I haven't seen may properly built/jetted LC 250R's do that but an air cooled version is allot more finicky IMHO. You had better have your clearances SPOT ON and your jetting pretty close too; Especially in hot humid weather.....
If it ran well for a long time, I wonder if you lost a main or big end bearing??
Regardless, it's gonna be a fun time figuring it out. I've even seen air filters fall off and create a lean condition or an intake boot deteriorate...............
Any more info??
All our government does is distract us while they steal from us, misspend our tax $ and ruin our country