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Timing Vacuum...

i_taz

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Currently, my initial timing at 16 with the vacuum adv plugged but it will jump about 8 deg or so when I hook it back up to manifold vac and will run a little rough. I turned the advance in 1 turn and it will only jump about 4 now and thrs no change in idling quality that I lower to 950. All in is at about 38 at 2k....

I read there should be 10 diff btwn vac plugged and connected but that doesn't seem like that...
 
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BriceRoad

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The way I learned it was that vacuum advance gets hooked up to ported vacuum. That’s the way I did it on all my engines. I know some people say you can use manifold vacuum. I tried that once and the engine ran worse. Just my experience.
 

BriceRoad

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Also all of your timing may not be in at 2K RPM. On my 440 I always set it at 34 degrees at 3K RPM.
 

i_taz

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I haven't really noticed a diff btwn the two vacuum signals as of yet really. I thought I found a sound
argument for timed vac since it's not supposed to affect initial timing at idle like manifold supposedly does but still saw it jump no matter which port I hooked to.... I'm running an Edelbrock performer
 

BriceRoad

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I have an Eddy carb too. The timing shouldn't jump if it is connected to ported vacuum because there is no vacuum at idle with ported vacuum.
 

Chryco Psycho

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My confusion is this the mechanical advance is zero at idle & increases with RPM , ported vacuum works the same way increasing with rpm / air flow through the venturi , if you connect to manifold vacuum then it works opposite so you get full advance at idle & it goes away with rpm so mechanical is going up while vacuum is going down potentially reducing the advance with RPM which makes no sense to me . either way at cruise the vacuum advance is working .
SO why ever connect it to the manifold vacuum ?
I know someone will argue this but I can't see any benefit .

Ok 1 more point I have found counter rotating dist on a few engines where the manifold adds full retard & it does advance as manifold vacuum drops .
 

BriceRoad

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That’s very interesting CP. I never heard of a counter rotating distributor but hey you learn something new every day.

Both Rick Ehrenberg and Uncle Tony (from You Tube) say to use ported vacuum along with my old shop teacher. You’re right though as I have read on various forums people will say you can use manifold vacuum. As I mentioned I tried it once (by accident) and the engine ran worse. Just my experience.

I_taz I wish I could come over and help. I am only 2 hours away but with my work schedule and the kids doing remote learning it would be tough.
 

i_taz

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I'm making progress slowly but surely. My biggest fear was put to rest today when my compression checked out 120 across the board on a warm engine. It started out hot but by the time your done standing on your head it's not for long. According to 'for c bodies' puts it at 8.2 comp. which makes sense since it's '71 block.
No wonder it never pings with reg gas...
When I was warming it up I forgot I had the vac adv disco'd and it was running OK. Chryco mentioned leaving that off till I get the initial straightened out so maybe I just did...

Uncle Tony's live demonstration from 06/17 shows how both work but can't post the link....
I say leave it off...
 

BriceRoad

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Rick Ehrenberg says all street cars should use vacuum advance. He says bad things can happen to the engine down the road. I don't know, I am just hobbyist. I see guys running without vacuum advance on their street cars at car shows all the time.

I just watched the video by Uncle Tony. He says it doesn't matter manifold vs. ported when the car is at highway speed but it is in the take off from idle that there is a difference. He says manifold works exactly the opposite as ported. He says ported does what you want. You want the timing advanced in part throttle take off/acceleration. When the engine takes off manifold vacuum will retard the timing when the engine needs it most.

This site is not letting me post a link to the video. Just go to you tube and search Uncle Tony "The vacuum advance debate" and it should come up.
 
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