Hideous question but I have a BB chevy that I am tuning up for a friend. Completely stock other then cosmetics. Spent a good amount of time getting the timing marks correctly index and going though the curve and distributor and well everything else. Vacuum looks good, tested and known good carb as well. Set the thing up 100% to book and the darn thing stumbles at idle and comes off idle terrible. Add in another 140rpm at idle and drop the timing by about 2.5 degrees off stock and it purrs like a kitten. So he is happy car is running great. I am not happy my career is built around building things to spec. So are modern fuels just to different to use the older factory spec's to the letter. Should I just assume the tuning is correct now as everything is running significantly better then before or am I missing some major issue with the motor that would let it run perfectly at factory spec. 99% of the stuff that I build has weird carbs and cams and all sort of other crap done to it so I use the book specs and go from there. IE better to tune to full book spec or tune by ear with the modern fuels.
if it stumbles off idle its either the pump shot is low ( didn't list carb type to adjust this ) , or the mixture is too lean ( use needles to adjust , turning up the idle is not the way to do it as it adjusts air and fuel ) when I worked with bikes back in the late 80's early 90's when e-10 started to show up the manufactuer ( jap bike name here) . they had us go up one number in size to help the lean condition that the ethanol made , and on a car carb often we had to drill out the squirter or change it if it was a holley or change the cam or bend linkage to get a quicker and bigger shot on others .
We have got to run that ethanol shit here in Jersey, it makes everything run leaner so I drilled out the jets a couple of numbers, also the idle mixture screws have some effect up to about 1/4 throttle....the old racer who taught me that called it " transition mode"
It's lean off idle. You didn't say what RPM it idles at. I would make it run right and forget about the book. But in Kalifornia...
Gimpy has the proper solution put a A/F ratio sniffer up it's ass and get it correct. I'm lucky I have a buddy with a smog machine, he re-jetted my super lean 40 Coupe 350/350/600 Holley (all new) to a perfect 13.6 between lean and rich, a little power and some mileage. Runs like a top. I always have my distributor dialed in at a local speed shops distributor machine before I install for vacuum and total advance. I believe he said he went up 3 jet sizes.
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