1950 Pontiac straight 8 Rochester 2-bbl carb, what initial setting should I use to adjust the two idle mixture screws? Also, in what order should I set the timing, idle mixture and idle speed? And, if it's not too much trouble can you tell me how to adjust the automatic choke?
Adjust screws out 1 1/2 turns Adjust timing with vacuum advance disonnected. Reconnest vacuum adv., adjust mixture screw until highes vacuum reading achieved on a gauge. adjust idle speed. If you have to change it much, check mixture screw adjustment again. Buy a shop manual... or look one up online. Too many variables on setting the choke to make a guess, but as a general rule, with the throttle open a little and engine off and cold adjust choke just until choke blade just closes. That should get close enough to get it in the ballpark. Did I mention getting a shop manual?
Basically what grey says, only make sure...that the engine is warm, and go back over the carburetor speed and mixture again (a second time). No need to readjust the ignition timing after warming the engine. Why, the engine will be running different warm than it does when it's cold. Also, if you don't have a vacuum gauge, no big deal, adjust the mixture screws (do this at least TWO times) to attain the highest rpm, realizing that if there is a large difference, that you "may" have to lower the idle speed to get the system back in the proper flow of things. Mike P.s. - If the transmission is an automatic, I "always" complete the mixture and speed adjustments WITH the trans. in gear (with someone holding the brakes). Why, you don't drive the car with the engine in...neutral !
The only thing I can say to that is in the past 58 years of adjusting idle mixture screws I have never once disconnected the vacuum advance while doing it. Not sure were that came from but I learned from and worked for and with some of the best in the business and have never upon never heard that one. Start by VERY GENTLY screwing the mixture screws in until they stop and backing them out 1-1/2 turns to stop is the standard procedure I learned, did and taught when I was teaching. That is the starting point and then you go from there to get it perfect. I say very gently because I have seen some screws that look like someone ran them down with an impact wrench before backing them off. It's just until they stop with two finger contact on the screwdriver and no further. A vacuum gauge is a great thing to have in your tune up kit along with a dwell meter if you have points. Plus of course a timing light.
The only thing that hasn't been mentioned is, what is the general shape of the carb. Has it been rebuilt? I ask because proper float level adjustment could be an issue as well along with just needing a general refresh. GET A SHOP MANUAL
Rochester??? Which one? Original was Carter type WCD. If it really is a Carter then: (1) freshly rebuilt engine - 1 1/8 turns (2) engine broken in, in excellent condition - 7/8 turn (3) engine with 4 million miles needing overhaul - 1 1/4 turns Maximum 1 1/4 turns If Rochester, and reasonably correct for the engine: Check the taper on the idle screws. If short taper (pre-1968), use the adjustments above. If long taper (1968 and newer) (1) 2 1/2 turns, (2) 1 1/2 turns (3) 3 1/2 turns. As to sequence: Dwell is always first. Then idle mixture, timing, and hot idle speed. To set the choke: Cold engine, ambient temperature from 65 to 70 degrees F. (if not in this range, wait). Loosen the screws holding the choke cover. With one hand, move the throttle to WOT, release, then to half throttle and hold With your other hand, rotate the choke cover in the "lean" direction until the plate hangs open. Now rotate in the "rich" direction until the plate just touches closed with zero tension. With your third hand tighten the cover retaining screws. Release the throttle you have been holding 1/2 open. Cold fast idle and hot idle may need re-adjusting, check them. Jon.
When adjusting the idle mixture screws, after achieving highest vacuum reading, or idle speed; I always finish with turning the screws In to achieve about a 20 rpm drop in speed, or a very slight drop in vacuum reading. This is the "Lean Roll", and will net you a very crisp throttle. And don't be afraid to do the whole process over again on both screws, maybe a couple of times, to really dial it in. The time you take doing this will result in a smooth idle and crisp clear acceleration off idle. If you're paying attention you'll notice it. On the choke, you may have to adjust it a couple of times. The first time you set it, it may be real close, but maybe not quite right, maybe the setting is too lose and the engine is hard to start, so you have to tighten it up a little bit. Or maybe it's a little too tight and the choke stays on too long causing black exhaust and rough running. But once the engine and the choke spring are warmed up you have to wait until it's all cooled back down to ambient temperature again, and that needs to be <65* (preferably a lot less than that). I like to mark the cover with something, like white paint, or "White Out" at the initial setting, then make fine adjustments using that as a guide.
On the WCD do not use the vacuum takeoff for the ignition timing as your vacuum source, as it has an idle cutoff. Disconnect the elbow on the manifold Bernie the carb that goes to the vacuum pump for the wipers and pull from there, through a restrictor if necessary to stop the needle dancing. Phil
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