BRIAN - a 1650 had 23-10.50X12 rear tires, turf tires were stock.
ROB - All hydro's whine to a certain extent. Changing the oil & filter sounds like a good idea, it was probably neglected for a long time.
The Onan engine has it's own set of problems. The intake manifold is two pieces, a "Channel" to connect the carb to the two intake ports, and the cover. They tend to develop vacuum leaks. You can remove the top and re-attach but I can't give you any tips on that, I've never had to do that. The governor on the Onan's is a pretty poor design IMO. The centrifugal weights are five ball bearings spinning in a ten finger plastic wheel that's pressed onto a cast iron hub on the cam drive gear. Plastic expands with heat FIVE times as much as iron. So when the engine gets hot the governor wheel slips, which causes the governor to allow the engine RPM to drop then surge. The manual says to replace the cam driven gear which includes the new governor, which requires the crankshaft gear to be replaced which requires complete engine disassembly. You're looking at over $600 for just parts alone. Some people have used Lock-Tight stud & bearing locker to attach the finger wheel to the cam gear, others have used a plastic piece extending into a notch on the gear to positively drive the finger wheel. I haven't fixed mine yet, so I run it with one hand on the throttle most of the time, I don't want the engine to race, the conn rods in the Onan are somewhat weak.
I fought poor running ALL last summer on my 982. New plugs, new fuel pump, carb rebuild, set the points, nothing helped. I finally ordered new points & condensor and installed them one afternoon and when I started the engine up I expected the same old ratty running I was used to. The new condensor did the trick.
You'll find that parts for an Onan will make you appreciate the Kohler engines even more. Onan has been bought & sold so many times that the parts for them are expensive. Some parts are cheap, I've seen complete carb's for them for around $100, but they don't fit the Onan in the 982. Think I paid $40 for the rebuild kit. The fuel pump I bought 9-10 yrs ago and paid over $100. Points & condesor cost around $40 plus shipping.