum.
2.0 400 rpm 17 hg
1.5 700 (after throttle bump) 19 hg
1.25 700 19hg
1.5 (final setting) 750rpm.. 19 hg

I then had a hesitation off idle on a full throttle start. Moved the accelerator pump up to the top hole on the linkage, and it seemed to clear that up.

Wondering how far I can advance the timing? I had it up to an initial 15*, and it seemed to run a lot stronger. I'm just afraid of going too far.
 

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skiz,

I'm confused with the 35* timing at 700 RPM on manifold vac. A typical vac advance can will add 12-15* to the initial at idle, putting you in the mid-20's with the vac advance connected to manifold vac.

Initial timing is basically to ease the load on the starter, not much else. A few degrees either direction isn't going to make a huge difference in the over all advance curve when the engine is running. It's usually best to err on the low side of initial, IMO.

Advance curves are usually checked with the vac advance out of the equation, so disconnect and plug the hose when checking your curve. Initial and mechanical only. In my experience with my 360, a good curve is 10-12* initial at idle advancing to 30-32* total at around 3000 RPM. That's assuming a basically stock setup. I run a little more than that due to my elevation.

Matt
 

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Oh yeah. Hammer and I went round and round about spark kernel development and flame front propagation across the chamber under varying charge densities and what the vacuum advance and centrifugal advance requirements were for each condition years ago.

I'm glad that's over! :cheers:


Shawn

Now that we have several advance theory and application threads, he'll pop back up after more than a years absence. It's like opening a portal with a ouija board, lol
 

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skiz,

I'm confused with the 35* timing at 700 RPM on manifold vac. A typical vac advance can will add 12-15* to the initial at idle, putting you in the mid-20's with the vac advance connected to manifold vac.

Initial timing is basically to ease the load on the starter, not much else. A few degrees either direction isn't going to make a huge difference in the over all advance curve when the engine is running. It's usually best to err on the low side of initial, IMO.

Advance curves are usually checked with the vac advance out of the equation, so disconnect and plug the hose when checking your curve. Initial and mechanical only. In my experience with my 360, a good curve is 10-12* initial at idle advancing to 30-32* total at around 3000 RPM. That's assuming a basically stock setup. I run a little more than that due to my elevation.

Matt
Now I'm confused too. I thought maybe that I wrote the wrong number down, but thinking back, I'm almost certain that number came at idle.

Looking back at my numbers, I am also only getting 7* of mechanical????. It topped out at 18* at a little under 1,500 rpm. I went up and over 2,000 and it never advanced past 18*, that with no vacuum hooked up and 11* initial.

It's a stock dizzy. Is there any way it could possible advance that far? Spring broken, or something inside?
I'll definitely hook the light back up tomorrow, and double check my numbers.

I thought maybe I wrote the "700" down wrong on the paper, and actually got the 35* at the higher all in RPM, but that would only put the vac advance at about 6*. Correct? 11*initial+18*mechanical+6* vacuum=35* all in.

So much for me to learn.
I'll start a thread on it tomorrow. Being very green at this, it would be a good way for other to learn also. I need a "timing for dummies", I tend to get confused the deeper into the threads that I go when you guys are posting. I'm getting there, little by little though.
 

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Quite possible that is all you are getting with mechanical advance, it is limited by the length of the slot in the advance head.

When you get your hands on the dizzie check you have a firm spring action when you rotate the rotor and then try to see the advance head arm through the window in the trigger baseplate. There is a number on it, if it says 6.75 or similair you have a late model CEC controlled ignition head and you may wish to swap it out or file it back.
 

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Man I wish I just had one of you guys fairly close to me here in GA. Would pick your brain and buy the beers until you had enough. I am very green and have been learning all about my jeep as I have gone along. Becoming more and more proud of each item I accomplish.

Today I went out and reset my timing. I then hooked up my vacuum gauge and set my idle mix screws. I am now getting a steady 20+ on the vacuum gauge at about 8* on timing. I am still on ported vacuum to the distributor but I have just been trying to get "Betty White" back to good, reliable and stock before I start tackling more advanced jobs. I plan to rebuild the Carter this winter (first time ever rebuilding a carb) but I have her running pretty smooth and steady right now. No hesitation and idles at about 750-800. But cranking can be an issue for it. Seems fuel starved. Plenty of spark and cranking power. Just takes a second or two to turn over when it's hot. No problem on cold. Two pumps and let her rip.

But oh I have so many more questions, lol. Just have that desire to learn and tinker. Which can get expensive and time consuming as well!

Thanks for all you guys do here!


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