So I'm trying to tune my VE table, and while I'm cruising, if I let off the throttle, the spark advance drops to 0, then back up to whatever, and causes this surge. I tried adding fuel to the 800 rpm, 50 kpa cell in the VE table which used to be in the 70's. That made the surge less severe, but I still have no idea why the spark advance drops to 0.
surge.hpl
Step 26.hpt