Further, on a c-accordion the F's are tuned sharper.

I have read about the F.
But also that some think that it doesn't have too.
On my accordeons(C and D) omly the B and the E are "out of tune", but it have to for Cajun.
On the D other notes are lower tuned, but the reeds under the same buttons.
Is it really necesserry to sharpen the F a little?
How much it have to be?
Does it change the sound very much?

Some builders sharp the F very slightly and others leave it standard. In early days of Louisiana builders and tuners it was a common practice to raise (sharp) the F (4 pull) until it was perfectly in tune with the A (5 pull). I find that the practice is not as widespread possibly because there is a bigger variety of performance keys (F for example) and a bigger variety of instruments that the accordion has to 'agree' with--chromatically tuned steel guitars and pianos. The players of the other instruments starting complaining about the F note sounding out-of-tune and being hard to play with so many modern builders/tuners no longer tune the F slightly sharp. Check with the builder if you are unsure...they can tune it either way.

Thanks for the information.
When we started to play Cajun, one of the members of my former band always complaning about the bass-side of the accordion.
Now we play traditional cajun with only guitar, fiddle, tit fer and accordion.
Is it better to tune the F higher now to get a traditional sound?

Thanks Jean...I appreciate it