Teleconverter question

[jeffde]

I have Canon 7D camera
I have a 1.4 and 2 x Kenko Teleconverter and when i put them on the 70-200 f2.8 i loose 1 stop and 2 stops as i knew i would. (i've had them several years)

I put the 24-105 F4 with the converter on the other day (first time) (i'm going on a cruise next month and want to keep the weight down and am thinking of leaving the 70-200 behind)

but i didn't loose anything in f stops - i still get F4 with both converters - the focussing slows but i still get maximum aperture.

But i don't know why ?? any suggestions ?