I think I've got req_fuel set up correctly. I used this equation:
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REQ_FUEL*10 = 36,000,000 * CID * AIRDEN(100kPA, 70°F)/(NCYL*AFR*INJFLOW ) * 1/DIVIDE_PULSE
from the megamanual and got 16.8ms for a 2L engine with 19lb/hr injectors. With that pulsewidth there's already a hard limit at 7143 RPM where the duty cycle would be 100%. Then, as you pointed out, the pulsewidth increases with engine speed (it looks like it goes up to about 23ms at high RPM) which puts that 100% duty cycle limit at 5217 RPM. I guess the limited resolution on my scope makes it look like that limit is earlier than it is.
Let me know if I messed something up in that req_fuel calculation. I really don't know if 16ms is a reasonable number, I'm just abusing the equation and I really have no way to reality check myself on it. Is 16ms a reasonable req_fuel time or am I an order of magnitude off or something?
It's actually really good to know that this is a logical problem to have with this setup and it's not a problem with my MS. Thanks for pointing it out to me!