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Help CB250 Nighthawk/CMX250/Rebel cam chain replacement

Discussion in 'Maintenance' started by Snod Blatter, Mar 12, 2019.

  1. Snod Blatter

    Snod Blatter Active Member Dirty Wheel Club

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    Hi all, I'm currently doing some serious work on a CB250 Two Fifty (MC26, like a Nighthawk but with a disc brake) and have split the cases to replace some worn gearbox parts. While I'm in there I'd like to fit a new cam chain as the old one has done 93K miles and seems to be quite bendy, suggesting the pins have worn slots into the plates.

    The crank is a 3 bearing affair with the middle bearing held by a carrier like so:

    MZq1YUr.jpg

    Is anyone here familiar enough with this engine to tell me if I'll have to remove the crank from the left case in order to hook a new chain round, or can I just unbolt the carrier and it'll give me enough room? I'd love to get away with doing half a job, I hear it's an absolute nightmare to get the flywheel off the taper as it runs dry and all corrodes in place..!
     
  2. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    I know the correct flywheel removal tool will make removing the flywheel easy
     
  3. Murdo

    Murdo The Good Doctor Staff Member Premium Member Ride and Events Crew Contributing Member

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    Try unbolting the carrier first. If it will not give enough room then looking at the parts manual you will have to get the flywheel/rotor off to allow the crank to move over enough.
     
  4. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    How did it go Shod? What's the answer?
     
  5. Snod Blatter

    Snod Blatter Active Member Dirty Wheel Club

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    I tried unbolting the carrier but there is no wiggle at all, no movement away from the case or anything so it looks like I'm either going to have to get the flywheel off and get the crank out or cut the chain and fit one with a rivet link.

    When I took the bolts out the carrier can be turned slightly but the top fastener is a nut on a stud and of course the stud stops it from turning very far at all, a bit of a nightmare really! I did vaguely hope the stud might turn out with the nut but no such luck and there's not enough of the stud poking out to be able to turn it out from the case with 2 nuts on it or something like that.
     
  6. Andych

    Andych Well-Known Member Premium Member

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    I am not sure if you can get a soft link chain in that configuration... it would be a cow to do.
    Personally I would be looking at getting the flywheel off if possible.
    I dont like the idea of soft link cam chains...
     
  7. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    I have a US spec Factory Service Manual, I could upload it to Dropbox if it'd be of use to you. From what I read the cases need to be separated and flywheel removed so the crank can be removed from the left case.

    I've got two of these engines from trainer bikes, not sure if they're in good or bad shape because of that, but one sat outdoors for an undetermined time without its starter, I'm counting on a full rebuild of that one. Maybe I'll even monkey with different gear ratios.
     
  8. Snod Blatter

    Snod Blatter Active Member Dirty Wheel Club

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    Thanks Alan but I already have an electronic version of the manual, though I wouldn't mind one with the pages in the right order..!

    Not having the starter motor in place shouldn't be a disaster, they run dry in there and there is a drain hole so it's open to the elements anyway. Maybe open it up to see if anything big has got in there but I'd totally expect to use it afterwards. I think you'll have to take the cover off anyway to put the chain and gear back on the starter motor so no great loss.
     
  9. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    Here's a link to that Service Manual, this is the first time I use it hopefully it'll work.

    https://www.dropbox.com/s/oyl8jbrtvoxxb11/91-97 CB250 Nighthawk FSM.pdf?dl=0

    Thank you for the advise, I haven't started in on these engines yet, too many little projects to complete first. One at at a time.
    I'm thinking kick or even bump-start only would be cool for the right build.
     
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  10. Snod Blatter

    Snod Blatter Active Member Dirty Wheel Club

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    Thanks for that Alan, really nice and clear too!

    In case anyone out there is interested, I just checked the thickness of the selector fork claws and they're all well within spec even at 93000 miles so I can only guess the poor gear change is due to the detent arm being completely worn out. The selector drum even looks like new, it's frankly bizarre. So with the gearbox now considered okay it seems all I want to do in there is replace the cam chain, time to look into special tools..
     
  11. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    Thanks, I scanned that one myself.

    Wow, I'm surprised those parts are in such good condition at that mileage, nice.

    Flywheel puller (though you already know I'm sure) is M16 x 1.5 RH

    Here's a link to an Amazon.com item where this puller is cross referenced to pullers from Kawasaki, Yamaha and Suzuki as well. Handy reference, some bikes can use their own rear axle as a flywheel puller, we're not that lucky this time.

    https://www.amazon.com/Motion-Pro-Magneto-Flywheel-Puller/dp/B0012TRH5E
     

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