Vacuum advance

TrackPack

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Quick update. HalifaxHops was right. I pulled the carb and found that the transfer slots were about 1/4 uncovered with my settings. I backed off the throttle position screw to see if the plates would cover the transfer slots and it wouldn't completely cover them. I noticed that the 4-speed dash pot was preventing the plates from closing all the way even though the throttle screw was backed all the way off, so I removed it.

So, the next step is to add some more initial advance in hopes of getting enough rpm and be able to back the plates off enough to cover the transfer slots, get a clean idle with the mixture screws, and verify that I have no ported vacuum at idle. Something tells me that I'm going to need more air to idle with the plates covering the transfer slots like Geoff2 said.

Thanks to all that have helped, more to come.
 
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Wietse

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If you are using a PCV valve (you should really) double check it is working.
A pcv valve will also add air into the intake, though being unmetered by the carb you need to enrich the mixture for that.
Not sure what your manifold vacuum is, but this is important to know the pcv valve will function correct.
A standard pcv has 2 flow rates, at high vacuum (idle) it will have a restriction and does not allow a lot of flow, at low vacuum the pcv will flow more air.
If required you could swap to a wagner pcv and you will be able to adjust the flow to suit the engine needs, but due to the cost it is not worth it if you can live without.
 

Dragon Slayer

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Some of the transfer slot should be exposed. The vacuum port hole is the one that would be covered. It is in line with the vacuum tube. Passenger side and should be to the right side of the transfer port. Some exposure of the transfer slot is desired.
 

biomedtechguy

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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
A good way to get to the place described in the quoted post is a FBO ignition system.
 

TrackPack

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Some of the transfer slot should be exposed. The vacuum port hole is the one that would be covered. It is in line with the vacuum tube. Passenger side and should be to the right side of the transfer port. Some exposure of the transfer slot is desired.

Thanks. I wasn't aware of the location of that vacuum port hole in the throttle bore, make sense now. The plate starts to uncover that port at about 1 turn of the idle speed screw.
 
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