Aldo, to adjust the vacuum advance you will need only the allen wrench, a timing light will help you know what you have changed. MSD has a kit available to modify the centrifugal advance by changing springs and/or weights. The literature with my Street Fire says vacuum advance can be adjusted from 1 degree to 22 degree's with a minimum of 20 inches of vacuum, if you have less than 20 inches then the amount of advance will be less but that's the benefit of the adjustable can, you can tailor it to the engine. Also, if your collecting tools, you do not need a dwell meter to set up this distributor, it has no contact points to adjust, it's an HEI unit that makes and breaks the signal with a magnetic pickup coil and permanent magnet pole piece.

 

If I were you, Aldo, I'd not only use the timing light, but would also hook up the vacuum gauge at the same time to see if you need any other adjustments prior to setting your timing at the end. I know that alot of us go by the specs for timing out of the book, but I don't. If everything is adjusted right, (especially the carb being in good working condition and adjusted correctly, also using the vacuum gauge) then you should have the best vacuum number when the timing is set according to specs.-not always the situation.

To me, a vacuum gauge ismy favoritetool for setting your timing, AFTER everything else is correct. The book timing spec is a good starting point. It's always worked well for me, and FOR SURE I'm not knocking anyone else's methods.

***A vacuum gauge is the only way to go when you have an engine with alot of wear on it.

You guys gave alot of input and thought to this topic, and "good looking out" for Aldo, as he has not only contributed alot to this site, butthrough his questions, he's generated / created some of the best brainstorming and input the C3 area has seen lately, alot of discussion under quite afew topics. And pg put it correctly, and even more so/hit the nail on the headin this case, there IS no such thing as a stupid question.
This situation, just like the relay can be if we get enough participation, has connected many of us through one another ending in a team effort as a team. LOL Am I advertising the "RELAY?" Damn Right! LOL But, all in all, great topic!!!

 

pg: This is the only link I could find: http://www.lbfun.com/Corvette/Tech/vettetech.html in my list. You do have to explore thru it to find everything tho. Was this the one?

 

Yes, that's the link, under distributor and timing, that is some great information. Thanks,PG.

 

The primary goal of any emissions system is rapid warm up. The hotter the combustion mix is, the less "bad" things are released into the atmosphere because more gets burned off. Every emissions device on a car is not funtioning until it reaches the recommended temperature. Even the catalytic converter. That being said, retarded ignition timing causes hotter combustion temperatures, and therefore burns off more "harmful" chemicals, and also causes the engine to warm up faster, therefore turning more emssions devices on. Retarded spark also produces hotter exhaust temps. therefore warming up the cat faster, and starting those chemical reactions. Thats why the smog engines have the advance hooked up to ported vacuum, so the ignition is retarded at idle, and resumes full timing at part-throttle, because at idle, there are no drivablity issues with retarded spark.

 

Thanks for the great explanation.

 

Hi everybody and thanks for all the tips!

I connected the vacuum advance hose tothe full vacuumport on the carb. What a difference!the engine , before this , was "sleeping" at 1000-2500 rpm.
Beforeto put a T adaptor , to plug together the vacuum adv hose and headlights hose to the full vacuum port, I plugged the headlights hose to the timed vacuum port. I needed to rev the engine to 2500 Rpm to open the headlights.So , i suppose that atthe timed vacuum port , there's not enough vacuum till 2000-2500 Rpm( more than just IDLE!)

Checked my street fire distributor and found the heavy springs on it. I found the MSD specs for this distributor and traced the graphs attached below. Actually none of the 3 springs set ups is close to the proper ideal behaviour ( the MSD pro billet distributor behaves far better).

So I have decided to use my street fire distr. with its stock heavy springs , applying 18° initial adv and 22° vacuum ( I hope the vac from my intake will be enough to get 22° , why not?)

I have found no lit about intake vacuum behavior( its slope) : I have read that vacuum is always high and decreases while approaching WOT. Expecting that my WOT is 6000 rpm , I traced on the lower graph that approxcurve.

Please tell me if i'm wrongabout the thoughts above.

I'm not going for timing lights , vacuum gauge... One of my mechs has a 1 Ton super machine to test Dwell , advance , vacuum , everything ( I think it makes coffee too). This mech has fallen in love with my car and searching now with my help something nicefor him ( a Camaro or Mustang : this is his style). So he will gladly help me in any tuning.He his a smart, experienced guy.

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Aldo,
The vacuum drops when you first open the throttle and builds back up quickly. Or when you quickly open the throttle, the vacuum drops way down and then catches up as your throttle opening levels out. Here in the US, the cars of the '50s had vacuum operated windshield wipers. When you took off from a light or stop sign and got in the gas, the wipers stopped until you got out of the gas to shift into second and then they'd stop again when you got back in the gas. They'd finally startconsistently workingwhen you were in third gear and cruising along at a steady speed. This was a perfect example of when vacuum would go away and / or come back.
This guy, Dry C, had a great explanation of the emissions system and it's workings. But for you, I felt, and you can see that, thefull vacuum port is the better way to go. Just "T" off again or find another vacuum source, like the manifold, to connect your head lights to.
It's great that you've found someone to give you the help you need w/ your car. Awfully good of him. Sounds like he enjoys his mechanics, someone I'm sure that I'd get along with, especially with his machine making good coffee also, espresso I'm sure. LOL We could hang out all night exchanging old tales and working on stuff. LOL
Great to hear that everything is going well w/ your 'Vette and please give "my best" to your mechanic. He sounds like a great guy.

 

Well done Aldo, you did a good job and you did it quickly.
If the green line is the the centrifugal advance than you are on the money at 24* at 3,000 rpm.
Now with the vacuum advance disconnected and plugged set the initial timing at 12*. 12 initial plus 24=36. That is the max that you want when you floor it.
78Buckshot pointed out that you have an adjustable vacuum can, if so than adjust that to give you 16*. Again 12* initial plus 24* centrifugal plus 16* from the vacuum can= 52* This is the max you can run at, and even here you risk detonation.
If there is any sign of detonation than back off the timing 2* at a time until you are sure.

Texas Jim, I remember grandpaw's old pick up truck, yes the wipers would stop until he shifted it. Didn't they also stop when he went up a hill? When did they invent electric wipers? PG.

 

Don't forget that your engine will probably never see full vacuum advance at the same time it sees full mech. advance due to the load on the engine which is the reason that pg's grandpaw's truck wipers slowed down/quit when he was going uphill. As the load increases, you have to increase the throttle opening which decreases the vacuum in the intake.

If I understood Aldo's post correctly, I think he is sticking with the slowest mech. advance curve. As pg said, the green line represents the best option. The slow advance was emissions related.