Check the appearance of the bolts of the block, also check for leaks or sweaty oil spots around the cylinder head and also look at the bottom of the block. Check the color of the oil and also check the oil level before you start. Is there perhaps white sludge on the oil that resembles mayonnaise? This can be due to many short trips, so that a lot of condensation remains in the block and a kind of mayonnaise forms on the oil. With a good long ride you can get this out. How old is the oil? Please check with the seller. Oil should preferably be changed around 6000km or every year. In case of the moto BMW this is important.
Start the engine.
Does it start decisively or with a lot of effort? Listen how the block runs, valves can sometimes be heard ticking, but the engine should not make strange noises. If this is your first motorbike, let that person you brought along with knowledge of motorcycles also listen carefully.
An air-cooled engine block makes a little more noise than a liquid-cooled engine block, the liquid dampens the sound. Look at the color of the smoke from the exhaust and if possible also during the test drive. White smoke is generally condensation and normal on a cold start. If it continues to smoke white, you may have a leaky head gasket and you are burning coolant. Blue is oil valve seals leak, black often indicates too richly tuned engine. Once at temperature you should not see smoke appear, so if you continue to see blue smoke, there may be something wrong with the valve seals or the pistons and valve springs may have had the best time.
Look at the exhaust from front to back and check it for rust. There should be no holes in the exhaust it must be gastight. Gently hit the back of the exhaust with your hand, do you feel some shaking? It may be that a stiffener is loose. Check the connection of the muffler from the exhaust and also check the connection to the cylinders.
Take a test drive
If everything looks fine, and you still plan to buy the bike, you can take a test drive. The engine should start well without a loud noise or a rattling noise. After the start, the battery light should go out.
Does the kill switch work? Does the jiffy security work? Fold out the side stand while the engine is running. If the engine does not fail, this must be repaired. Sometimes it is a connector that is not connected sometimes a metal rod is stuck by thick oil lubricant of the front sprocket near the jiffy. Throttle, see if the lever snaps back properly and doesn’t get stuck. There should be some play between no gas and turning it on. It is therefore not the case that the engine with 1mm throttle open, should already give a lot of gas. Try to drive on different road types.
Shifting should just go smoothly
With many engines you hear a neutral from 1 to 1, sometimes from 1 to 2. This should go without problems. If you have trouble getting into a gear, there is something wrong with the gearbox. The clutch plates can be checked by accelerating in a high gear at a low speed. The clutch should not slip, so the engine should not shoot unnecessarily high when the clutch is released and then accelerate at normal throttle. With 125cc engines this may be a bit more difficult to accelerate from a high gear.