1984 Aspy idle and low speed miss

[royando]

1984 Aspy with 32,000 miles. Left front cylinder dead at idle up through mid throttle. Have done the following:
new plugs, good spark, no arching at night
good compression, cam timing is correct
carbs have been cleaned 3 times, synced, float bowl is full, 2 tanks of Seafoam
adjusting pilot screw makes no difference because cylinder is not firing
does run better with choke on and rpms above 2000
rpm picks up with carb cleaner sprayed into sync screw hole

I am sure it is the slow speed jet or the idle mixture circuit, but how do I check these circuits? Can I test the idle mixture circuit on the bike?
Now what do I do???