to me, this sounds classic of the throttle-blades being, EITHER;
up TOO-FAR on the transfers, or
DOWN too far.
>>In the case of, the throttles being open too far;
Here's what can happen:
With a manual trans, the tendency to downshift each gear is pretty normal. So as you are coming to a stop, with the throttle closed, the pistons are seriously pulling on the transfer slots, and gobs of fuel is being sucked into the engine. Some of this fuel ends up on the floor of the plenum, or sticking to the port walls, etc . Just before stopping you clutch it, and the transfers return to normal.
But the engine is in a partially flooded state so wants to stall. The only thing keeping it running is the amount of advance. Over the next few revolutions, all that fuel in the plenum, works it's way thru, and the idle returns to normal.
>>In the case of the throttles being closed too far
Here is what can happen;
With a manual trans, the tendency to downshift each gear is pretty normal. So as you are coming to a stop, with the throttle closed, the pistons are seriously pulling on the transfer slots, but with them closed so far, the engine is pulling in gobs of air around the throttle blades, which dries up the transfers. So then when you push in the clutch, suddenly the engine has nothing motoring it, and she wants to stall for lack of fuel.
Now, ready to idle, but a falling rpm, the manifold vacuum is very low, and it takes a few revolutions to bring the transfers back on line, or a blip of the throttle.
>> Ok so the solution to all this is usually a three-parter;
1) the transfers have to be set just right, and
2) then the idle speed adjusted with some combination of Idle-timing.
3) And with Idle-Air bypass, to maintain idle-quality. This last part is a function of the installed cam.
Notes;
Ok first, you gotta understand two things;
1) your engine does not actually suck anything into it.
Your engine, by virtue of the pistons falling on the intake stroke, tries to create a low-pressure area inside the cylinder. And the atmosphere, at a higher pressure tries to get in there.
At idle, this is past the nearly closed throttle valves.
2) All engines have the same or similar timing requirement which is required to create from the expanding gasses, the maximum amount of cylinder pressure possible, at just the right time, in terms of crankshaft degrees. This position is around 25 to 28 degrees AFTER TDC. All your timing systems are designed to start the fire early enough to create this moment.
3) At idle, this can take 30 or more degrees of advance. The closer you get to that perfect point, the more power your idling engine will make.
This can be a problem in a manual-trans car, because it makes it difficult to drive slowly, especially if it has a lot of CCP. It just gets too jumpy as the rpm goes down. To counter this, my advice is to retard the timing until it has just the right amount of power to pull itself around in first gear, without adding throttle nor riding the brake. In my car this is about 5*@550rpm. It will pull itself along for hours on flat, level, pavement at exactly 4 mph.
Of course, if you are not into parading, this may not matter to you, but the advice is still the same. Too much advance will only cause problems in the fuel delivery from the too-far closed transfers. And too little timing will cause problems in the fuel delivery from the too-far open transfers.
4) AT IDLE, a very-late closing intake valve is gonna push just-inducted A/F charge back up into the intake plenum, so the idle has to be set quite rich. But at idle, with low manifold vacuum, the engine may be short of air when the transfers are set right. Therefore, you may have to provide a way for more air to get into the intake besides past the nearly closed throttles. Your first go-to is the PCV valve, which, at idle, will be on it's minimum flow rating. Some guys will crack the secondaries to get that air, or in your case, the outboard carbs. But I've never had much success with that, as the idle-quality usually suffers. What I do is drill small holes in the throttle blades themselves, right near the transfers, which reduces the tendency for them to dry up. and, as the throttle is applied thet go towards vertical and are thus defeated.
5) As to Big overlap cams, running with headers, operating at idle;
In this case; the headers, on the overlap cycle, will draw A/F charge down to the intake-valve and pull it straight accross the chamber into the primary pipes. That charge was supposed to be a part of the idle charge, but now it is gone forever. So then, you gotta compensate for that, at the throttle valves. But-but, this changes with rpm, so you sorta got to pick one rpm and stick to it, until you understand it. Then you can fool around, knowing what to do.
6) heaven forbid that your headers are sucking air at the flanges. When this happens the fresh air mixes with the raw fuel that is there on the overlap cycle, and if it catches fire, you get an instant pressure signal that goes in both directions, including backwards across the piston and up into the plenum, where it wreaks havoc.
7) If you run a long overlap cam with log manifolds, your overlap cycle gets killed; so right away, you could have installed a smaller cam with nearly zero consequence, and enjoy it's better manners. But with the long-overlap cycle, and the logs, it is easily possible for the exiting high-pressure exhaust gasses, to turn around and enter into a different cylinder that is on the intake cycle. Instantly you have a runaway EGR event, and that cylinder will misfire for lack of a combustible mixture in it.
Ok so as you can see, at idle, with a long-period cam, there are a lotta things going on, that you the tuner, have to be aware of.
And now you know why the factory 340 cars got a 114Lsa cam with a mediocre 44* of overlap, to go with their log manifolds, and why they idled the with that wannabee a-rock-star sound, that never actually delivered accordingly.
Ok so then lets define long-period.
This title is usually given to cams with long intake durations. Like over 268 or so. But the truth is that it should, IMO, be more accurately applied to long-overlap cams. And here's why;
You can have a 268* cam on a 114LSa that might have 44* or less over lap. So even tho the 268 says almost a long-period; the 44* says loser cam. Take that same 268* cam, now on a 106 Lsa, and the overlap goes to 60* which now has the potential, with headers, to make a lot more power.
So how does one define a long-period cam?
IDK, it's sort of ethereal.
But again, IMO, if your cam has an Ica of more that say 66/68*, the compression cycle is getting pretty short, and the required Scr to make it a HotRod engine is getting harder to achieve.
Back to your carb issue;
Let's say your idle-speed is 750, and you experience a tip-in sag, which is a stumble or hesitation, with a small amount and slow rotation, of the throttle-shaft, I'd be willing to bet that your transfers are not open far enough.The transition has to be smooth, else it will drive you crazy.
I would recommend to take the carb off, tip it upside down, and reset the Transfer slot exposure under the throttles, to between square and a bit taller than wide, both the same ........ but I know what a pita that would be.
So in consideration of that.....
Increase the idle-speed 50 rpm, and back up the timing, back to 750rpm; then try again. If you still get a tip-in sag, do it again as many times as needed to get rid of the sag. If/or when, the timing goes to under 12 degrees with say a 220/225ish cam, stop!, something else is wrong.
If you do not have a tip-in sag at all at 750 rpm, congratulations your carb is rich enough; crank the idle speed lower, and go hunting for it .... the same way...... lol.
When you get this right, and your cam is smaller than the MOPAR 292/292/108, (the largest cam that I have tuned) then the engine will behave itself when coming to a stop, and she shouldn't need the throttle-cracking solenoid; cuz you know, with that solenoid engaged, parading at walking speed will be impossible..
and Finally
With a manual-trans; who cares what the idle-timing is. It is impossible to engage the clutch at idle so your engine will never be asked to pull from there.
My 11.3 Scr 292/292/108 idled just fine at 5*@ 550rpm, pulling itself, and the jumpiness was delayed to 50 rpm lower. Let your engine tell you what it wants ..........
Ok here's another one;
with the engine at operating temperature, and again with a modest cam, it should be possible to just twist the key and she springs to life in a split second without any help from your big right foot. Shut it off and do it again. and again; this is so satisfying when you get there.
