I did something like that on my motorcycle... I over oiled the K&N air filter, instant plug fouling and it barely idled, giving it gas would make the engine bog real bad. I removed and re cleaned/oiled the filter and it still ran crappy. My spark plugs were blackened and fouled to the point that even with a clean filter it wouldn't run. Replaced the fouled plugs and fixed it right up. Have you replaced the spark plugs?