1983 Honda CB450SC Nighthawk Low Compression Help
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1983 Honda CB450SC Nighthawk Low Compression Help

This is a discussion on 1983 Honda CB450SC Nighthawk Low Compression Help within the Technical forums, part of the Caferacer.net Forums category; Been working on getting my old 450 nighthawk back in shape to give to a friend for his first bike. Compression reads are 110 / ...

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  1. #1
    Member Phaedrus's Avatar
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    1983 Honda CB450SC Nighthawk Low Compression Help

    Been working on getting my old 450 nighthawk back in shape to give to a friend for his first bike. Compression reads are 110 / 80. She's ain't got the kick that she used to and is having trouble getting up long, gradual inclines. I geared down the main sprocket 1 tooth when I replaced the chain (from 17T to 16T). I'm wondering if this could be adding to the poorer top end performance? I used to cruise around at 75/80mph all day with a little headroom, and now she maxes out at ~70.

    Plug check shows a lean condition as well. Is it possible to tune some more umf into her? Maybe throw a dual pod K&M and re-jet or just richen the stock set up a notch or two and see if she's any happier?

    Basically, ya'll know any tricks to breathe a little more life into an ailing engine? A rebuild is on the table, but I'd rather just get this thing to putt along and let my buddy learn how to ride. I doubt he'll use her for too long before he wants something bigger.

  2. #2
    Banned Witworth's Avatar
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    Forget taking the airbox off that model and expect it to ever run well again, regard less of how much time, carb jets and money on pod filters you throw at it. Get a new OEM filter, if it's paper and old, you can't clean it, throw it.

    I'll hazard a guess that the carbs need a long and careful clean out. Forget just blowing compressed air or some wonder carb spray through them: that ain't going to work jack. Clean the in tank fuel filter as well.

    It sounds like the motor needs a pro valve seat job and maybe even some new valves. I guess you would have adjusted the valves to spec.
    I think you need to go back to a 17T front sprocket.

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  3. #3
    Member Phaedrus's Avatar
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    Right on. I'll throw the 17T back on. Haven't touched the valves yet either. I'm willing to bet that those two alone should give her a bit more umf.

    Valve seat job sounds expensive... Hopefully it doesn't come to that.

    Cheers!

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  5. #4
    Senior Member 8ball's Avatar
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    Valve seat job is expensive...probably more than the bike is worth to you. Keep in mind that valve clearances generally wear tight, not loose, so if they are very tight, one or more valves may not seal and you will lose compression. You need to do a bit more diagnosis, but start with a valve clearance check.

    As far as the sprocket: When you say maxes out, do you mean hit the rev limiter, or just looses steam at 70 mph. If it's the latter, changing back to a 17T will make it worse.
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  6. #5
    Member Phaedrus's Avatar
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    Okay, good to know. Will adjust valves tonight and see what I find. Though, in my experience, super tight valves make it really difficult to start a bike. This 450 starts instantly, doesn't seem to have any trouble breathing. We shall see what the clearances reveal though...

    And, it's the latter. The bike just runs out of steam ~7k RPM. She just feels tired, winded. Could check the timing too after the valves. Maybe the advance is off the mark.

    So stick with the 16T then, eh?

  7. #6
    Senior Member Geeto67's Avatar
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    have you done a leakdown test? Betcha the rings are done in that engine, esp if you ran it 70+ mph for long periods.
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
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  8. #7
    Senior Member Cyorg's Avatar
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    If you are getting 110 and 80, then like Geeto said a leakdown test would help. Don't recall what pressure it should be, but a wing of that vintage would be around 170. Did you do the compression test with the throttle wide open? If so, no wonder it feels a little tired. I don't think the sprocket is worth worrying about at this point, especially if you're down on HP. You can squirt a bit of oil through the plug hole and do the test again. If compression jumps, then its a good indication your rings are due for replacement or perhaps a rebore and pistons. Either way that head is probably coming off in the not too distant future.
    "Non urinat in ventum"

  9. #8
    Member Phaedrus's Avatar
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    Haven't tried a leakdown. Will do. I'm almost certain the rings are shot, or worse. Just wanted to see if I could get her to cruise at 65 safely for him to travel on the freeway. I used to ride her excessively hard back in the day. It's no wonder she's feeling worn out.

    Will adjust clearances and do a leakdown tonight. Thanks for the advice.

  10. #9
    Member Phaedrus's Avatar
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    Whelp, the valve clearances were bad, but not tight enough to affect compression. Though, she's definitely happier now that she's breathing healthily. Leakdown test verified that either the rings are shot, or the cylinders need reboring.

    I'll take her out for a joy ride tomorrow and see if the valve adjustment was enough to make her pull better at higher speeds. Piston rings will be ordered in the meantime. Thanks for the guidance.

    Cheers

  11. #10
    Senior Member Geeto67's Avatar
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    Don't order rings until you take it apart. Why? If you need to bore to the next largest size piston then stock ring sizes are useless to you.
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

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