1971 CB350 K3 Motor Rebuild
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Thread: 1971 CB350 K3 Motor Rebuild

  1. #1
    Member doctorruckus's Avatar
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    1971 CB350 K3 Motor Rebuild

    Hi all,

    I started a thread having some fuel issues and I've traced that all the way back to poor compression. I measured around 90psi in each cylinder and only 110 with oil on the rings. So I'm tearing down my first bike motor. Kind of starting a thread here just to chat and ask questions. So far all I've done is take the head cover off and disassembled the rockers and lifters. I'm taking the cam out tomorrow when I get an extra set of hands to keep the motor from spinning. I'm mainly doing the rebuild as a hobby and because it's leaky. I won't be riding for another few months in Seattle anyway

    So far it looks like the
    *Exhaust valves have so much carbon, they may not be seating. Rocker arms have a few pits, but nothing major. I might just buy an ultrasonic cleaner and lap the valves myself if everything is in spec.
    *Cam chain tensioner was stuck so the wheel is all ground up. --Can't find a replacement just yet.
    *Dropped a metal spacer (see attached pics). It fell off the left side of the motor when I was pulling it out and I can't tell where it goes right away.

    https://imgur.com/a/zzkTU8j

  2. #2
    Sensei 66Sprint's Avatar
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    Bottom left front motor mount spacer....
    Last edited by 66Sprint; 11-01-2019 at 06:23 PM.
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
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  3. #3
    Member doctorruckus's Avatar
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    Quote Originally Posted by 66Sprint View Post
    Bottom left motor mount spacer....
    Thanks. That's where I stuck it because it fit that bolt and was covered in a similar amount of old gross oil. I need to show you guys my rockers and cam when I get them out. They have a few pits, but I think I can get by without doing anything. This is just going to be a weekend hobby bike.

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  5. #4
    Senior Member Boomer343's Avatar
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    1971 CB350 K3 Motor Rebuild-dsc02914.jpg

    These are the parts that I replace or check over very carefully

  6. #5
    Senior Member Boomer343's Avatar
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    When you are checking your valves the exhaust valves are usually ok. Intake valves on the other hand fail about 90% of the time. If there is any dishing of the valve where it contacts the seat it is no good. Valve seats also go out of round. Valve guides need to be checked as well and that is usually done by feel.

    You also need to check the spring heights, my experience is most of the inner valves are under spec and half the large ones are under as well. I've gone through 20 inner springs to get 4 that are close to spec. I would expect to go through 10 to 15 of the outer springs to get 4 above spec.

  7. #6
    Super Moderator longdistancerider's Avatar
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    Delta Cam in Tacoma can fix the cam and rockers easily.
    slofut likes this.
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 nuts and bolts restore in progress


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  8. #7
    Member doctorruckus's Avatar
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    Quote Originally Posted by longdistancerider View Post
    Delta Cam in Tacoma can fix the cam and rockers easily.
    Thanks! Any suggestions for a hone and potentially head work if they're too bad for me to fix myself?

  9. #8
    Member doctorruckus's Avatar
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    Ordered a bunch of parts today and I'm finishing the teardown when my offset 10mm comes in today. Just a few thoughts:

    How is the bike running with 90psi compression when I read they won't run below 130?
    How much will mufflers affect my tuning and riding? The ones on the bike are pretty roached and I'm not sure I can spend the cash right now for aftermarket or new. Common motor has aftermarket for $125+$60 shipping. Would I be better off not trying to get it all going until I can source mufflers?

  10. #9
    Super Moderator ancientdad's Avatar
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    Quote Originally Posted by doctorruckus View Post
    How is the bike running with 90psi compression when I read they won't run below 130?
    It's not that they won't run at all below a certain threshold of compression, it's just that they won't run properly - lack of power, harder to start, and poor idle/won't idle
    Tom

    CL450 project reboot, street legal this time
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    CB350K1 full patina project
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    running points... because I'm too old for mysteries that begin with pushing

  11. #10
    Senior Member jakec's Avatar
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    If you're rebuilding the engine you have some time to worry about exhaust, I would get reproductions if you want it to run and look the best. As so keeping the engine from spinning you can put a long wrench on the rotor, turn till it hits the ground, and then the engine will be locked.

    where in seattle are you located?
    1975 CB400 F0 Stock
    1974 CB550 K0 Chopper
    1970 CL350 K2 Stock

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