built ka has cold stumble/miss

[struckinc]

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i have a problem with my new engine build.. when started cold it idles ok but when you try to give it gas it has what feels like a miss or a stumble.. i thought it was timing but timing is dead on 20 deg. idle is set at 775. it also has a brief pop through the intake when accel when cold. after it warms up it runs great..very strong. i changed the coolant sensor thinking it may be running too rich when cold but it didnt change anything. i have rechecked timing. no codes stored in system. now the engine is a ka bored 40 over, 10.5:1 pistons, port and polish head, no egr/emmissions, fuel pressure reg. set at 43 psi, walbro 255lph fp, intake ported, underdrive pulley, clutch fan removed, throttle body coolant lines removed, pacesetter header with 2.5" exhaust, stock cams for now, 170deg thermostat. this is driving me crazy so someone please help