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Discussion Starter · #1 ·
85 F150 w/ 95 302HO crate
625 AFB
Carter Adjustable vacuum
Lightest springs (from the Carter Kit) in the Dist.
3-sp. auto

I read some of the "detonation" post so I hooked up my manifold vacuum to the dist. (directly off the back of the manifold)
and I get 25deg.!(btdc)

Sure idles better, I had to close the throttle plates to get it (idle) down to about 800rpm. It kinda bugs me gettin 25deg. at idle but at the same time it sure runs smooth. Would it hurt anything to run it w/ manifold vacuum? (I run short burst on the hi-way, 70mph=3000rpm)

Thanks,
 

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Dist is not hooked to runner or plenum vaccuum.
Dist is hooked to vac pickup on carb metering block or throttle plate which picks up vac from just above the throttle blade opening at idle.

My understanding of it

Using runner/plenum vac results in timing being instantly dropped to static initial timing if you mash the pedal as plenum vac drops to near zero.
Putting it in other two places results in vac signal being "transmitted" to dist due to vac caused by air demand below the throttle body ie the greater the air flow to the motor the more vac created in the metering block of the carb which creates the pull on the dist advance.
May be a bit simplistic but thats my grab on it.

Shoot me down if I'm wrong guys
 

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For the most part it isn`t a huge difference. The factory has done it both ways. The difference is usually a smog motor versis not. If you notice light throttle pinging you can try backing off the vacumm adv some. We had several TSBs to switch from carb to manifold vaccum to cure some off idle stumbles in the late 70s. When you mash the throttle the timming is supposted to go to just mechnical, (and will with either set up.
 

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Discussion Starter · #4 ·
So, with my base timing @ 6deg., it really dosen't matter what the timing is w/ manifold vacuum @ idle? When throttle plates open, vacuum drops and timing goes mechanical to 6 or so deg.? Am I understanding this right?

Also, I recurved the dist. w/ light springs, (mechanical 30deg.) which means I should only go as high as 8deg. base timing. (38 deg. total @ 3000rpm)

I have an air pump hooked up pumping air to the cat, and the heads and no EGR. I did all this plumbin to pass Texas inspection and now I don't need it. Should I remove the pump and the plumbing?
 

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Max timing of 38*?? What is that based on. Have you had the motor on a dyno to check what the max timing should be? That really is the best way of determining that.
Each set of specs and fuels can result in differing max timings.
Rule of thumb on initial is it should be whatever the car can handle at a hot start. You know you are right on the edge of initial if it kicks a bit when you try start the motor in its hottest conditions. ie summer heat and recently run warm motor.
My motor runs at 14 static for example.

I dont know if your vac induced idle timing of 25 is bad or not
but have been steered away from manifold vac source due to pulsing and irregular vaccuum signals during decelleration etc (vac goes HUGE when throttles snap shut at 5000rpm)
I run without vac at all and have a curve to put 24* in by 2200rpm (but mine a propane only motor)
 

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After a fair amount of experimenting with my milds 9.5:1, 351W, I settled on 10 degrees of mechanical advance in the distributor, which is 20 at the crank. I use 16 degrees of initial, 36 total and it's all in at 3600 RPM. Very occasionally when the engine is quite hot, it'll kick back on the starter, but doesn't ping under hard acceleration. I may go to a 12 degree limit plate for 24 degrees of mechanical advance at the crank and set the initial at 12 degrees. That would cure any starting problems. As for where you plug your vacuum advance into, it doesn't make much difference. At full vacuum the vacuum canister is at the end of it's travel so it doesn't over advance it when it's hooked to manifold vacuum. I usually try straight vacuum and ported carb vacuum on every engine I have or build, and use which ever one that it's easier to get a good consistent idle with.
 
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