also being that carb i would say that engine has been run very lean unless the jet was adjusted to suit 160cc
Not really those carbys went on a lot of different mowers.It would just have to have come off the right mower to run a 160.In-fact I have a couple here that came off a Scott Bonnar and they were 155cc engines.I reckon that could easily run a 160cc engine.Yes it's not hard at all to change from a G4 to a G3,but it is impossible to change from a G3 to a G4.Unless you change the cylinder to one that had a G4 on it.
