Wild guess, but you might have to press the button on your keyboard where the plus key would be on an American keyboard. Try the key next to backspace (and minus would then be the key to the left of that).

If that doesn't work, set keyboardlayout=none in the dosbox config and try again.

I tried every key on my keyboard and none increase the throttle. I'm not sure if the keyboard layout thing helps as I doubt I'll be able to bind a regular "+" on the keyboard?? It's worth a try once I get home though, do I just type keyboardlayout=none on dosbox?

If anyone knows how to bind + on dosbox it would be much appreciated. As said, the keypad + "[+]" doesn't work in this game. (regular "-" works but keypad - "[-]" doesn't)

Do you have a numpad?

Ah sorry, I meant to say the numpad + doesn't work in this game.

Any way to bind a regular +?

Ok solved it. Had to take 'mod3' off from the button.