You can try GM top end cleaner. Have to remove the plugs and put a few spoonfuls in then let it sit overnight. Afterwards crank the engine so it spits it all out. Can help with carbon deposits and sticky rings if that's the case.
https://www.amazon.com/gp/aw/d/B0013...rb_top?ie=UTF8
Just came across this:
https://m.eeuroparts.com/Parts/46712...xoCrykQAvD_BwE