I'ld say that the tunning was out 'cause you are getting to 270 with out the use of full throttle. What i'ld personally do in that situation is set all the needle back to factory and start again.

how far was the throttle opening?

When i had the msn set wrong and was having problems trying to get it tuned right what it was doing was comming out of corner fine then at about mid revs it was bogging as if it was rich then it would finally get going again. I stuffed around for ages trying to get it going right and found that there was a real fine line between it going alright and it just getting to hot. I've had to of these motors and never had them hotter then about 230 except when the msn was wrong. the first one i had said that the msn should be one turn in from flush (and i had my problems when i set it flush after cleaning the carby) and the new one says flush with the case. I've always found this motor runs better when the msn is between a 1/4 and 1/2 a turn in from flush and the other needle tuned to suit.

but once again i'ld set all needles back to factory and go from there.