Don't worry folks, I've fixed it
I forgot how hard the game is !
It turns out the issue was with the key mapping, so I couldn't actually press fire to start a new race.
I disabled the 2nd joystick and set the 1st joystick to keyset 3, which should have been the arrow keys. Anyway, it strangely works with the num pad keys and right ctrl.
Another question though, say I had a tape image with a few games/demos on, but don't want to play the first game, I want to skip it and load the next game on the tape image, how do I do it ?