Timing and idle rpm . . . on my '66 Newport 383 2bbl, the base timing is 12.5 degrees BTDC. With it set there and the carb idle speed optimized, I can change the idle rpm just by increasing or decreasing the base timing. In a linear fashion, but I never ran it more than 15 degrees BTDC. This is with the stock "closed chamber" cylinder heads.
When I got my '70 Monaco 383 4bbl, the base timing on that engine is 5 degrees BTDC. With the carb optimized, I can move the distributor to change the base timing significantly and the base idle speed barely changes. This is with the 906 stock heads (open chamber).
First thing to do is get the engine warmed up (run about 20 minutes or so, or drive it that long). You can shut it off momentarily to hook up the timing light and tachometer, if it doesn't have an inductive hookup. Then start it back up and let it stabilize a minute or so. Set the timing to factory spec, whatever that might be.
Second is to set and verify the hot base idle speed at factory spec.
Third is to verify the idle mixture and that the two needles are adjusted the same number of turns "out" from being lightly seated (you can check this when the engine is off). From there, turn the mixture screw in or out, noting which way increases or decreases the idle rpm . . one at a time. Aim for the max rpm with each screw individually and collectively. Readjust the idle speed down, if needed. From this point, screw each screw "in" until you get a 20rpm drop, and then turn it back to where it was (max rpm), for each side.
Some like to use a vac gauge to set the mixture with, but every time I'd try that, my results didn't always seem accurate for what I wanted. And if you get excited about trying to achieve some "max vac" level that's allegedly attainable, you might not hit it. I just like the precision of the dwell tach in this area.
Tweek it a few times to see if you can get a little "tighter" in the adjustments, but don't be dismayed if it's already as good as it can be.
For my final check, I'd nail the parking brake and foot brake, gently put it in "D", pat the throttle to see if response is good and if the idle quality is smooth and stable. From there, with the parking brake fully engaged, I'd get out and check the pulses out of the exhaust pipe (works best on a single exh car) to see if the flow was smooth or pulsating. If it pulsates, then tweak the idle speed up about 1/8th turn and recheck. Just when the flow smooths out will be your best hot idle speed, in gear. Then put it in "P" and see where the idle speed goes to. Then put it back in "D" and see how smoothly it goes into gear. If it's a little too abrupt, you might take the base idle speed down a smidge and see how it acts.
If desired, with each speed change you might re-check the idle mixture settings, for good measure. If you haven't don't this before on this car, spending some time getting things to act the best can give you a baseline should something change later on.
So, the final adjustment can take some back-and-forth tweaking to get to the best "sweet spot" in those idle adjustments. There might be a little "shake" at idle in gear, but that's fine as long as the off-idle response is positive and eager. AND, you'll know that YOU did it yourself. Once you do it and get the knack for it, it doesn't take as long as the first time tends to.
Back when we didn't have the amount of "stuff" in our gasoline, I'd put my hand into the exhaust flow for a second, then remove my hand and smell where the exhaust had flowed over it. IF I smelled too many hydrocarbons, the mixture was a little rich, so it needed to be leaned a bit. Goal was "no" or "very slight smell". But usually, after I was done, it was basically "no smell" . . . and that was good. Not sure if it works with current fuels? I"ve been driving my modern cars too much, of late, to have had to deal with a carburetor.
Enjoy!
CBODY67
