Yes..... checking the plug is a must, so no matter what you do be sure to use the plug as your guide so you dont lean it out to much and cook your engine.
I have three of these carbs. and in all of them the main jet size was to large, I could put a #71 wire gauge drill bit through each of them, which was allowing my engines to run just a little rich, ( a little rich is a lot on these little motors I found out) so anyway I just soldered my jets up and redrilled them with a #73 wire gauge bit and SHAZAM.....! problem solved, they will still 4stroke a small amount when running with no load on them which like it was said earlier in this thread is normal, but when I crank the throttle my engines come alive and just take off, my bikes run WOT in the 36-37 MPH range and cruise real nice in the 28-30MPH range with a 41T sprocket, I've made some 50+ mile trips without a single snagg, they just squall, whine & Buzz...along like they should, so......... I said all this just to make the point that you may have over size jet in carb. that could be reduced just a little which may solve the to much 4stroking problem, it sounds like you were having. Just my experience and 2 cents, I fought my carbs for about 2 weeks before I did this and solved all my issues with them. Good luck, and get that mount fixed quick..... Fun is just waist-in away....!