To tune the Idle;
Set the T-slot exposure underneath the throttle plates to between square and slightly taller than wide, them leave it alone.
set the idle-mixture screws to in the middle of their adjustment range
Make sure the Power-valve system is not flowing
Make sure the PCV system is working
Make sure that whatever air the engine is getting, it is coming thru the Primary venturies, or if the Secondaries need to be cracked, that there is at least some fuel in that air. If your carb does Not have a 4-corner idle, then close the secondaries up tight but not sticking.
Set the idle speed by timing, as low as it will idle in gear to between 550 and 700, depending on the Convertor stall.
Make sure that the Vacuum advance is not affecting the timing below ~1200rpm
The idle-speed goals are;
1) To not have it bang on the Neutral to in-gear shift, and
2) To not have a stumble when tipping the throttle in, and
3) To not have a stinky idle.
A/B) At idle, the more timing you give it, up to around 25/30 degrees, the more power she will make. If it the trans bangs when going into gear, she is either;1) making too much power, (rpm is too high); just take timing away; or 2) either the low band or the hi-drum, is coming on too fast, or the Convertor has issues. If the rpm gets to be too slow with reduced timing, she wants some Idle-air bypass. If she simultaneously has a tip in sag, then she wants a tiny bit more transfer fuel first.
Remember, at this time, the Power-valve system must be shut off, and the timing has to be stable.
If your idle mixture screws wander more than 1/2 turn from the center point, then the Transfer-fuel needs adjusting. Your idle mixture can be any combination of Transfer fuel plus mixture fuel, that it takes to eliminate the tip-in sag. But, if the mixture screws are too rich or too lean, then the entire slow-speed system, coming off-idle, will be wrong, which is why I recommend to set them in the center of their range.
C) if the exhaust is burn your eyes stinky, either; your valve lash is too tight (if solids)/ or the engine coolant is too hot or too cold/or the AFR is wrong/ or the timing is too much.
If you have to add bypass air, the idle speed will go up, same as if you had adjusted the curb-idle screw; but with one difference; whereas the idle screw will open the Transfers, adding fuel; the bypass air will not unless you are cracking the secondaries of a 4-corner idle. If your cam is smaller than
[email protected] you should not need any bypass-air. If your cam is bigger then 240* then it will for sure need bypass. Between 220 and 240 degrees it gets tricky, with varying amounts of air.
D) if you just can't get it, get the back wheels off the ground; maybe you convertor is too tight. Put it in gear, then drag the brakes until the wheels stop spinning. If the engine stalls or nearly does, either; the TC is going out, or the engine lacks low-rpm power and needs more tuning time.
Just so you know, my 230*@.050 cam will idle down to 500/550 in First-gear (manual-trans) and has enough power at 5* advance to pull itself (10.97) roadgear, on a flat/level/hard surface. At one time, I had a 292/292/108 cam (249*@.050) that would do the same but at 8* idle-timing. The point is that mega idle timing is NOT required. However, once in gear and the throttle tipping in, the more you have the snappier it will feel. and that is where the combination of Vacuum advance and a fast timing curve, and/or hi cylinder pressure will really shine.
Happy HotRodding
