I have a 1997 Yamaha Riva 125 and need to change the oil. The manual calls for 20W40 SE grade which I cannot find at any automotive store. Is there a safe alternative that will not damage the engine?

SOURCE: oil change on my 1990 fzr yamaha 1000

20/50 shouold be fine oil drain plug is under the belly pan

SOURCE: what typpe of oil it takes

10w30 is recommended and just under a quart is needed

SOURCE: Where is the oil filter on a Yamaha XT 125?

if you take off the clutch cover and look to the bottom right of the engine you will see a black ruber strip. Pull this out and thats your oil filter. Unlike your usual filters you can just clean it and place back in.

SOURCE: yamaha tw 125

The noise you hear is valve clatter. You should not adjust your timing chain. Instead get a manual and find the vlave clearances for that bike. (Ex. The exaughst valve may be .006" and the intake valve may be .004") Then you'll need a feeler gauge, and to do this you'll remove the valve covers and get the engine to TDC (Top Dead Center) This is where the piston is at the very top of it's stroke and both valves are shut. (compression stroke) You must then loosen the rocker arm nut to the point that you can insert the feeler gauge between the rocker arm and the top of the valve run the nut down tight to the feeler gauge and then pull gauge out and repeat on the other valve.

SOURCE: how do i decode yamaha yz 125 engin id number

You don't decode it. It is only a production run series number. Mororcycles are like cars in that respect. It is the frame number, AKA the "VIN number", is what carries all the information. Call a dealer with the frame number and he can tell you year, paint color, specific model and more. They may have a problem if the bike is too old. Yamaha changed to a newer code system a few years back.
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