Its very important they you test all the basic mechanics of the tractor, so take time to test the steering, gears, forward and reverse, PTO, lift, lights, indicators and hydraulics.
You should have a detailed check list which included at a minimum the following: does it start easily from cold, run well when hot, all the electrical components work well, does it have good traction If possible you should go to your test armed with a toolbox containing a compression tester, expensive Fluke digital multimeter, hydrometer, and other specialty tools.
How easily does it start from cold? One key thing to watch out for is if the tractor is out and warmed up prior to your arrival, you lose an important checklist item, namely the cold start, because as we all know a warm engine starts much more easily then a cold one..
How well does it run when warm - Getting it hot is a must if you want to find out how it will work after you plowed the first row.
Are the brakes working well - Although the brakes are inexpensive to replace, they are inaccessible on many tractors and will require extensive teardown to get the new ones in.
Does it smoke - Blue smoke spells trouble like rings, pistons, or valve guides. White or black smoke can oftentimes be fixed with carburetion or ignition changes but still means work. The clunk should be more pronounced under load. Chattering noises from the pump while lifting indicate the pump is not getting enough supply of hydraulic fluid. The pump may have experienced excessive wear when run this way for long periods of time and may be about to fail.
