Mid RPM Stumble / Die

Have a brand new 914 on a Sling 4.  Everything has been installed/setup according to the Sling 4 manuals and the Rotax Manuals.  The engine has been sitting on the airplane for about 2 years while the airplane was finished.  Went to start it for the first time and it fired up right away, sounded great, smooth, good oil pressure, good temps, good fuel pressure.  Ran at low RPM but our tach signal is unreliable at the moment, so not 100% sure of the engine speed.  Went to advance the throttle to attempt a run-up to check ignition and as soon as the throttle advances beyond around 1/8th to 1/4 throttle, the engine RPM starts to come up and then around 2500 to 3000 (again, not certain but guessing) the engine starts to stumble really dramatically.  If the throttle is not reduced immediately, the engine will die.  It repeats exactly the same every time.  We've run it for about 30 minutes total trying to get it to come up above a high idle with no luck.

Upon shutdown we did have a significant backfire once.  This makes me suspicious of the fuel mixture/carbs/fuel pressure regulator.

Checked vacuum/manifold pressure lines to fuel pressure regulator, no problems there.  Have not taken the carbs apart yet, but before I dive into a brand new engine I wanted to hear some other opinions.

I'm thinking about just continuing to run good fuel through it and hope it's just a sticking regulator or a gunked up carb that will clear itself out.  Am I wasting my time?

Hoping for something simple.

Thanks!