2000 600 XC SP edge chassis conversion

I took a 2000 XC SP 600 VES edge chassis and put a 700 non VES motor in it. The 700 motor runs but about 6200 rpms on the stand. Some back firing also at lower rpm and when pulling the starter rope. The guy I got it from was trying to put a 700 motor in it he never got around to accomplishing that task .The pipe that was with the chassis I believe it's the original 600 pipe so I beleve this may be the problem of why I cannot get RPMs or stop it from backfiring I thought it might be the carbs that were in it they were a set of of keihins so I switch them out with another set I have that were originally on the motor that I put in there.this came back with the same result I was wondering if there may be some type of wiring that I'm not catching on to maybe a voltage regulator somewhere any help would be greatly appreciated
Motor I put in is an 870-70-1770A2 what year is that???? Block stamp #0073562