Overheating problem - thermostat breather hole.

[torino68]

Hi, thought I would post this just in case anyone else has this happen and wonder why. Got my bike going about a month ago now, after having replaced engine, so thermostat housing and connected pipes had been removed. Ran fine for about 250ks or so at constant 65 degrees once warm, then one evening riding home at constant 100kph, temp guage rose slowly from 65 to 85 degrees about every five minutes, then once higher temp reached would drop straight back to 65. Outside air temp was cool so no need to overheat. Kept doing this all the way home. Just pulled thermostat apart and found what seems to be the leg off some insect stuck through blocking the breather hole in the top of the actual thermostat itself. Something must have crawled in there while it was all apart and mostly disintegrated apart from that leg. Removed this, cleaned it all up and resealed etc all back together and took for ride now all perfect with bike again at constant 65 degrees once warmed up. Interesting thing is I am guessing that the obstruction must have caused complete blockage of breather meaning that although the bike normally either doesn't need the thermostat open at all or only slightly during normal use, the blockage meant that it wasn't getting the normal bit of extra flow, so each time the temp got up, the thermostat would fully open, causing a sudden rush of cooler fluid past the temp sensor which is right next to it, causing the temp to be going up and down so much. Anyway all okay now (hopefully!)