J.W. we don't have the LS400 in America, just the LS650, we have a 40mm carb stock jets being 145 main and 52.5 pilot . That being said ,if all you changed is the filter and your running a stock muffler,I wouldn't think there would  be a  the need to change jets ,if the jets you have are stock .You probably run a smaller carb on the 400 so the 70 pilot sounds very large to me. The motor running badly may be caused by it being dirty, a bad petcock ,old fuel line ,bad float valve /float level , etc. You might for a few dollars try the next larger main jet (127.5) but I'm just guessing.What condition is your exhaust system in ? the bike has to breath in and out.

If your changing mufflers ,and the new one is less restrictive then moving to larger than stock jets may be needed.