Odd... Regardless of whether the object is a weapon or an engine, especially if it's a weapon, a button press should always result in a reaction. Did we ever put speed boosts in Kartman? I would assume they were set up a similar way to what we're trying to achieve if that's the case.
If I'm wrong, then perhaps weapon objects can only accept weapon inputs?
Maybe a vehicle is only limited to 2 weapon controls. I'm not really sure.
Perhaps try using a really beefed up plane engine for main thrust instead of nitro, and keep just the left/right nitro?