My 2003 Suzuki rm 125 kick starts with brand new spark plug then eventually bogs down and dies, does the same when push started, does it need new piston and rings?

The plug you need is an NGK R6918B-9. they aren't cheap. I would think this may be the problem. 2 strokes rely on the correct bits to keep them running smoothly. Spark plugs are more important on 2 strokes than 4 strokes due to the oil in the fuel/air mixture.

the way you explain this, the fuel and air mix is wrong, is the spark plug wet, if it is check the carb air mix screw. remove the mix screw and use a can of air and blow it out. before doing this remove the air filter and the spark plug then spray some wd (brand name) in hole and do again with the air can. then screw back in. leave the air filter off. and put spark plug back in and start the engine you will need to adjust the screw until you can get it to start. when it does give it a few good revs and keep it running for a bit. replace the filter and start engine and you will need to adjust the air screw till the engine sounds right again.

it's prolly somthing electrical

Make sure you have ignition spark and just wer yhe tip of the plug with gas. Puttting gas in the cylinder is too rich and will add to you problem. Good Luck

best spark plug is iridium plug, takes the heat and lasts alot more rides. could be flooded, take plug out , stick finger over hole , kick start till it drys out. take match stick and clean inside of plug, should be good. otherwise probably a fuel problem.

Turn the air screw and throttle screw all the way inward. IMPORTANT > do not tighten these two adjusters down. Only screw these in until they LIGHTLY seat. Now turn each adjuster one and one half turns outward. The carb is now set back to factory specs assuming no one has changed out the main jet or played with the float level..

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Pull the slide out of the throat of the carb then move the clip at the top of the needle down one notch, two if needed. This will richen the low end fuel mix.

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check compression

Scatter gun approach > thoughts to check out > > >
Clean and dry the air filter? Go back to a stock main jet, at least to check things out? Are "O" ring seals on the main jet going bad?Partial internal collapse in exhaust (remove exhaust pipe & do a quick run down the alley), Install a new stock NGK spark plug, Be sure battery connections are clean and tight, Be sure intake manifold and carb are good and tight.

Hope this helps!