throttle wide open

It took me a m inute but I'm guessing the piston you speak of is the slide. Visible when looking into the input side of the carb? Is the throttle cable free to move or relatively tight? Look for debris in the ends of the cable. Is there and freeplay in the throttle? Look for any binding or pulling of the throttle cable. Perhaps you caught a stick or somethng.

Does it increase revs when you turn handlebars.Badly routed cable,wrong size cable,incorrectly adjusted cable

Just buy a new cable, it cost me $20 Aus dollas so it might cost u about $14-16 US. Is it a 2 stroke or 4 stroke. If it is a 2 stroke, smear a little bit of 2 stroke oil on the throttle slide.

Take the cable off, spray some kind of penetrating fluid in it as best you can if you dont have a cable oiler. Pull the cable back and fourth afew times untill it seems to lossen up. I had to do it with my old cltuch cable untill my new one came it.

Now, take apart the throttle tube housing thing. (Sorry, I dont really know the name), and give it a good cleaning in side where the cable attaches. There could be some sand in there binding it up. Then, slide the throttle tube out about 1/8 of a inch, just in case its rubbing and binding on the bar end. If theres a gap in the throttle tube housing, put some kind of sealer around the gap (just a thin layer) so if you lay it down on the right side, you dont get sand in there.

Hope this helps.