I have a JD x475 with a kawasaki FD671D-AS02 engine and when it runs at idle through mid range it pops back through carberator on left side only. Had it in the shop and they did not really know what the problem was they said possible a intake valve staying open. I have it apart but it does not appear to be sticking. Any suggestions on what could be wrong.

Not sure why the shop stopped their look at the issue. If it was a sticking valve they could have confirmed it with a compression test. That way the shop makes more money. What kind of shop was this? The sticking valve makes sense and may only stick when the engine is warm. Since its torn down might as well replace the valve guides but I have not worked on this engine so it might not be fun unless you are a skilled mechanic.

Him
The water-cooled Kawasakis were very picky about clogging air filters and fouling the plug on one side. That is where I would start on a FD engine.
Peace,
Clarke