Too big it will sputter as I tested a few jets on my build . How many MM is your carb ? Did it have the variator washer which limits the speed removed ? I also replaced the 40 idle jet with a 45 IJ in my 19mm TK carb and it really improved starting and idle . I bought some jets here since I could not find a jet kit at that time for my carb
http://www.jetsrus.com/a_jets_by_car...11-22_main.htm Remove the sparkplug and compare it to this chart
http://www.4secondsflat.com/plug_chart.html If you change out the jet again get a new sparkplug , run it at WOT while riding and let it hit max RPM then hit the kill switch and inspect the spark plug color again to see how the new jetting is .
I replaced the 68 MJ with a 75 and it ran great when it was new and still a 50 . I bought a Uni vent cover which is smaller than the big red Uni airfilter and went with a 90 MJ . It ran great as bottom end and midrange were better with the same top end . Well I later bought the Uni airfilter and my dealer and no other shop in town had anything in my jet size bigger than a 90 . I started it up and it only had a slight bog but nothing like it was going to stall . Being impatient and anxious I wanted only to ride around the block to test it . Well dang it I had a seize and it melted the prong off of the sparkplug . I have never had this happen with all of my many years of riding and modding two wheelers .
When I put on the 70cc BBK I also put on a PM Racing pipe and first tried the small Uni vent cover it sputtered so bad I could not ride it with a 140 . I put on the proper Uni it was better . I tried a 130 it sputtered but not as much . A 120 MJ it was great . The PM Racing pipe was not restricted like the stock pipe so I lost backpressure and takeoff power but midrange and top end it was beasty . With a 2T you always want to run just a hair rich rather than be dead on it to avoid a soft seize . When my 50 seized it fried it . I did have a mild soft seize with the BBK while testing a 105 MJ but after a few minutes it fired up and I rode it home while being easy on it . I also went from running a NGK BRP(7)HIX to the (8) version which is a cooler heat range .
You may get a gain by replacing the intake with a Hoca intake with carbon fiber reeds . It should have a bigger venturi , CF reeds respond better than the stock metal ones which have been known to break and eat up the soft piston .
I put in Hoca 1.5k clutch springs so the clutch engages at 1.5k higher RPMs than the stock springs to help on take off . It came with 6g rollers and I tried 4g but they were too light so I mixed them to equal 5g when it was a 50 cc . With the BBK I could redline while going up a tall and steep hill . I put in 8g DR Pulley sliders since I loved how they performed in my Echarm and I wanted heavier weight to try to keep from over revving . Well takeoff was less , midrange just a hair less but top end was faster ( only registers 50MPH and it goes beyond the gauge ) but still would scream when I went above 50 MPH even uphill with a running start . So I will have to get taller gears and lighter sliders . I also put in a Hoca BB torque spring base which has BBs that prevent the torque spring from binding and it seems like I have more " gear shifts " and the shifts are more smoother too .
http://www.partsforscooters.com/169-...category=74117 I tried to put on a Hoca 1.5k torque spring but I could not get it to compress enough by hand so I reused the stock one