I'm not clear on what the question is, aussietrev. If you connect a Bowden cable directly to the throttle butterfly, you will be able to operate the throttle like on an old carburetor-style car engine, but like on the car engine, if you open the throttle wide with insufficient load, it will either valve-bounce or blow the engine. In the case of race engines you usually stiffen the valve springs, which makes it quite sure it will blow, not valve-bounce.
Preparers of mower racing engines usually accept that they won't last long, and make a series of design changes aimed partly at increasing the output power, and partly at holding them together for the time it takes to complete a race. Then they get back on the spanners and rebuild them for the next race.
I can remember Jack Brabham being interviewed on TV, right after his engine blew in a Formula Libre Cooper mid-race at Sandown, when he was in the lead. The interviewer commiserated with him over his bad luck. Jack said it wasn't luck, he'd missed gears twice in the race, and the tell-tale on the tachometer was pegged 500 rpm above the red-line, so there was every chance it would blow, and it did.
