If you are still using the stock muffler, I'd grab a 150 main jet.  That shouldn't affect the backfiring but could give a bit more top end power.  If it's too rich, go back to the 147.5.  If you go to the K7N filter, you'll definitely want the larger main jet (maybe even a 152.5 but I think the 150 will do it).  Remember to start the tuning from scratch when you change the air filter type.

Tomv wrote on 12/31/69 at 16:00:06:

A K&N will make your bike leaner. Leave the 1/2 spacer in place. You need the richness to compensate for the freer flowing filter.