Regarding my 1982 Volvo 240 B21F9-CI, NO emissions are very high on the low speed 15 mph test, but nearly zero on the high speed 25 mph test after installing a new cat converter. I'm using 91 octane plus a few gallons of E85. All other emissions are very low. How can I reduce the low speed NO emissions?

 

High NOs indicates high combustion temps - which is usually indicative of a lean condition or an overly advanced spark. I'd be looking for intake air leaks as I don't think the ignition timing is adjustable in your car. Given you say the NOs are high at low RPMs but come into spec at high RPMs, then an air leak (tubing, idle air valve etc) would make sense to me as the wider the throttle, the less impact a smallish leak would have.