I've had real bad carb ice two times in a C-150 on perfectly clear
days. Both times the conditions were similar to yours. One time at
6500 feet; one time at 8500; about 70 degrees on the ground and about
42-44 at my cruising level each time, and also in the south. I got
detailed weather information from Shaw AFB WX after I got back each
time which showed relative humidity at my cruising altitude was just
over fifty percent on both occasions, despite the beautiful clear skies
and great visibility. This is when carb ice sneaks up on you and hits
you hard, leaving you shocked and in disbelief, trying other solutions
and cursing your mechanic.

One time the carb heat cured the situation very quickly, the other it
took quite a while and I had to keep adjusting mixture. When you have
carb ice the mixture changes as the ice builds up and again as it melts
away which sometimes has you "chasing" the engine with throttle and
mixture manipulations to keep it running.

I didn't read all the other posts and it may have been mentioned
already but it's important to get full throttle in right away when
dealing with carb ice also.

In 3000 hrs flying mostly small piston planes I've learned to
immediately pull carb heat full out and leave it out at any burble,
hesitation, slow down or silence! Once you've got the carb heat
engaged, go to full throttle if you haven't already and adjust mixture
for best power -- you may have to keep adjusting the mixture several
times as the shape of your venturi/level of blockage changes from ice
melting or, (GASP) growing because your carb heat is inop or
insufficient.

- Brett Justus, ATP, G/S MEI/CFII, ASC