@stevepax Hi, I have the same setup as you so I am hoping I can help.
There are two keys that I know of in MAME that could be doing what you suggest, "F10" which toggles throttle on and off, and "insert" which throttles when it is held down. As mitu suggests try pressing tab to get into MAME menu and check the Input (General) and see what the throttle key is assigned to.
Setting up the controls for Street Fighter 2 and other similar six buttons games took a little trial on error (as least it did for me!). If it helps I have my buttons set up like this:
Player one
Top row from left to right
LP - lctrl (P1 button 2)
MP - lalt (P1 button 3)
HP - space (P1 button 5)
Bottom row from left to right
LK - lshift (P1 button 1)
MK - z (P1 button 4)
HK - x (P1 button 6)
I set the keys up from within MAME using Input (this game).
Hope that helps