The old stalling when put in gear syndrome

Thanks for the information. PCV air is one of those things that is calibrated into the total carb calibration. Which affects the location of the idle discharge ports and transition slot. Specs on flow are hard to find, but I did find some in an old Exxon service station service manual back in the earlier 1970s. It can be a tuning device as Chevrolet used a higher-flow valve on their LT-1/L82 motors than on the normal engines. Had a purple die on it for ID purposes. Even so, seems like the CFM on them is under 2cfm.

Yes, at idle, the increased vac bottoms out the internal valve against spring pressure. With a clogged pcv valve and/or hose, the idle speed will only decrease about 25rpm and get a slight bit rougher, all which is better when the system works.

There is an old SAE paper on PCV flow which GM did back in the 1961 era. They had two test vehicles, which were '61 Chev sedans with the 235 I-6, which were in their security fleet and saw lots of low speed use, which was known to accumulate sludge as the engines never got enough road speed for the road draft tube to work.

In more recent times, in another forum, there is a place which builds adjustable valves. Allegedly supposed to work. I do not feel that is your issue, though. As the carb should work with any PCV valve ever built. They are much more generic, I suspect, than anybody desires to admit to, which means they all have the same flow. As one valve can fit anything from a 225 to 440cid engine. The adjustable valves are more for the realm of highly-mnodified engines than stock engines, although some in the rother forum feel they can solve a rougher idle situation with the adjustable valve on a stock engine that idled a bit rougher from the factory when new.

Well, I have been doing this as avocation, and some other related work on steam and gas turbines and more for a long time. But, I am always surprised to see how poor my skill is when folks are so well informed as seen in this note above. Impressive. So, I had thought that during hi vac idle, the flow was cut off, and apparently not so, just low. And, when I put my finger over mine, it has huge vac and pretty substantial flow, and the engine rpm dropped significantly. After all of my recent digging and your note, I now will check the rpm drop for fun. And, I ordered a Delco valve to try as well. All fun, and we will see. But, I am still at odds with this engine that does not like to idle well in gear....More work. thanks
Raymond, forgive me if I get this wrong, since you and I discussed a lot of different options, some relating to your great rebuilt distributor....cant thank you enough. I am very happy that you have limited my mech advance to 16 so I can keep the initial high at 18. And yes, I did go back to 15 on initial and did not find a change to my in gear idle issue. And now, I am also trying the CBody options, one to go to ported vs manifold vac, and readjust, then to look at the PCV valve again. And, I will say, that this has made some difference. The ported moves idle down, obviously due to less advance, so had to go from 550 back up to 700, and adjust the mix. When in gear, it managed idle much better, with just some grumbling, so raised up to just under 750 and at this point it seems better......need road tests now. I will note too that when I plug this PCV valve, idle drops 150, not the much lower numbers quoted before, so am thinking too much flow at idle?? One other point on the ported is that the response is off just a tat. Hit the throttle and there is just a hint of lag before it launches....not awful, but a note that is not there on manifold vac. So, I am really curious to change the PCV now, and once again, thanks to all who have added thoughts on this subject, esp since it seems to be somewhat common.
One more happy note this sunday morning.....just went out for short hot run. And recently, after my front engine work, it would hardly run when hot, in gear. So, after more adjustments as above, down the road, and again, I am feisty and like to include a slow start in first, hammer it, shift and let it run out in second, then go back to normal cruise. All the bases, except for hot trans oil, which I wonder about too. But, this morning, the car is back to its normal for the most part, and had no issues in gear. So, I am thinking that the ported vac has made a big difference, because on the manifold, I could not adjust it to work at all. Not sure why the cam timing changed all of this, but that was the only change that resulted in my problem here. So, a happy day for an old guy after this slug of work.
Well at least you listened to advice. Thye lag might be the Acel pump needs adjustment on the carb.
What I need is more parts from you, when the bank says I can go for it!
The vac would affect it because ported is completely off in all these states but manifold it's pulling the VA but then the vac changes when you drop it in gear.
The vac also is very high when idle deceleration but will drop as rpm drops
Exactamundo.....but how in the world does this help in this case....aargh. And further, the points made by CBody regarding the air jets and flow around a closed throttle plate....esp since now, with no added vac at idle, it means the throttles must be opened more to get the speed back up. I continue to learn that I know nothing. But again, I am having a great sunday with no stalling.
It's not about air speed flow, at idle, it's more about volume. Ported is better for most engines, especially stock engines. Some people use it to band-aid an idle situation with a wilder cam, but then discover that they have an off-idle "lag" or "flat spot", which means their wilder cam needs more air, so the valves are opened wider, so the relationship of the throttle plate and the idle ports is out of whack. The fix? Drill holes in the throttle plates to allow more air in, but then close the throttle plates back to where they should have been, so the engine gets the additional air it needs and to also operate as it should when the throttle is opened. Because the desired geometric relationship between the throttle plates and the idle ports was re-established.

Holley 4bbls have a secondary throttle plate set screw, hidden on the rh side of the carb under the secondary diaphram mechanism. A small set screw that keeps the rear throttle plates opened just a hair, to allow the secondary idle system to work and keep fresh fuel in the secondary float bowl, rather than let it stagnate. An additional tuning tool for some, but few know it's there. One of the newer series of Holley 4bbls have "4-corner idle mixture adjustments" to help address this situation. But to me, it CAN be another way that an inexperienced person can get themselves in trouble with their carb and engine. In prior times, these things were "race only" items only on full-race carburetors. As there was no secondary idle system on Carters, no need for these things.

Ported vac was always used by everybody (typically) to run vac advance. It usually took about 8-9" Hg to initiate the vac advance actions, maxing out at 14" Hg. This covered the bulk of normal suburban and highway driving. Kept the total timing up, too, for good fuel economy and throttle response.

In the later 1970s, GM started to use manifold vac to run the vac advance units. On my '77LT 305 2bbl, it increased the lower-rpm response a good bit (once the EGR was "limited'). It had great off-idle response as there was more timing in the engine. BUT the downside of such appeared with the '85 Chevy pickups. They'd come in for their first tune-up and could have a complaint of "lower power", too. The new spark plugs did not fix that. Finally GM put out a TSB that toally fixed the issue and that's when I figured out what was going on.

With the lazier centrifugal advance of an emissions engine, the mechanical advance took a while to get going. With the vac advance running on manifold vacuum, the vac started out high at idle. When the throttle was opened to accelerate, that high vac was reduced, as expected, which meant that total engine timing advance went away, too, until engine speed got high enough for the mechanical adv to start working more. So the GM factory-fix was to take two of the dreaded Ford-style "vac delay valves" and turn them backwards (so they would hold vac rather than bleed it off), put the two valves plumbed in parallel, and replace the vac line to the vac advance with this new part (two valves and related rubber lines). This kept the vac advance can charged with full vac until the rpm increased enough that the mech advance was operating. Everybody loved this, but it did not work for everything, although it livened-up the 305 4bbls in 1/2 ton pickup trucks a good bit.

Had they used ported vac to run the vac advance, the off-icle response would have been more appropriate for a 305 V-8. Higher throttle opening at idle, vac advance didn't
happen until the throttle was opened, quickly advanced and stayed there while driving, WOT power not affected as that was lower vac anyway, as lower-rpm response was increased. Total advance at 2500rpm (base + vac + mechanical) was still past 50 degrees BTDC, so general operation not really affected.

In normal driving, many people like to floor the throttle for acceleration, for the feel of power. In reality, they could do better with less throttle by using no more than 3/4 throttle and using manual downshifts (unless the automatic was programmed for automatic part-throttle downshifts) if needed. Reason? Keeping the vac advance can charged means more advance in the engine for better power without the richer mixture that WOT would result in. Once you figure these things out, adding just enough throttle to trigger a part-throttle downshift, it becomes "second nature" and everything works well. Yes, this even works on computerized engine controls!

On our '66 Newport 383 2bbl, I learned to do a manual downshift into "2" with a slight throttle increase when I got onto the freeway on-ramp. I learned to do that as others did not notice I had done it. Just enough throttle to make s smooth merge, than a manual upshift into "D" and all was well. On our '72 Newport 400 2bbl, we tweaked the transmission kickdown linkage adjustment to do the same thing, automatically.

I'm not sure why you feel anything slower than 700rpm is "too slow", but our '66 Newport would idle just fine all day long at 550rpm in "D" with the a/c blowing frigid air in the summer time. Many cars of the middle 1950s were spec'd to idle at 400rpm, by comparison, but even the 4bbls were small, in the 450cfm range. So, by observation, your OEM-cammed engine can idle at those lower speeds just fine.

The aftermarket AVS2 you have (650cfm?) is rated at engines from 300-450 CID, so it is more "will work" as to the idle port placements than the OEM Carter AFB, which was more specific to the designed application.

Enjoy the tweaking!
Still amazed at your analysis....well done. I dont disagree with the point on the size of the new AVS, and that may be true....not sure, but the OEM AFB was a bit smaller, and of course, designed for street work at low rpm and not performance......and the new stuff moves to performance. So, for this work, probably correct. My point on idle speed is based on seat of the pants. Some of the old harleys and a couple of the old Farmall tractors would idle at almost zero, and were amazing. This engine will not, under its current work ethic....when turned down to the 600 I began with, it was not happy and almost stalling in gear. Given some newness in a rebuild, who knows. I do recognize that back in the day, this was not an issue, as I recall in my years of drag racing in the 60's, and lots of cars....that is the only reason I had jobs from 15 on. So, no argument here that they might work well at the lower idle, but I have to deal with what I have been dealt, for now. I am going to the hotter plugs.....probably what it should have, the champion RJ 14....I had used 12's and are a bit sooty. I am also curious about the next PCV valve, still an unknown for me. I think it was your point that the idle should not drop more than about 50?? Mine drops 150 with this yet another hill to climb. Yes, more tweaking, thanks for the fine reading. I forgot to mention the ported vacuum. I still am not educated on when it began....your point is early. I had thought it did not show up until this point is also very interesting, and needs brain tuning for me.
My 50 rpm drop with the pcv plugged is my estimate. I never did pay attention to the dwell tach reading for that, just noted that with everything adjusted to spec, the rpm did drop a bit, but not drastically.

Ported vacuum has always been the default for decades, going well back into the 1950s and probably earlier.

The reason hot base idle speeds trended upward toward the 1970s was more for emissions purposes than not. The Chrysler CAP engines for California had a higher hot base idle speed than did the federal-spec motors. A bit better atomization of the fuel at the slightly higher idle speeds, they claimed. Some of the earlier 1970s factory hot rod motors were in the 900rpm range, but only a very small number. Anything over 800rpm was not typical for Chrysler engines. Seems like our '72 400 2bbl was 700rpm? But that was during the emissions era and it would idle slower.