BaDaBoom there it is!
The T-port sync, is the position of the throttle blades relative to the bottom of the transfer slot. If you don't have it right, then it's really hard to get the idle, the off-idle, and tip-in, working right.
The transfer slot is a vertical slot next to the idle discharge port. You cannot see the Idle port with the throttle closed,cuz it is right under the plate. Both of these are at the bottom of the primary throttle bores in the front walls close to where the mixture screws are. In fact if you screw the mixture screws all the way in, you will be able to feel the points as they enter those bores.They are best seen with the carb on the bench and upside down.
If the throttle is open too far, exposing a lot of transfer slot, more than the engine can handle, then the idle goes rich. This is often accompanied by you having to screw the mixture screws nearly closed. When it idles too rich it is often accompanied with missfires.Then because the mixture screws are closed, tip-in goes lean and you may get a lean hesitation on tip-in.
If the throttles are too far closed,in relation to the transfer exposure, the engine will not idle properly until you maybe max out the mixture screw adjustment.Again accompanied with missfires. But now, on tip-in, the mixture goes rich.This usually just makes a slightly mushy tip-in.
So the key is to have the throttles adjusted so that they are in just the right place,to have the mixture screws in the center of their adjustment range,at idle.
Depending on the size of the cam, this often seems impossible.Your options are idle-air-bypass,timing, and cylinder pressure. Since cylinder pressure is sorta fixed at this point, you have to mess with the timing and bypass air, to get a reasonable idle speed and quality. By doing this, getting the idle right, the engine starts to make a bit more torque, and then putting it into gear doesn't bother it quite so much.
The faster the idle-speed is, with a stock TC, the more likely the engine is to misbehave. This is because of the sudden drop in airspeed past the low speed discharge ports,(the transfers and the idle enrichment),due to the trannys oil-pump load. With the loss in airspeed, the fuel dries up and the engine stalls.
So the slower the engine is able to idle, the longer it will take for the trannys oilpump to pressurize all the circuits, and the more gradual will be the load applied to the crank, and so no more sudden loss in airspeed, and thus no more stalling.
The 268* cam is medium sized cam for a 360, and is usually an easy tune. So, crank in about 18* of idle timing.
Then pull the carb off, flip it over, and set the curb-idle screw to adjust the transfer port exposure to square to a bit higher than wide. Make sure that the fast-idle cam doesn't mess you up.Flip the carb back right side up, and set the mixture screws to 1.75 turns out on all 4bbls; except Holley types to 3/4 turn. Reinstall it. Now from this point on, do not mess with the curb idle screw. I suggest you figure out a way to get back to this position, in case you do not heed my warning.
So now fire it up, and warm her up.
Adjust the idle speed with timing. I like 750N/650 in gear. I like 12/14* on that cam, but if your cylinder pressure is down, you may need quite a bit more timing.Some say into the 20s. I am running 10.9 Scr @ around 180 psi. If you are down at 120psi, take the motor apart and swap out those dead slugs! You are never gonna get decent low-speed response out of her.
If you are up close to 165psi,(a good place to be) then with iron heads you shouldn't need more than 14/16*.
But let the engine chose it's own idle-timing, by not messing with the T-port sync from where you initially set it.
>That cam likes quite a bit of bypass air. Once the PCV is known to be working, you may need to introduce air into the intake from another source.The usual spots are secondary cracking, or airholes drilled through the primary plates.
Some engines do not like dry air scooting around the secondary blades.The back cylinders may end up lean enough to cause issues.
>Drilling holes in the primary blades is a good option. I like it cuz as the throttles are tipped in, the air no longer passes thru the blades, but follows the normal path.And this air can be introduced near to the low-speed discharge ports, allowing plenty of time for thorough mixing.
>Sometimes the engine cannot be slowed down enough. In this case it is probably sucking air someplace. The first go-to is the PCV, then the brake booster, then whatever else is attached to the intake.
BTW, the PCV needs to be sucking from the carb between the primary throttle plates,NOT from any manifold port!nor from the brake booster port which is occasionally ported at the back of the carb.
After that you will hunt around the intake ports, the carb base-gasket, and finally the underside of the intake, in the valley.
Adjust the idle quality with bypass-air, starting with making sure the PCV is on there and working.You can fine-tune with fuel level in the bowl.
At this point she will be running pretty good. Now is the time to revisit the mixture screws and the T-port sync.
So adjust the mixture screws and see what happens.
If she runs better with more mixture, then return the mixture to the "base" setting, and increase the transfer fuel be cranking in the curb-idle screw 1/4 to 1/2 turn. Then try the screws again.
If she runs better with less mixture, then we go the other way. Return the screw to the "base" setting, and reduce the curb-idle opening 1/4 to 1/2 turn.
Now if the idle speed changes much during this, it's back to adjusting the timing this time, cuz the mixture is just right. Just add or subtract as may be required.
Once this is all dialed in you will have to check your power-timing and limit it to whatever it takes to stay out of detonation. Don't get hung up on forcing her to take 36* or 34* or whatever degrees; just keep it out of detonation for now. This may require you to modify your distributor to achieve both goals,namely; the previously established idle-timing and the now required power-timing. And you may have to do this more than once. Make it easy on yourself and order up a limiter from FBO.
Ok, I have, in my 360, a Hughes HE3037 cam which is;276/286/110 and lifts of .549/.571 advertised with 1.6 roller tipped arms,and it runs very nice with 14* initial, and very little bypass air. At first I tried secondary cracking, but she did not like that.So, I figured out how much air she wanted by introducing air into the PCV circuit, through various sized orifices teed into the line. A little math later and I had figured out what size holes to drill in the primary blades. And then I had her running down to 650 dragging the clutch.Good enough I thought.
Some time later, when I swapped the 4.30s out for 3.55s, I needed the engine to idle slower. Pulling the 750DP apart, I noticed that it was sorta set-up for delivering fuel to the secondaries, along the top of the baseplate. Well it didn't take me long to figure out how to deliver some fuel there, and then secondary cracking was back on the table. And so it came to be.Of course I had to close up those pesky holes that I had previously drilled.
A little more tuning and I could drag the idle down to 550 with the clutch. The result of that was that now she was pilling herself along at 4 mph on flat hard,level ground. Pavement actually. Then by retarding the timing, with my dash-mounted, dial-back timing-device, I smoothed her right out and now down to about 3.5mph@ a tic under 500rpm. Parading was back!
So what I'm telling you is this; I have the next bigger cam from a 268, and I occasionally have it idling down to 500@ about 5* initial timing, and it has enough torque to pull itself on flat level pavement, with a 10.97 starter gear.
What I'm NOT saying is that your combo needs mega-timing, nor a 900 idle.
You just need to spend more time on the idle. Don't be discouraged if it takes several hours.
Once she's making torque at 650 in gear, she will quit stalling.
BTW I had to go thru this 3 times; once for each cam I have run. The hardest was the 292/292/108. From there I installed a 270/280/110, which of course was a completely different animal. And from that cam to the current 276/286/110, was just the secondary idle fuel delivery and 2* more idle-timing
That's my story and I'm sticking to it.
