i had a similar problem when i got my tank for my 86t3...i used some superglue around the nut and let it seep into the plastic around the nut area....then after letting it set up for a night, i soaked it with some pb blaster, let that soak a few hours and gave it another try, came out witha little coaxing. see if it will work for you.
1985 KXT 250 Tecate - kx cylinder, kx ignition, bassani headpipe, answer silencer, v-force reeds, 38mm airstyker carb, +2 franks swinger, raptor rear shock.
1996 YSF240 Blaster - maxxed out!!!
Life's journey is not to arrive at the grave safely in a well preserved body, but rather to skid in sideways, totally worn out, shouting "HOLY SH-T, what a ride!"