Wow, what a beautiful Tigercat!
Luckily I am not experiencing near the difficulty that you are describing, otherwise my knees really would be knocking!!!
Both of my engines are running smooth- no sputtering or hesitation. But from the experience running it up and down the runway, its important that the throttle be very gradually advanced. Until you build up some speed, if you "goose it" even just a little the stronger engine can try to throw the plane off track before the other engine catches up. I get this same effect on my Twin-Air with the tuned mufflers so its not a new experience, but it seems that the P-61 with the two-blade props is going to take "overpowered twin" to a new level for me. I hope the airframe is good and strong on this bird!
For some reason one of my engines is running incredibly strong even for a YS. Its turning the APC 15x8 just shy of 10,000 RPM and throttle response is near instantaneous. The other is a few hundred RPM weaker all the way through the range and just a tad slower to spool up. Its probably nothing that you would even notice on a single-engine plane, but with the two running side by side the differences are very obvious!
I compensated by dialing down the strong engine using the throttle curve which seemed to work fine. But I plan to tinker with the setup on the weaker engine to see if I can find the "sweet spot". It was interesting that the strong engine is throwing a lot more smoke when spooling up- opposite of what I would expect. I'll start by verifying that I'm getting full throttle, then tighten the valves a bit and maybe richen up the regulator. Just part of the fun!
Wednesday afternoon is still looking good! Of course I could just wait for my replacement 4-blade prop to arrive... naaahhh.
Walt