I have a 1978 cb750k 30k on bottom end fresh rebuilt top end. I was riding not to long ago after just changing oil and filter and tuning the carbs a little because they were to rich. i believe i tweaked it to much and leaned it out i was riding with my girl on the back and the bike had no signs of any thing wacky going on great power with great oil pressure 75psi + and then it squealed really loud i grab the clutch and it shut off, was still coasting and let out the clutch and it locked down the back tire and the motor did nothing. i pulled off the road thinking the motor was locked down in the bottom end i let the bike cool off for about 10 min and it started right up like nothing ever happened no ticks no knocks nothing, seems like it over heated..... took off down the road and made it about a mile and it lost power miss firing and died, pulled off again and let it cool off while we waited for a rescue to come pick us up. i figured the bike was screwed so i started it up just for the hell of it and it was missing like crazy shut it off and got it home. Drained the oil, cause it smelt burnt but not black still golden color. changed the oil 20w50 of course. i tuned the carb rich and i readjusted the valve because i checked them figuring since it got "hot" my clearances might be off, found almost all the adjuster nuts and screws were still tight but the lash was like .005 int and .006 exhaust, when i adjust my valves i always do it cold and .002 int and .003 exhaust on the compression stroke of each cylinder. i readjusted them like i always do and started the bike up and it ran great!
so heres the kicker ever since then i can start the bike cold and it runs amazing all kinds of power and smooth idle......unit it get all the way to operating temp and then the valve lash starts getting looser and looser and the bike starts ticking like crazy of course. check the lash while its warm and it alwasy around .005 int and .006 exhaust the bike will cool off over night and the lash is still .005 & .006. ill readjust the valves and it does it all over again. my only thought would be weak valve spring i didnt change them during the top end rebuild because the machine shop said they were good at that time im wondering if i overheated the motor fatiguing the valve spring maybe... im lost can anyone help before i ether tear this motor apart again or just drain all the fluids and make it in to man cave art!
below are a few pics of my build
