People the above portable unit is powered by a briggs & stratton engine model # 257412.
Ever since the unit was new this problem presisted, despite being to the supplier a few times ( my dad got fed up with them after seeing no results ... but that is history now ).
The initial problem was that the exhaust manfold used to get red hot after about 1 hour of runnning and the speed would start to change from high to low but the unit would not stall.
( the unit is rated 8000W and was used to power 10 twin tube 10feet flourscent lamps and 4 computers with LCD displays. The computers ran on UPS for when switchover to generator is done ).
It was sent back under warranty and all I see was done is that they drill a hole in the gas cap. This issue with red exhaust disappeared but the varying speed still comes.
This evening I ran it and after approx 30 mins the speed issue returned.
Correct plug was replaced and regapped according to specs, fuel filter replaced, air filter replaced,all throttle/ choke linkages lubricated and checked for free movement.
Oil replaced with specified body.
The unit gives no problem to start , one pull and it starts but after a while the speed problem returns resulting in a severe variation in output voltage. (120 v to 90 v).
Removing the loads results in same problem.
People who repair these units any suggestions.