Idle Mixture Adjustments . . . .
In the earlier days, the desire was to use the highest rpm/vacuum reading as "the place" to set it. That was good, back then, and felt to be optimum. Using the factory specs as the cars were still "used cars" at that point.
As the "emissions era" came to be upon us, I noticed several different orientations, from different OEMs. So I pondered how'd they compare. On the tune-up decal on our then-new '69 Chevy C-10 350-4bbl (regular fuel on the pickups), it mentioned adjusting the mixture for the highest rpm with each idle screw, then going leaner until you got a 20rpm drop on each screw, then turning it rich until that 20rpm came back. Which I believe was later termed "Lean Best Idle". In this method, the mixture was leaned a bit from optimum rpm/vacuum, but then leaned to "the edge" of the top of that mixture adjustment bell curve. Lean enough to still be smooth and responsive, but not too lean to cause a rougher idle, on the "lean side" of the bell curve. Just as there was a "too rich" situation on that side of the bell curve peak.
What I later developed was something I did on my own, as a quality control check. This works well on single exhaust vehicles and a little less so on dual exhaust vehicles. Doing it first on our Chevy pickup, which was the newest vehicle at the time. NO need for a tach, either, so it's "pure shade tree" in nature.
I noticed that (with a standard/OEM cammed motor), when setting hot base idle rpm, a rougher idle would usually result in feeling separate pulses at the end of the exhaust pipe in "D" (Parking brake fully applied!!). Which might vary a bit with a looser factory torque converter. On manual trans vehicles, not a good way to simulate these same things.
So, starting with the carb at Lean Best Idle, I'd put the vehicle in gear with the parking brake strongly applied. After easing off the foot brake, to check for creep, then I'd put my hand over the exhaust pipe end to check for smoothness of flow. Sometimes, it might need an additional 20rpm on the speed screw to make that happen. Then, when removing my hand from the flow, I'd smell it. A hydrocarbon smell would indicate it needs a leaner tweak, with the desire to minimize the hydrocarbon smell. Necessary to wash that hand afterward!
So, after getting the idle speed/mixture tweaked, with the a/c compressor running, Dad could sit in the field, talking to whomever was working/plowing that day, for hours with NO overheating problems. As the pickup just sat there in a smooth idle, in gear, foot on the brake, and the a/c blowing cold.
Only thing was that in '69, Chevy upped the line pressure on the THM400s, so they ALL had a more intense initial engagement when put in gear . . . which Dad didn't like. BUT with my settings, although it did well, when in "N" or "P", the resultant idle rpm would be 780rpm, which seemed a bit high to me for a stock motor, but otherwise worked very well. SO he'd find some mechanic to decrease the idle speed for a nice, smooth, car-like gear engagement. Which lowered the idle speed too low and it's get hotter in those field conversations (which might last an hour or so, sometimes, in the summer).
I did the same thing with the '66 Newport, which generally resulted in an "in gear" idle speed of about 550rpm and the "P" idle rpm a bit higher than the factory settings. So I tried to tweak the Stromberg WWC for a happy medium of sorts. But getting "the smell" minimized was possible. The exhaust temp was cool, too.
When we got the '72 Newport 400 2bbl, after it got some miles on it, I did my settings on it, too. ONE big difference was that the exhaust temperature (with the 8.2CR motor, vs. the 9.2 CR motor in the '66) was much hotter. The base timing was something like 7.5 degrees plus or minus 2.5 degrees, so I (and the dealer) usually put it at the maximum number. Due to, best I could determine, the higher heat transfer to the exhaust due to the lower compression ratio. Whereas the 9.2+ motors had a cooler "burn" than the lower compression ratios did. Such that the greater heat in the pipes might also help cook the emissions before they arrived in "ambient air". No AIR pumps on these particular motors, either. The increase of coolant temps form 180-185 (OEM 400 t/stat in '72) or the later 195 t/stats should have had no affect on ultimate exhaust temps, either. That's just the way it was, for whatever reason.
When I checked the spark plugs every so often, they always looked "light tan" which further indicated things were good. As did the light gray color of the end of the exhaust pipes, when the gas had some lead in it. So, all was well.
So all was well, from the shadetree method of looking at the color of the inside of the tail pipes, UNTIL unleaded gas arrived, with its "black" coating, no matter what, even on catalytic converter cars.
Now, initially, my tweaked setting usually would consume the better part of an hour. NOT something that a dealership tech might want to do, especially with their higher-tech instruments of that time. BUT something that a motivated individual could do with no problems, usually. PLUS keep them out of trouble for a while. Unless something broke!
Not unlike spending a while cleaning, filing for sharp edges on the electrodes, gapping, and reinstalling spark plugs. Had a really nice uniform pattern on the scope, as a reward. Those were the days! AND some of the rewards of knowing that you did a good job that made the car work better (even if just by a miniscule amount).
Enjoy!
CBODY67
