I have an MVVS 2.15 gasser and I was going throught the same thing. I tried everything and everything that I thought would resolve the problem. I even replaced the darn carb. But still no improvement. To cut it short I pretty much did everything you did. Heck I even tried changing out the spark plugs to NGKs, champion, and other types of spark plugs that would fit. Non worked. Then I went out and got a brisk spark plug (UR17 to be exact, like the original). And guess what? Problem solve. Hope this helps.