vacuum advance
Could you elaborate on this please?
The thing to keep in mind is that I set the initial timing by the T-port sync. I let it tell me what the engine wants. I don't even pay attention to the timing lite or a vacuum gauge, at this stage.You may end up with idle timing anywhere between 10 and 20-25, depending on your particular combo.
Look.Down. To the sig area
Cams from about 215*@050 and up Will want varying amounts of extra idle-air. If you give it this air by cranking up the curb-idle screw, you may expose too much transfer port, and then the idle mixture screws go numb or just plain quit.So now you have an engine with a high enough idle, but the mixture is impossible to get right.So one solution is to crank in more idle timing. The extra timing brings up the idle speed, so now you can close the throttle blades, again.And mess with the mixture screws to get a fair idle. If you give it too much timing, and subsequently crank the curb-idle screw too far out, then the throttle blades fall below the transfer port. While it may idle this way, they usually have a tip-in problem; like a hesitation or a stall.In front of an automatic, when shifting into gear they just flat stall.
So what to do?
Well the first thing is to get the T-port sync reasonably close. And then adjust the actual idle speed with timing, and the idle quality with bypass air.Bypass air is just air that gets into the engine, bypassing the butterflies.You can bring it in through the PCV circuit or by cracking the secondaries, or by drilling little holes in the butterflies.
So after I ballpark the T-port sync, and bolt the carb back on, I tug on the Vcan, cuz it probably won't stay running with out that. Then I hit the key, and warm it up.
Now here's a thing you gotta know; Most sbms I have worked on with cams in the 215 to 250 range will idle on timing anywhere in the range of 10 to 25/30 degree range. Just by twiddling the screws you can make it work. Well, after the T-port sync is set you got no more screws to diddle with. So if the idle is not smooth or the speed is not right you only have timing and bypass air. See how easy this is getting?
So then you just crank the dizzy to the lowest stable idle speed,that the engine will give you. Then you adjust the A/F with the bypass-air, until it smooths out.Then you pull timing out until a new lower stable idle-speed is achieved, and smoothen the A/F with the bypass air again. And so on until you achieve the idle speed and quality that lets you put it in gear and drive away. Whatever that idle-speed might be.
Occasionally, you might reach a point, where the idle speed is just not smooth enough.Or you cannot get it low enough. Well, you might not have got the T-port sync right on, at the first try.Maybe it's just too lean, or too rich. If/when this happens you just crank in the curb-idle screw to expose a little more or a little less transfer, and continue fine-tuning the bypass air.
Now here's another thing. Some guys know from experience what various engine combos take or want , for idle timing, and can say with authority what to set your timing to. And when you follow their advice , then the T-port sync might naturally fall right into a decent zone.And then you don't have to go through all this rigamaroll. And I give the guys who know this, much respect, cuz they had to find it out;and that took time and thought,tools,and experimentation, so when they give it for free, they have a big heart.
As for me, I don't do this stuff as much as I used to, and plus it seems as I get older, I seem to have trouble retaining some things. But this method I developed years ago, so, it sticks, and gets results; albeit sometimes the long way around.
Another thing I can tell you is, stick cars are much harder to get right; and hi-stalls are very forgiving.And stick-cars with small gears are the hardest, while hi-stalls and big gears don't need me, cuz almost anything goes.
I stick to what I know, which is cams of 215 to 250.And of these I am intimately familiar with 230*@10.7Scr-aluminum heads, with a stick and any rear gear.
Vcan assisted idle-timing is another story
