Bonk, sommetimes blowing out the jets isn´t enough. You might have to unclog the air passages too. Another possibility is that some of the fuel stabilizers can dilute the volatility of the gas enough that it won´t ignite, especially if you just dumped some in, and got the mixture too thick. You might have to drain the tank through the petcocks and balance tube, and don´t forget the fuel bowls. You´ll run out of battery long before you use up the bad fuel in them. Even if this is the problem, you might still have a problem with roughness in the idle/off idle and high speed circuits, but you´ll know where it is then. I think there are specific sections in the Haynes and Clymer manuals on this.