I've had/have both 250 and 300 on both '18 and '21. I havn't had what can be called the best results with the 300 (7 seizures in less than 12 months) but things do now seem to be running fairly well.
Out of the 2 the 300 kit has worked best on the 18 (giving more power) than it is on the 21. However the fastest bike is still the 250 F3 barreled '18.
The 300 Kit as it comes is a great road bike, but like andy I really like to rev a bike and have a hard hitting top end. The 300 just doesnt really do this for me. Its great fun to ride, but I like the 250 more even though the 300 is now reving as good as the 250, It still just doesnt have the ht of a 250.
Power wise, I had over 71bhp out of my 300 '18 with stock carbs running a hrc jet kit comapired to just over 64bhp out of the 250 '18 running tyga pipes and stock needles with abit of a tweek on the old mains.
If i was building something trick (which I am) I would go 250 not 300. If you want something that gives you a whole load of middle with very linear power then the 300 is perfect.
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If I have to take the carbs off once more...