Speaking of things clean how is the air filter dirtyy filters are the most common cause of rough idle on most engines because of the age of this machine I would also check the inlet manifolds they may be perrished & sucking air this can be checked by spraying wd40 onto the mannifolds while the motor is running if the engine speed increases you have found the cause of the problem of to the dealers for a set of new mannifold rubbers
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Speaking of things clean how is the air filter dirtyy filters are the most common cause of rough idle on most engines because of the age of this machine I would also check the inlet manifolds they may be perrished & sucking air this can be checked by spraying wd40 onto the mannifolds while the motor is running if the engine speed increases you have found the cause of the problem of to the dealers for a set of new mannifold rubbers