Regardless of the colour as soon as I get a mower the plastic needle goes straight in the bin, I don't even attempt to see if they work or not. MF I think it is the black ones that are the shortest, this is why you have to be careful with the one length viton ones, the primer caps vary and on some the float comes up too high and hits the body of the primer cap and won't shut the needle off. I have to visibly check every one up against the light to make sure it works properly. Even a new primer cap and needle with an old float can have the same result. I have to mix and match to get a set that works and even then they can still fail and not always straight away, sometimes a week after fitting a new set they can start spewing fuel out the primer bulb
