1981 CB750C If you have had this issue save me the headache!

[samcraig182]

You're not the first one to be here by a longshot, most early DOHC end up that way, nobody ever sets the valves and perfectly good engines go to crap over it. Almost every one you buy will have the carbs apart over this issue, the owners commonly telling you it ran fine. Well, it did, and likely in recent memory, they run fine then begin to lose their edge and then suddenly it's running like crap.

The cams even on a brand new bike are super loose in the head and the valve springs then push the cams all over the place to change the effective valve clearances, which are commonly at .003" if by the book and .002" is considered useable. But it's not, .002" is pretty much zero and valve held open and why if you can recover the engine you loosen them up to a setting of .005" instead, then the engine goes whoppingly longer with no issues. A Honda mistake that they knew of but rectified it by changing up the valve setting marks on cams, that did nothing to fix the problem.

It is a 50k engine. Yea I know you probably saw that one coming right haha.

So I am thinking rebuild or new engine. A few people are selling salvage 750 dohc engines near me but If I decide to go rebuild do you think I am looking at extensive work, time and money? Tough question without seeing the condition of anything but I have never really dealt with this situation so I am curious as to what someone with experience thinks that road usually leads to.

If I decide to get another engine, what can I do to tell whether the condition is good ? Mileage I suppose can help to an extent. But as you said if clearances aren't set these can go bad fast so you never know how quality with quantity varies. I can get compression numbers as well because the engines I am looking at are still in frame and hooked up. But to avoid this situation again what's the key here?

 

[amc49]

All of these will pretty much be worn at this time in their history unless you find one locked away in the corner of a garage somewhere. If you can get compression reading it is more important than mileage although mileage in hot climates has the cam chain tensioner parts cooking pretty quick. Here in Texas they are already cracking at the rubber tensioner blades at 35K miles. If the valves leaking don't bring them down then tensioner breakage is next. The alternator rotors go out at the blink of an eye too and why you see so many for sale on Ebay and commonly bad ones too.

50K is a lot of miles on these although I've seen fluke ones in very cool climates and very easy drivers go up to 90K, pretty unusual that. They aren't like cars being aircooled. The rings go much longer than the valves do. The valve seals themselves cook pretty quick too.

You should know that these are some of the most expensive engines to totally rebuild as well. If you go the whole distance. Why drag racers never flocked to them in big numbers, the head port issues stop them from making big power like other engines did too.

The engines were really a stopgap measure, Honda dumped them pretty quick to push the new V-4s which then brought other problems with them. Only 4 years of production, Honda late realized the SOHC was dead saleswise and had to throw something new out there until they got newer and different stuff ready, the public being as fickle as it is.