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  1. #1
    CB750 Member Mars's Avatar
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    New member and new owner of 1979 CB750K-Limited

    I thought I might as well join so I can learn some things about this bike. I'm very new to motorcycles in general and taking a safety course at the end of the month. I'm throwing a lot in here and don't expect anyone to answer all of it, but if anyone has any comments about anything it might help me.

    The bike sat in a shed for something like 15-20 years and has more rust than I'd like. There is a hole in the exhaust which I'd like to fix, but the whole system is rusty so I might spring for a new or new to me replacement eventually. There is also a sprinkling of rust all around. My only concern really is that there is a bit of rust that the fork seals will rub on. Does anyone know how big a problem that is? I can get pictures up eventually.

    After evicting the family of dead mice from the airbox, changing the air filter, installing a battery and cleaning the carbs (twice) I got it running and a local shop synced it. I haven't checked compression or valve clearances yet, but I know I should before too many miles. I believe the previous owner took good care of it when he was alive, but I should take a look inside myself. The throttle response is still a bit wonky and it backfires on a quick throttle-down, but I'm happy with how it runs considering the limited work I've done on it.

    The lights get brighter with engine speed and I've seen conflicting ideas about how normal that is.

    The horn doesn't work but I can't imagine it being too hard to fix.

    The tires hold air, aren't cracked, have no bulges and have lots of tread, so is there anything to worry about with them?

    The chain has lots of surface rust but seems structurally okay. I lubed it and it isn't overstretched, so is it good for now?

    So hello from Rochester, NY. Please feel free to point out any and all fuck-ups I am making or might make based on what I wrote.

  2. #2
    Old Bike Nerd grepper's Avatar
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    >The lights get brighter with engine speed and I've seen conflicting ideas about how normal that is.
    Usually means a charging or battery issue

    >The horn doesn't work but I can't imagine it being too hard to fix.
    Could be simple, but could also be a charging or battery issue. It takes a lot of amps to make them go beep. Also the button internals can get coorded.

    >The tires hold air, aren't cracked, have no bulges and have lots of tread, so is there anything to worry about with them?
    Old tires are dangerous, old hard rubber doesn't grip well, I wouldn't ride them.

    >The chain has lots of surface rust but seems structurally okay. I lubed it and it isn't overstretched, so is it good for now?
    When chains break they can wrap around the hub and lock up the back wheel and can kill you.

    When ever I restore a bike, chain, sprockets and tires are always on the list. There's a date stamp on the tires, if they are anymore than 5 years old, they should be replaced.
    Drive fast and take chances

  3. #3
    CB750 Member Mars's Avatar
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    Thanks for the reply grepper. I appreciate the advice.

    Something else that is going on is that I have super low compression on just the #4 cylinder. The engine seems to run really well once it's warmed up but until then will hesitate or stall at idle and if I quickly roll on the throttle it will act as if I didn't at first and then after a long pause rev up. I left a little Marvel Mystery Oil in the cylinder to see if it loosens a possibly stuck ring because someone suggested it, but not super optimistic.

    I'm not real sure how far I will go restoring this bike. It will need an entirely new exhaust probably and everything else is spotted with rust. I may just put new tires and a chain on it and ride it for the season or until it really won't run properly. Perhaps over winter I'll pull the engine and clean and refinish all of most of the bike but I haven't at all made up my mind.

  4. #4
    CB750 Addict Dave's Avatar
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    Quote Originally Posted by grepper View Post
    >The lights get brighter with engine speed and I've seen conflicting ideas about how normal that is.
    Usually means a charging or battery issue

    >The horn doesn't work but I can't imagine it being too hard to fix.
    Could be simple, but could also be a charging or battery issue. It takes a lot of amps to make them go beep. Also the button internals can get coorded.

    >The tires hold air, aren't cracked, have no bulges and have lots of tread, so is there anything to worry about with them?
    Old tires are dangerous, old hard rubber doesn't grip well, I wouldn't ride them.

    >The chain has lots of surface rust but seems structurally okay. I lubed it and it isn't overstretched, so is it good for now?
    When chains break they can wrap around the hub and lock up the back wheel and can kill you.

    When ever I restore a bike, chain, sprockets and tires are always on the list. There's a date stamp on the tires, if they are anymore than 5 years old, they should be replaced.
    Yep, headlight should be steady once bike is running.
    Horn, test it by hooking it directly to a 12v battery. PS They are adjustable. If you "feel" something, turn the little screw until you "hear" something.
    Yep, Old tires = death trap.
    Yep, Old chain = death trap.

    Safety courses, (please wait, I fell out of my chair laughing). I'm sure, well I hope, they are not all equal but the ones I've seen personally are jokes. Here in CA the written test has several blaring errors that could get someone killed. (You should cross railroad tracks at an angle, not perpendicular!) is the one I remember the most, f#$%ing idiots. Oh, you should ride in the center of the lane (where all the oil is you morons) yeah, that's another good one.

    You want to stay alive, pretend that you are invisible and no one can see you.
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  5. #5
    Moderator dirtdigger's Avatar
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    Need to check valve clearance, more then likely have a tight clearance holding the valve open...do not run till it is corrected, you will burn the valve if it hasnt done so already.

  6. #6
    CB750 Member Mars's Avatar
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    Quote Originally Posted by Dave View Post
    Yep, headlight should be steady once bike is running.
    Horn, test it by hooking it directly to a 12v battery. PS They are adjustable. If you "feel" something, turn the little screw until you "hear" something.
    Yep, Old tires = death trap.
    Yep, Old chain = death trap.

    Safety courses, (please wait, I fell out of my chair laughing). I'm sure, well I hope, they are not all equal but the ones I've seen personally are jokes. Here in CA the written test has several blaring errors that could get someone killed. (You should cross railroad tracks at an angle, not perpendicular!) is the one I remember the most, f#$%ing idiots. Oh, you should ride in the center of the lane (where all the oil is you morons) yeah, that's another good one.

    You want to stay alive, pretend that you are invisible and no one can see you.
    Thanks Dave. I will be changing tires and chain before riding. Everyone here recommends the course, I've already signed up (paid) and it is a convenient way to get the license.

  7. #7
    CB750 Member Mars's Avatar
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    Quote Originally Posted by dirtdigger View Post
    Need to check valve clearance, more then likely have a tight clearance holding the valve open...do not run till it is corrected, you will burn the valve if it hasnt done so already.
    I guess I didn't mention it here, but I did a wet compression test which sent it from 30 psi up to 205 which made me think it is the rings, but I will be happy if I'm wrong and really don't know much. Does that change what you think? Either way it sounds like I will have to pull the engine. I was hoping to ride it for a while but perhaps it is optimistic to think that is a good idea.

  8. #8
    Old Bike Nerd grepper's Avatar
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    Quote Originally Posted by Mars View Post
    I guess I didn't mention it here, but I did a wet compression test which sent it from 30 psi up to 205 which made me think it is the rings, but I will be happy if I'm wrong and really don't know much. Does that change what you think? Either way it sounds like I will have to pull the engine. I was hoping to ride it for a while but perhaps it is optimistic to think that is a good idea.
    I'd still check the valve clearances. They are often neglected and the cause of a lot of issues, not to mention some damage if not addressed. It's not too difficult, there are some YouTube videos.

    Sent from my XT1254 using Tapatalk
    Drive fast and take chances

  9. #9
    Moderator dirtdigger's Avatar
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    If compression went that high it sounds like you used to much oil in the cylinder and got a false compression reading. Very very little oil to be used with this small of an engine, too much oil takes up a lot of combustion chamber volume bumping compression ratio sky high giving a false high reading.

  10. #10
    CB750 Member Mars's Avatar
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    Quote Originally Posted by dirtdigger View Post
    If compression went that high it sounds like you used to much oil in the cylinder and got a false compression reading. Very very little oil to be used with this small of an engine, too much oil takes up a lot of combustion chamber volume bumping compression ratio sky high giving a false high reading.
    That's possible. It definitely was not "very very little". I'll be real happy if I can get away with just a valve adjustment! I will try to check clearances tomorrow. Thanks for the advice grepper and dirtdigger

  11. #11
    CB750 Member amaskedman's Avatar
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    Quote Originally Posted by Mars View Post
    That's possible. It definitely was not "very very little". I'll be real happy if I can get away with just a valve adjustment! I will try to check clearances tomorrow. Thanks for the advice grepper and dirtdigger
    I am in the same boat with the same bike. Ill let you know how mine goes. Bought the tools, just have to do it. Do you live in WA state by any chance?

  12. #12
    CB750 Member Mars's Avatar
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    Quote Originally Posted by amaskedman View Post
    I am in the same boat with the same bike. Ill let you know how mine goes. Bought the tools, just have to do it. Do you live in WA state by any chance?
    Have you gotten a chance to do the valve adjustment yet?

    I checked clearances on mine and confirmed I don't have a stuck valve, so I think I the piston rings on one of my cylinders is shot. I bought a gasket kit, which maybe I should have waited to do, and will be pulling the engine at some point.

  13. #13
    CB750 Member amaskedman's Avatar
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    I did the valve adjustments. Mine were all over the place. I may just pull the engine and go through the entire engine. Its a good winter project.

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