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  1. #1
    CB750 Member MrTaurus's Avatar
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    '79 CB750k Possible Rebuild

    Hello to everyone. I am new to the forums here. I am looking forward to learning as much as possible and making new acquaintances. With that said, here is my dilemma:

    I recently came into a '79 CB750K engine without the carb assembly. Just the engine. The cycle shop that sold it to me stated that he will look for the carb assembly in his storage and I'll update on that should it happen.

    As far as the engine goes here is what I know:
    It has been sitting in his non climate controlled storage (Kansas) for over 2 years. When I walked up to it the spark plugs were gone and it had written in sharpie on the engine "80 honda runs". A bit of a hopeful sign. The oil res is bone dry. There seems to be a bit of noticable rot on the top end seal. The exhaust chambers look gunked up a bit. The outside looks like its intact and in relatively good shape. No noticable dents or damage. A good cleaning would get it looking good but I have not done a compression check yet. I will very very soon and update on that as well. The cycle shop owner did attempt to run the starter and it did kinda turn.

    I have opened up anything yet or taken anything apart. I have a feeling that even if it passes a compression check, considering that it's been sitting without plugs, who knows what has fallen in those chambers.

    I'm thinking I may need to tear it down and clean her up. I know if the cycle shop cant find those carbs I'm looking at an easy $400 plus on that alone. So, what do you guys think?

    I would post a pic but I dont see the option.

    Fyi, this engine is meant for a custom build I'm making.

  2. #2
    CB750 Member MrTaurus's Avatar
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    Click image for larger version. 

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    Figured out the picture part. If you guys want more let me know.

  3. #3
    CB750 Guru amc49's Avatar
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    You should be able to get carbs for less. Any early DOHC '79-82 will work if 750 carbs. Do not use ones off the SOHC they will not work.

  4. #4
    CB750 Member MrTaurus's Avatar
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    Got a good direction to point me in? I've tried local and not so local dealers and part yards and I only found one place 2 1/2 hours away that has one for $350 and it needs cleaning. Ebay has some but judging by the pics they dont look in that good of shape or missing parts. Finally other online part sites either dont have the full assembly or sell really expensive after market carb assemblies.

  5. #5
    CB750 Guru amc49's Avatar
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    I myself use ebay, but I approach things differently from you likely. I intend to rebuild and clean them myself as that type is wildly prone to still having problems after being cleaned 3-4 times by 'experts'. I will not pay for work that is substandard but then I've never paid ever for a car repair either, I do it all myself.

    I've bought them there needing work for less than $100 but the parts were all there, and all I need.

    $350 and still needing cleaning? The guy is utterly nuts. People trying to create a big money market for early DOHC parts like the SOHC but it won't happen, the bikes destroy too easily in the hands of the unlearned and I've had two given to me for free in the past.

    You might want to look closer at your base engine, if left unplugged then condensation may well have ruined the cylinder walls, only an idiot removes plugs from an engine sitting for a long time. Hope you didn't give more than $75 for it. I myself would have passed as soon as I understood the plugs had been gone for a while.

    Part of the problem is covid, when people lost their jobs their stuff on ebay went up as much as 300-400%, it's NOT the time for good deals right now. Can't believe some of the prices I'm seeing. They want $180 for a small battery for my Nissan, just worked out today for very little effort how to change to one with almost same warranty but $80. Another quick $100 bill in my pocket, I do that stuff all the time.

  6. #6
    CB750 Member MrTaurus's Avatar
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    I very much intend to do all the work myself. Learning and getting my hands dirty is part of the process of restoring this engine. I've worked on engines here and there but never done a rebuild. As someone who does a lot of diy stuff, I'm actually excited to see this engine come back to life.

    I've attached a closer picture of the plug holes and the intake/exhaust holes as well. I suspect you are correct. There may be some buildup/corrosion in there. I had a feeling that I would need to tear her down but in a way I am happy to do so I knowing that every nook and cranny is clean and serviced and dont worry, I paid $50 for the engine.

    If the cylinder walls have mild corrosion does that usually mean they will need to be redone or would a good cleaning take care of it?

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  7. #7
    CB750 Member MrTaurus's Avatar
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    Another question. Considering that I'm probably running into a very dirty engine im wondering if I should even bother with a compression test since it's likely im going to tear it down anyway. I don't want to do any more damage that I'll have to fix.
    Last edited by MrTaurus; 07-23-2020 at 10:31 AM.

  8. #8
    CB750 Guru amc49's Avatar
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    Your call there. Can't tell about cylinder walls without them exposed anyway. Last pic of exhausts shows problems if somebody did not oil the motor before spinning it. Dead valve seals or valves.

    You should know up front that rebuilding one of these can be very expensive as compared to others. Nobody ever set the valves because they require shims to do so yet everybody thinks the engines are still in fine shape many years later when they commonly have burned valves in them. Parts are quickly drying up too as the engine does not have the following the SOHC did.

    If you haven't picked up on it yet then you MUST mark all parts as to exact locations so they can go back exactly where they came from, it will kill you on the head for sure not doing it. I use heavy duty freezer bags for the valve parts to keep them separate, each is marked.

    Another note...........the alternator rotor MUST come off to split the lower crankcases, you cannot escape it. The cylinder block may be pure hell to remove and often damaged doing it, the cylinder studs fill water around them from rain to corrode down low next to case and then the block acts like glued down.

  9. #9
    CB750 Member MrTaurus's Avatar
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    Thank you for the advice. I'll do just that. The freezer bag thing is a good idea. Label as I go. I'm going to take my time with this. Asses the situation and make notes of everything that needs work and/replaced. Take notes and so forth.

    I called honda motorsports and a csr was kind enough to give me details about the engine and even went so far as to mail me a copy of the service manual free of charge. I found a copy online but having that hard copy on hand will be very helpful.

    I'll post pictures as I go. I'm sure I'll have a ton of questions.

  10. #10
    CB750 Member MrTaurus's Avatar
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    I decided to pop off the valve head and the clutch cover to get some clues as to the condition of this engine. Here is what I saw.
    First the valve head:
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    It looks to be in good shape at a glance. The seal had a lot of spring to it as well. In the third pic I could not find the name of that part in the shop manual but it's part of the B bracket inside the case. Looks like that will need replacing. Doesn't seem like the valves are leaking as the oil pooled inside is staying inside. I cleaned the outside channel of one of the exhaust chambers and it appears to be carbon buildup and not any oil or grease residue. When putting the top back on the screw that goes in the upper right corner of facing the engine from the exhaust didn't quite grab fully. Upon further inspection it seems that it was held in by some kind of bond that flaked off the screw so when it comes back off I'll check to see if the screw hole is stripped.

    Now the clutch:
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    There's some buildup there but not terrible. Doesn't seem like rust, more like oil buildup but what concerns me is what saw behind the clutch in the second picture. I'm not sure if the case was stressed or if that is normal or what. Some input on that would be appreciated.

  11. #11
    CB750 Guru amc49's Avatar
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    Not cracked behind clutch, rather sloppy casting by Honda.

    The tach drive cam cap may still be useable. The bolts holding it on break super easy, don't go over 8 ft.lbs, manual says 10 and they break. All cam caps are machined at one time while bolted to head making them matched parts. The chain tensioner bolts that go into aluminum and the valve cover bolts also strip aluminum out super easy. If talking B cam chain that tensioner set bolt at an angle on front of head is commonly stripped and you have to helicoil the head. It has a nut to jam it and too tight on the nut then jacks the threads out.

  12. #12
    CB750 Member MrTaurus's Avatar
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    So its called a tach drive cam cap. Well the outside is chipped as you can see but the tip of the nut inside is chipped as well. So it looks like its going to be hella fun getting it out to replace it. Here is a closer look at the inside:
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    On the head that seems to be the only bolt doing that so ill take your advice on the helicoil. It just tells me someone has been inside this head before.

    Do you think its wise to use compress air to clean out the spark plug chamber since there were no plugs for a while or will that possibly create a bigger problem?

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