Those symptoms are more akin to secondary spark. Plug wires, leakage, cracked insulation. If it were a 1200, I'd say you were lugging it. But the 1500 should pull decently even at those rpm's. Connect a timing light to the plug wires and see if the signal gets erratic or goes away.
Ah!!! Interesting that I never performed that test. So you think perhaps the spark could be jumping from one wire to the next? Wouldn't this also be felt at higher RPM's? Also, I know the bike has never had the timing belts replaced. I wonder if that might be it.
Here is another symptom I've noticed. When I replaced the IASA valve, there are three hoses attached to it; one side has a large vacuum hose with plenty of pull; the other large hose pulses. The 3rd hose is a very small vacuum hose. I believe constant vacuum from the small hose is applied to the valve keeping it closed. But when you snap the throttle closed, the momentary drop in vacuum pressure moves the switch in the open position, thereby allowing exhaust gases to temporarily flow back into the left and right intake manifolds; this allows for a reduction of fuel delivery from the carbs to high RPM deceleration, and also reduces the chances of rear wheel lockup. Once you come to an idle or stop, the IASA valve closes and the engine then receives fuel from the carbs.
Is it normal for the large lower left hose (as indicated on the pic) to pulse the way it does? Meaning neither vacuum nor push is happening from the lower right hose of the ISAS valve....refer to the picture for reference, although the valve depicted in this picture is facing backwards in relation to its actual position when installed. In the bike (installed), the single sided port faces left, and the double ports (small and large) would actually be on the right.
The small vacuum port allows for opening and closing of the valve. The single-port on the left side (in the picture) is the vacuum flow, whereas the large port on the right side is the hose in question that pulses yet neither pushing nor pulling air; just pulses. Is this normal?
I just replaced this valve 2 days ago, and the bikes pulls much harder. However, I'm still jerking below 2,000 RPM under 1/2 to full throttle load.