what is the best way to know when a 2-stroke dirtbike engine needs a top end rebuild?

Answer 1

If you don't have a compression gauge take out the spark plug and seal your finger over the hole nice and tight. Next make sure that spark plug wire is away from you (it will shock you!) and kick it over. 2 stroke motors are high compression so if it doesn't blow air by your finger/off it's in need of rebuilding. That's usually a good rule of thumb for compression. If you have a clanking noise every time the motor makes a revolution its likely that theres rod slop going on. Rod slop occurs at the crank or the wrist pin.

Answer 2

NEVER re-use the circlips holding the piston onto the piston pin. Once they are removed - throw them out. A good habit to get into when rebuilding a top end, is to install a new piston pin and bearing (upper connecting rod bearing and pin). The parts are cheap enough. Install a new head gasket. Remove all of the old gasket material from the gasket surfaces. Clean the surfaces with contact cleaner and a clean paper towel (contact cleaner is available at bike and auto parts stores). Microscopic dirt particles will prevent the gaskets from making a good seal. The surfaces must be as clean as brand new. 2 stroke engines with more than one piston ring - the chrome, or silver ring goes on the top groove.

Answer 3

The best way, if you have the equipment is to run a compression test on the cylinder.
You should be able to get the spec. off of the maintenance manual.
Run the engine until it is warm, pull the plug, screw in the guage and kick the bike through about five needle movements on the guage, compare your reading to the spec. something around 150 is good.

Answer 4

generally if you have been riding that bike for awhile u know how it starts when you notice you got kick it a bit more to get it started is a good sign, a compression gauge is the best . if you are the type to pull it apart yourself pop the top and send it to machine shop for a refit requires new rings piston wrist pin and bearings also nessisary gaskets dont forget new cir clips for wrist pin. should be like new again if bottom is good and con rod is good

Answer 5

Along with some of the other answers, when the spark plug fouls often.
The low compression will not fully burn the oil in the premix.

Answer 6

Before ya do a top-end,,, Check everything else. Intake reeds that are worn (or sumpin stuck in'em ) will take away some of that crisp throttle response. =^ )

Answer 7

1 if the engines starting to get noisy
2 low compression test
3 lacking power

good luck best to fix it asap to prevent further damage

Answer 8

if the engine is starting to get noisy (rattling) hard to start, down on power. then it is time to rebuild it. if you are racing the bike then fresh rings every race meeting.

Answer 9

Power loss and backfiring through the carburetor.