Just picked up a new TRX400X and I have a Lexx mxe slip on coming in a few days. The guy at the dealership said just to swap the stock 148 main jet for a 150 and thats it. Does that sound about right? Elevation is 0

 

What ever jet you use check the color of the spark plug after a high speed run. Needs to be a light to medium tan color on the electrode. If it's whitish color,need to bump the main jet up. I always started out with the stock jet to see how it ran,plus checked the plug.You may or may not have to bump the jetting up. Not all after market exhausts require rejetting in a lot of cases. It's when you open up the air intake with less restrictive filters and the exhaust is changed is when changes in jetting are most needed.

 

OPT gave you some pretty good advise. I'm going to add to it. The way you check the plug is start the ex. Let it warm up. Then take it for a short rip. In 2nd or 3rd gear get it up to 3/4 throttle and shut it off while its at 3/4 throttle. Let it cool down then pull the plug. You want the plug to be a cardboard brown in color. If the plug is white you are lean and need bigger jets. If its black you are rich and need smaller jets.