Re: merc. 140
The serial # would be on an I.D. plate within the transom clamp screw area, on top. Some later models do have it on the side of the steering yoke (the middle portion of the motor), below the lower cowling.
Your motor most assuredly uses a 50:1 fuel/oil mix. Use a mfr's oil (merc, OMC, etc) or good aftermarket such as Penzoil (you can get it cheap by the gallon at Wal-Mart). Any oil you use should be rated TCW3 or better.
Mix with Mid-Grade or premium unleaded (no alcohol if possible). If all you have is gasohol, Merc doesn't recommend using premium as it has higher levels of the Evil Ethanol.
Use a carbon-cleaning additive in the fuel, such as OMC Carbon-Guard, Yamaha Ring-Free (can't remember what Merc calls theirs but they have one). Seafoam is also an excellent additive if you can get it in your area. This will keep the rings carbon-free and Very Happy.
If you have no idea when the water pump was last serviced, do yourself a favor and have a pump kit installed. Impeller, pump insert, pump plate, and a few gaskets will save you thousands of dollars over replacement of a toasted powerhead due to pump failure.
While the impeller is being serviced, have the lower unit gear oil drained/refilled with a mfr's premium lube. At minimum use Marine-Grade "Hi-Vis" gear oil. I'll take much less than a quart to do so.
Have the carbs and ign linkages adjusted/synchronized and adjust timing to approx 18 deg BTDC. Note this will vary as to the type of ign system your motor has. If you're doing the work yourself, pick up a good manual.
Here's an excellent writeup for this Link 'n Sych procedure:
http://forums.iboats.com/showthread.php?t=168855
HTH & Happy Boating........ed
