Top End Rebuild - Is it difficult?

[86FJNJ]

How would you rate the level of difficulty to rebuild the top end of a 1976 CB750F SOHC engine and redo the timing etc? Fresh gaskets, rings, valve seals etc

I'm mechanical and can do all other work on bikes etc, but I've never taken the heads or jugs off and engine. My engine has over 35k miles and I think the head gaskets are original, so I would like to know how difficult it would be to do a refresh over winter. I'd like to avoid a can of worms and don't want to turn into a full on performance build. I also don't want to get too deep into something I cant finish. Engine is a 76F but rest of bike is a 78F

 

[dirtdigger]

Its not too bad just need to use all new o rings and stuff. Make sure the surfaces are all flat and if not have them machined. These things can go many more miles then yours if it has had good maintenance and everything checks out mechanically , if its not leaking and compression is good I wouldnt mess with it. The f models are getting hard to find rings so check on that first, the 76 f models are kind of oddballs. There are other things I would change if you had the cylinders off. I would check compression and if its good i wouldnt mess with it.

 

[86FJNJ]

Thanks dirtdigger, compression is 125-130 across all four cylinders. I got the bike from somebody and it sat for years so maybe after a few hundred miles the numbers get better. I think it might have small leak under the head but I may just run it for a while as is and see how it does. Out of curiosity what would you change if you had the cylinders off?

 

[dirtdigger]

Thats not terrible compression. It would have decent performance with that. If they are even I honestly wouldnt mess with it unless you find or have some more issues needing top end removal.

If you remove the top end I wouldnt put it together without boring, the cylinders like to wear out of round on the 750, if you just hone and install new rings and the bores are out of round the new ones will never seal right. I would replace stock studs with heavy duty studs....the stock ones are one of the reasons the head gaskets seep oil after some miles. Cylinder and head surface I would have machined for MLS gaskets then never have leaks again. Have cylinder head rebuilt while off. Do a little touch up on the ports and smooth them a bit, not a full blown port job although that would pick you up more power then anything, the 750 suffers from restrictive ports that got worse in the later years other then the F2 heads. Timing chain tensioner wheels are probably hard and getting ready to break apart so that needs replaced. Condition of rocker arms and cam tower bearings would be checked when apart obviously.

This would obviously be the time to step up to a 836 kit if boring is needed. maybe step up to a drop in cam and you would have a motor that runs like stock just a little stronger.

 

[86FJNJ]

Dirtdigger, thank you. I'll plan to run it as is and see how things go. Good info on the bores out of round and a 836 with a cam sure is tempting if I open it up.

There is a lot of carbon in the exhaust ports. Would it be a horrible idea for me to go in with a small wire wheel and clean the carbon out of those exhaust ports while I have the engine out? I could do it with a shop vac to help suck out all the carbon that breaks loose.

 

[dirtdigger]

i mean it wont hurt anything to clean it out but its just going to get carbon build up again at some point. Guess it depends on exactly how much build up there is as that can have some indication on engine condition as well.