It really is process of illimination.
1, Remove the safety cut out wire from right hand side of the engine, this will bypass all safety cut outs, if engine runs ok then you need to go through all the safety cut outs on the machine one by one to see which one is bad. (check the one in the vac tank first)
2, Check / replace air filter (even a clean looking air filter may be restricting air)
3, Check / replace spark plugs. Spark plugs these days are pre-set.
4, Check / replace fuel filter (even a clean looking fuel filter can restrict fuel flow)
5, Remove belts from engine to vac blower. If engine runs ok then there is a restriction in vac blower (maybe burnt foam on the lobes) or the pre heater under the vac blower is clogged....remove pre heater, clean then replace
6, Unbolt the exhaust from the engine, if engine runs ok then there is a restriction in the heat exchanger.
All of the above are the easiest and cheapest ways to do a self diagnosis. If you do all of the above and the engine still running rough then you will need to contact a professional.
Hope this helps.
Richie.