400 ex is more cc's so need more fuel but not much. if anything would make 350x run rich. the jets have a # stamped on them. see what the 400 carb has and 350x carb. i would say toss it on and ride it for a while and then check your plug. if heavil sooted go with 350x jet or something in between. i imagine 400 carb is bigger (flow more air) so would think 350x jet would be lean. better to be rich than lean.
Trikes: All Honda: 84 250r, 85 86 and 87 250sx, 84 and 85 125m, 79 82 84 and 85 110's, 73 us 90 and 77 atc90, 75 atc70
Cycles: Honda: 71 cb100, 78 cb750 coming soon cb750 trike. Yamaha: 67 yds 3 (250cc, 2cyl, 2 stroke), 82 750 SECA
Other toys: Chenoweth VW sandrail dune buggy, old race go kart, racing mower, 76 Arctic Cat Pantera 440 snowmobile
KEEP IT ON ALL 3 OR AT LEAST 2 AT A TIME