Aerodynamic forces come into play and make progress above 100mph really slow. The engine is under a lot of stress too. I've seen engines blow when pushed flat out for too long. I'm presuming you'd dropped to 4th gear for this off road speed test
Twin spark engines will need 8 plugs. It is a clever system, they fire one after the other on the power stroke increasing compression as the flame front travels across the combustion chamber. The second spark fires slightly later right in the extra compressed mix. This ensures a much cleaner burn as there is less unburnt fuel in the exhaust.