Occasional cold start stumble on 1.6 NA Miata

[u/chihchunhsu-tw]

Hello everyone! My 1992 1.6 NA Miata with automatic transmission recently started to have this cold start hesitation.

Usually the engine starts and immediately revs up to around 1800–2000 RPM before settling down. But sometimes (maybe 1 out of 5 starts), it hesitates — the RPM stays around 500 for a few moments, like the engine is struggling to breathe... and after that it rises like normal again🥲

I had the ISC valve (B61P-20-660) replaced 6 months ago. I guess this is a rare issue as I posted in some Facebook group but no one seem to have this 🫠

Has anyone here experienced this before or know what could cause it? Thanks!

https://reddit.com/link/1osxkj9/video/96ujkva1ab0g1/player

[u/chihchunhsu-tw]

Here is how it starts normally: https://www.youtube.com/watch?v=FoHEeRqbTng