But if it stumbles when quickly opening the throttle cannot be cured with mixture screw then the #38 idle jets would be the next logical step.
I gotta disagree. The 77 and 78 PD carbs were mandated to idle lean to pass emission's PPM hydrocarbon test at the exhaust. They were mandated so lean, that sudden throttle opening ALWAYS caused stumble as the mixture went way too lean with the sudden slide opening. Later models went to CV type carbs, in part to alleviate this, but for these mechanical slide carbs, an accelerator pump was the only way to get the enrichment needed for snap throttle response.
If the accelerator pump is working at proper capacity, the pilot jets can be completely plugged and the engine will still advance vigorously when the throttle is twisted. (Yes, I've tried it that way.
It won't stay at idle without choke, but if you catch it with a throttle twist, it will pick up. )
I've had it the other way, too. Pilots jets clear, but the pump working weakly. Idled great, but with throttle advance stumble. In my case, the mouse ear holes on the diaphragm had swelled almost shut. Drilled them to match the housing's holes size and wow, nearly got shot in the eye.
This bike has the stock F muffler, and a K&N filter in the stock air box. Stock jetting and carb settings, of course.
Cheers,