mikemnh,,, Nogyro is right,,, every "new" gas model plane motor I have been around has spit this black goo !! even my MVVS 35, my friends BME 105 spit really bad for like 5, 6 gallons,,, I don't think your MVVS will spit goo as long as that BME because your MVVS was made differently,,,

if your motor is rich and it stops spitting, it will spit more when you lean it, then after a few flights it will stop spitting, then you lean it more and it will spit more, as it brakes-in more and more it will stop spitting,,,

a MVVS thing,,,
MVVS should tell there customers but they don't,,, when you got your MVVS did you look in-side at the black piston ?? your MVVS piston is coated with graphite, this graphite will come off and be in the black goo, and it will get in the spark plug, so you will need to clean your plug after a gallon or so (make sure to check the plug if you get a flame-out), as I found-out when I removed my plug to look at it, the graphite will chunk-up in the muffler too.

Jim