I currently have a Pro-Circuit pipe with a slightly higher flow foam filter, the stock intake screen that is on the cone under the filter is cut off, and it is jetted for having the airbox lid off. I am upgrading my intake to a High Flow K&N with the wider flange airbox adaptor. So, I will still have the airbox, but should have better air flow. Do I need to re-jet the quad after I swap the intakes? Any help is appreciated.

What quad?

Generally speaking yes you need to rejet. You are letting more air in, so you need more fuel. In sensitive quads like the YFZ, rejetting is a must for it to run perfectly.

Man, it doesn't matter what kind of pipe you run. Or what intake. You should always check your jetting at the begining of every season. OR if you wan to be anal, at start of evey trip to change for different conditions. Anyway, there is no "jetting for having the air box lid taken off." If you change something to the combustion cycle, check your jetting. You might not have to change it, but then again, you might. Oh by the way, what kind of quad is it?

This is on a 2004 YFZ450

What jets are you currently running?

I honestly don't know. I bought the bike used with the pipe, altered intake and re-jetted. Now I want to switch to the K&N intake, but am not sure what all adjustments I will have to make to the carb.

So, you have a full pipe, and an intake? I would jet 172 main, 45 pilot, ncvq needle on the 4th clip, and try 1.5 turns on the screw.

Actually, I have the slip on Pro-Circuit T-4 pipe (stock header), with the screen cut out of the intake, a better foam filter and no airbox lid, and it was rejetted for this setup. I now am adding the K&N wide flange filter with the airbox adaptor (so I will retain the stock airbox, only with the larger opening). Just in case it matters, I will be mostly riding at Glamis, which the altitude is probably about 1000 ft.

Try the jetting I recommended above. That should get you very close.

Thanks, 24V. Much appreciated.