CB 750 K 1979 Rebuild Amateur Needs lots of Help

[Monshall]

Ok this is going to be a laugh a minute as I learn going along.   Bought this 1979 CB 750K for the huge sum of $700 dollars, she is totally original with 36 000 on the clock, was very dirty with lots of surface rust, did a bit of a strip down and cleaned her up, got her serviced and running sweet and put a few thousand kilometers on her.  What a blast.  Unfortunately seems she has a valve seal problem or a broken ring on piston 3 or 4.   Not using noticable oil but smokes at idle, plug oils up idling at traffic lights and when you pull away dumps a lot of blue smoke then settles down.   Every mechanic I talk to gives me a different story, all want lots of money to open her up and look.  Anyway decided rather than driving the motor into the ground this would be a great project bike to get back to good looking.  Have pushed her into the garage and plan a complete rebuild. Only bikes I have worked on in the past are 50cc two strokes in my youth so please be kind and free with your advice.

[Re-run]

Hey welcome to our humble little forum! Don't worry about being laughed at! We laugh because we have been there and know how miserable it can be!

First thing to do is download the manual for your bike here in the manuals section.

Unfortunately your issue is hard to diagnose. It could be a broken ring, it could be a bad valve or seal, heck it could just be the rings are worn.

The manual will help immensely and we will do our best to keep you moving along!

[wvhardtail]

I am in your shoes. Bought a donor bike for my chopper and when we got it running it did the same thing. Pulled it apart and thank goodness just some bad rings. Hoping to have the motor back together this weekend. Good luck!

[Monshall]

Thanks for the warm welcome and support and advice, decided to get the motor fixed before I strip her apart, have eventually found a good and reasonable bike mechanic (I cannot do motors).  Will take her there on Monday.  Problem is once the motor is done I will probably end up just riding her as I am addicted, so addicted that I sold my other reliable bike, hehe,  old toys are such fun.  Oh thanks for the link to the Shop Manual, very helpful indeed.  When the motor is out might get the exhaust headers re-chromed, they are not too bad but need doing.  Maybe I will just rebuild her as I go along until everything is sorted then do a quick strip and powder coat the frame.

[wvhardtail]

It's a lot of work. I stripped my frame and it is at the powder coaters now.The more you do yourself, the more money you will save. It is always good to have a good mechanic. I have a friend who does all my work. I do the work and he supervises. Keep posting pics as you go along. I like following other builds.

[Re-run]

The other benefit of doing as much as you can is that you learn your bike. In the off chance something happens, you will have a good idea what it is an how to fix it or what it will take to fix it.

[Monshall]

yes I agree that Photos are great and the more you do the more you learn.  Soon after I bought this bike I stripped down the whole back end,  found a new/2ndhand pair of shocks, cleaned off all the rust, dirt, years of grime/oil, polished all the chrome and treated all the surface rust on the frame/under the seat/ fixed the front and back brakes, had to flush out the front fluid about 3 times, repaired a number of little things like indicator wires, and repainted a few parts like the chain cover. it was such fun and the bike looked so much better for it.  She also got new sprockets and chain, had the carbs cleaned and balanced and did a service. Learnt a huge amount.  Need to get the garage sorted out so I can work better in it.