amc49
CB750 Guru
Look here......................any time the engines got to where they would not rev all the way out on me and popping once getting up there they had bad valve seal when I yanked the head. The exhaust seats will be dead. Another way to tell if you are sensitive enough to engine note is that they will seem to act like carbs not synced, there will be a burbling note at cruise that sounds like one carb is off sync but is actually at least one cylinder not putting out as much as the others. Other than that the engine will run on all 4 cylinders and seem pretty good, it will just not rev all the way out. It really IS making less power but they make enough you think it's fine at first as the loss is not much, later though it gets steadily worse. One other indicator is that everything you can think of is right, there is nothing else that can be wrong. In that I don't think the advancer is your issue, I've had no trouble with them at all. You should be able to tell it anyway by the amount of spark scatter you get while setting timing with a light.
The bikes were built in a era of low lead fuel which actually was not as bad as zero lead, even a little helped tremendously. The valves are heat treated only and a very thin layer, it's not remarkable to have to change 2-3 valves every time you pop a head on these; they are shortlived in modern zero lead gas and ethanol only made it worse. I call around 25K miles as about the time to need valvework if you playbike it enough. Why the compression numbers matter more than usual. I consider the engine type to not be as robust over the longterm as the SOHC, the bigger single valves have half the number to leak and bigger ones stabilize better in the valve guides anyway.
How most of them die, they begin to not pull out max rpm and then it slowly drops lower and lower.
The bikes were built in a era of low lead fuel which actually was not as bad as zero lead, even a little helped tremendously. The valves are heat treated only and a very thin layer, it's not remarkable to have to change 2-3 valves every time you pop a head on these; they are shortlived in modern zero lead gas and ethanol only made it worse. I call around 25K miles as about the time to need valvework if you playbike it enough. Why the compression numbers matter more than usual. I consider the engine type to not be as robust over the longterm as the SOHC, the bigger single valves have half the number to leak and bigger ones stabilize better in the valve guides anyway.
How most of them die, they begin to not pull out max rpm and then it slowly drops lower and lower.
Last edited: