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Discussion Starter #1
If I'm using the manifold vacuum instead of the ported vacuum on the carb., what happens when crusing and lifting when vacuum picks up? Does the timing go way too high? Will this cause me problems down the road?

302HO
Carter AFB 625
Crane adj. vacuum
6 deg. base timing
25deg. w/ manifold vacuum

Thanks,
Meaux
 

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Guys ... This is some info I had ... It pretains to Rocky50's post ...

How accurate it it?

as follows:

If you run a vacuum advance … two things:

#1 - While adjusting timing, vacuum line to distributor must be disconnected and plugged.

#2 - While vacuum line is hooked up, there should be no vacuum while the engine is at “idle”. Reason is … If you have vacuum at idle, vacuum will advance timing (at idle) and idle will speed up. You will then most likely be inclined to adjust idle down which means you are closing the throttle body cutting fuel (and air). Once this has been done the engine will not idle well and throttle response from idle will suffer … plus you run a high risk of total advance ending up way too high (maybe 45 degrees or more) depending on where you set the initial advance. Too much total advance can lead to Detonation and harm to your engine (not good). If you then retard timing to try to regain some low end throttle response … initial advance will at times drop too low, maybe even to an literal “retarded” position when no vacuum is present … no engine can run well in that circumstance.

Whatever you set your “Initial Advance” at … that is where the engine should be at idle … Also, timing advance should never fall below that setting at any time.

All these problems can be avoided by choosing a vacuum port on the carb that supplies “zero vacuum” at idle. This factor is also reason to keep your “Idle Speed” low … This will help insure that vacuum is lower at idle … Keep in mind, as constant engine speed increases, vacuum will also increase (even at idle).
 

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If that Carter AFB is the same as my Edelbrock 750 (I think they are), you want to hook your vacuum advance up to the small vacuum port on the right (drivers) side of the front of the carb. That is the timed vacuum port.
 

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Discussion Starter #4
Well Guys,
The reason I ask this question is: I have an "off idle" stumble and a bad idle that I can't get rid of while using the "port" vacuum on the carb. When I use a manifold vacuum (port off the back of the manifold) source, the idle increases, so I close the throttle plates to 900rpm, and I get a very smooth Idle, no "off idle stumble", timing is now at 25deg., and the thing just plain runs better, but I'm not comfortable doing something strange w/ the timing.
I'm gonna try the "timed port" on the carb and see what happens. When I check this port w/ a vacuum gauge it seems to do what the other side does, No vacuum at idle.

Maybe I should buy a MSD timing computer to go with the MSD 6 ignition. I was really trying to build the simplest truck I could.

I bought this crate motor new, 95 302HO, RPM performer manifold, from FORD, and installed it in my 85 F150. I don't have an EGR valve but I did hook up an air pump and cat. I have the air pump hooked up w/ valves to the back of the heads and to the converter. Had to pass TEXAS inspection but I moved to Alabama and don't need to pass an inspection anymore. Should I remove all this crap? Just had a new 3:55 gear installed and I'm gettin back into my project. AOD is next.
 

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There are no hard and fast rules about running ported or straight manifold vacuum to your distributor. Above idle speed the vacuum readings at the distributor will be very similar from either vacuum port. During the 70's many vehicles came from the factory with straight manifold vacuum going to the distributor, so you can be sure that it won't harm anything running your engine that way. I always find out which port that works the best for my combination and go with it. If your car works best with straight manifold vacuum there will be no harm done by using it. The story that the vacuum advance will over advance the timing during deceleration just isn't true. Most vacuum advance canisters reach the end of their travel at 17 or 18 inches of vacuum, so pulling 20+ inches of vacuum won't over advance it. Just go with what works and don't worry. It will work fine, just as long as you make sure that you set your timing with the vacuum line unplugged.

<font size=-1>[ This Message was edited by: mach1morgan on 3/25/02 2:33am ]</font>
 

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Discussion Starter #7
Hey MonsterMach,
This ought to crack you up.

>… initial advance will at times drop too low, maybe even to an literal “retarded” position when no vacuum is present … no engine can run well in that circumstance.


The factory timing on my 72 Porsche 911T is:

32deg. @ 6000rpm to get 5deg. "After"TDC. (vacuum retard)

Then again this is a 2.4 horizontal aircooled engine. That 6000rpm timing setting is kinda scary at times, with your face in the fan, but it's easier after a few Coronas.


Meaux
 

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Yeap ... 5 degrees retarded sounds a little strange for sure ...

I have been trying to get a decent handle on "Tuning" for a while ... I keep gathering info and trying to test it out and ask questions ...

Sometimes it almost seems as though there are no damn rules
 
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