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Vacuum advance

TrackPack

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Low compression big block 400 with AVS carb and mild hydraulic cam. What should the timing be at with the vacuum advance hooked up at idle? Total advance set a 34 on pump gas.
 
Disconnect vacuum n plug it so no vacuum leak. Set timing at whatever your engine likes 10°, 14°, 17° etc. Then once set, hook vacuum back to the distributor. Recheck timing, it should not be any different. If it is, you are pulling vacuum from manifold, not carburetor above throttle blades.
 
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When you checked total advance, was the vac advance connected, or disconnected and plugged?
 
Timing was intially set with vac advance disconnected and plugged to 34 by 2400 rpm. Then hooked up vac advance on AVS port and timing advanced past my scale (over 40) at idle.
 
Ported or manifold vacuum? Ported should not move it.

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I have a stock Carter AVS..............port is next to the PCV specifically for vacuum advance....it should be ported, right?
 
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Yes and zero vacuum when at idle. Looking at the front it is on the passenger side. If it has vacuum the trottle plate needs to be aligned or for some reason the idle is up.
 
If you are going to run ported advance then you shouldn’t see any change in timing at idle with vacuum advance connected or timing with vacuum advance disconnected. You might want to experiment with hooking it up to the manifold vacuum port for a test - you might like it.
 
Yes and zero vacuum when at idle. Looking at the front it is on the passenger side. If it has vacuum the trottle plate needs to be aligned or for some reason the idle is up.


Thanks! For some reason I have vacuum on this port with idle at 850 rpm. Idle air bleeds work as they should and the throttle position screw is only 1-1/4 turn in.
 
If you are going to run ported advance then you shouldn’t see any change in timing at idle with vacuum advance connected or timing with vacuum advance disconnected. You might want to experiment with hooking it up to the manifold vacuum port for a test - you might like it.


If I connected to manifold vac that would advance the timing like it is doing now, correct? What timing is acceptable at idle?
 
10-15 depends on the cam and how happy it is really. Sounds like 34 was the full mechanical to me.
 
10-15 depends on the cam and how happy it is really. Sounds like 34 was the full mechanical to me.


Yes, I set mechanical to 34. With no vacuum advance and port plugged I'm about 17 at idle. Distributor has light springs and slot stop plate.

Again, vacuum port connected advances distributor past 40 at idle.
 
No idea of your cam so if it likes it that way without pinging without your vac advance hooked up your close. Tons of variables.
 
Yes, I set mechanical to 34. With no vacuum advance and port plugged I'm about 17 at idle. Distributor has light springs and slot stop plate.

Again, vacuum port connected advances distributor past 40 at idle.
What number is stamped on the vac advance arm? At full manifold vac that will add double that amount to the crank also.
 
Cam is hydraulic flat tappet 227/235 @.050 480/488 lift. Runs really nice, just trying to get vacuum advance to work if possible.
 
What number is stamped on the vac advance arm? At full manifold vac that will add double that amount to the crank also.

I wrote that down somewhere, will have to look. Can't see it with just the distrib cap removed.

I take it that the choke pull off port is supposed to have full vacuum at idle, while the distrib vacuum port does not?
 
Yes it is above the throttle plates and the pull off is below them hence full vacuum. Prob just need to adjust the lower plate.
 
Yes it is above the throttle plates and the pull off is below them hence full vacuum. Prob just need to adjust the lower plate.

Thanks for the help, appreciate it.

I'll try and drop the idle to 750 ish..............it may not like it though.
 
Seriously prob the plates a bit misaligned, not hard to re-align them there are threads on it here.
 
Track.
There will be good advice &.....bad advice. I will give you 50 yrs worth of good advice...
[1] Forget about the carb for now
[2] Forget about the curve in the dist for now
[3] The first thing you do is get idle timing correct. Everything else THEN follows...
[4] The low comp ratio & big-ger cam reduce low speed efficiency, require more air to idle; t/blades are now open further at idle to get more air in, so that you are pulling vacuum through the Ported VAc ADv [ PVA ] port. PVA is useless by the way, see link below.
[5] To start set initial timing close to factory number, not critical. Disconnect & plug VA.
[6] Engine idling, in gear, wheels chocked. Turn dist slowly CW. idle rpm will increase. Use eyes, ears common sense, no AFR or vac gauges. Keep going CW until you get the highest rpm. Toggle dist to make sure you have the highest/smoothest rpm. Now check the timing.
[7] Say [6] is 34*. You use vac adv connected to manifold vacuum to get this number. You will also need an adj VA unit, stock will NOT work. Get the adj unit & turn the Allen Key fully CW [ softest spring setting ]. These units have about 30* of movement, so to get the reqd 34* at idle you can use any combination of init + MVA to get 34* [ Eg, 12 + 22, 8 + 26, 18 + 16 ].
12/22 is probably a good choice. You will need to fabricate a stop for the VA plunger arm to limit travel to 22*.
[8] This is all part of the tuning process. It is a bit of work. It is needed to get the best out of the combo.
[9] Test [6] will result in a large increase in idle rpm. After [7], the idle speed screw can now be backed out & mixture screws reset. Usually after setting up as above & improving efficiency, the engine will now idle at a slightly lower rpm & smoother.
[10] The centri curve in the dist will need to be modified. You do that LAST, not first like so many do. If the engine runs best with 36* @ WOT, & you chose 12* for the static initial [7], then the dist curve needs to be 24*. You will probably get all sorts of charts thrown up but THIS engine is no longer than stop & only dyno or track testing will get perfect numbers.
[11] I suspect that even after getting the optimum idle timing as above, too much T slot will be showing above the pri t/blades. 0.060" is max, & less is better. The fix is to drill the t/blades. I would leave that for now & do the above & report back.
[12] Man vac supply for the VA unit: any port or fitting in the intake man runner [ eg power brakes ]. Any port on the carb below the t/blades; you can tee into any fitting that is connected to man vac [ except PCV ] such as choke dashpot etc.

David Vizard has written 30 + auto books. This is a quote about idle timing, but it is in a Holley carb book. Think about that...

' The optimum idle advance is typically about 35-40* for a for a short cammed street engine & though not commonly realized as much as 50* for a street/strip engine'.

MVA benefits, scroll down to post #6.

www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html
 
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