I just picked up a used GRP .28 MRT with grp exhuast (2053). I got it for $100 bucks and its not in the bad of shape at all.

Until i thought. I have a really high idle after high speed passes. This is my first GRP so i had to do some research to get the correct needle settings and to read about all the different things people go through. Well the guy i got it from used OS p3 which i found out after is a bad idea. Im assuming the plugs will leak. Would that be the cause of the crazy idle?

When i do a high speed pass. the engine will be revving high then it will go down after 20 seconds or so. then if i just tap the throt it will go right back up.

Well i couldn't figure it out for the longest time and it was about to just be a shelf queen at this point. I really wanted to use this mill. so i picked up new p/s, rod, pin, clips, front and rear bearings. cleaned up the inside put her back together than started her up. Well same problem. I sealed the engine's carb and backplate while it was apart too as i do with all my engines.

I still dont know what is wrong with it, maybe its the plug that im using. i just ordered a new head button so i can run the grp plugs. Im hoping that is what the problem is.

has anyone ever had this problem? If so, what did you do to resolve it.
i just got my untuned grp .21 yesterday. So if i can't fix it, im just going to swap the parts that i can with the .28 and .21.

thanks for all your help guys.