4 stroking eh? What color is the plug? All the above speculation is just that, until you check the basics. Like a plug chop. 4 stroking is almost always rich not lean. First off VM20 is way too much carb for a stock case making tuning problematic. I have had best results on not matched kits with a 15mm. But you need to know which direction to go with jetting and the plug should speak to you