The cutaway numbers are richer the lower the number. The 5.5 is richer than the 7, just another tuning option, I run the 7 on my 39 and 38a.s.
The 38as being smaller will give you higher velocity (Airspeed) through the carb.
I primarily run singletrack through the mountains, old skidder / two track jeep and forest service roads, 3000 to 5500 ft or so elevation. I've been running the 38a.s. for quite a while, no complaints. I saw an increase in throttle response (good for where I ride).
All the guts in the 38 will swap with your 39 if you decide to try one, slides jets etc.
I have gotten the Chineseium clone for a buddy who couldn't afford the real Keihin 38as for an old KX250 . we swapped out the guts from a real Keihin and has been working the last two years.
I swapped out carb my CR500 and liked it, so when I got the KX it worked just as well. (I shoulda never sold that thing.. oh well)
The jetting on the 38 will be leaner than the 39 due to the increase in velocity.
I have a 52 pilot in the 39 and the 38 likes a 40. (just for comparison)
I put the 39 on for my kid when he went to the sand dunes last fall and he said it ran great. There were very few drag races he lost in the sand and there were some high dollar buggies and sand bikes running against a basically stock 20yr old motocross bike.
Theoretically you should be able to get more top end out of the 39, More air/fuel, You can also get a 41mm pwk but the way its done is kinda hokey . Sandblaster has been testing lots of combinations and found the 38 made more power for him. (I'm thinking improved filling of the cyl. but that's just conjecture on my part)
He has lots of info here and on his site KXguru.
If you have some discretionary funds and like to tinker with your stuff. (that doesnt sound right) give it a whirl. Start out on the fat side with the jetting and you won't hurt anything. If you have plenty of discretionary funds, look into Lectron Carb....
As an aside, I'm wondering if the carb (that does not run well) you currently have is not "worn" out for lack of a better term.
There is a small block in the carb your not supposed to tinker with, (tamperproof heads) that has two "o"rings and a "D" shaped ring that are notorious for giving the tuner fits when you can't get the bike to run right and everything else checks out, along with slide being worn etc.
So many options... Good luck!
Tuck\o/
