CB750 Nighthawk Engine Questions
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CB750 Nighthawk Engine Questions

This is a discussion on CB750 Nighthawk Engine Questions within the Technical forums, part of the Caferacer.net Forums category; Hey, guys! Okay, so I've been around this forum a bit and I've come to realize that your brains are big. Mine is not, but ...

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  1. #1
    Junior Member UniHalf's Avatar
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    CB750 Nighthawk Engine Questions

    Hey, guys!

    Okay, so I've been around this forum a bit and I've come to realize that your brains are big. Mine is not, but I want to learn. Engines fascinate me. I figured the best way for me to learn might just be for me to get my hands dirty, so here we go.

    I went on craigslist, and I found a cheap example of a Honda CB750SC engine to mess around with, and maybe clean up a bit.

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    So I've got nothing but questions here, if anyone is up for helping me out.

    The first thing I want to know is, what exactly do I need to do to get this thing to run? Apart from attaching the carbs, which I do have on hand. What exactly needs power for this thing to start?

    Thanks.

  2. #2
    Senior Member TrialsRider's Avatar
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    You sure did dive right into the deep end :|
    Did the person you purchased that from suggest it was an operational motor, is it a complete unknown, or is it a boat anchor?

    What have you read or been taught about 4-stroke motor and transmission principals so far?

    You say you have something that resembles carburetors, how about ignition coils, battery and exhaust?

    … and Luck, you need to find lots of luck.
    On the other hand; If you bought it dirt cheap, maybe you can just take it all apart, learn a little from that, itemize all the parts and put them right back up on CL
    I wouldn't break even doing that but maybe you can.

  3. #3
    Junior Member UniHalf's Avatar
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    Carbs, ignition coils, and wiring harness are here. No exhaust though. I can get my hands on a battery.

    I heard it turn over before I bought it, so it's not a total brick.

    As for engine and transmission principles, I know the basics of internal combustion engine operation, but that's about it. With a bit of luck, I won't actually have to open the case to get it running.
    Last edited by UniHalf; 02-27-2016 at 06:54 PM.

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  5. #4
    Senior Member TrialsRider's Avatar
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    Cool, you're an optimist so am I

    First thing you're going to need to do is bolt it onto something better then the rubber tire you have it sitting on now. Next I would recommend attempting to test for compression on all cylinders. Then maybe assemble it all together so it looks like more of a complete motor with everything more or less bolted together in place, you're going to need some kind of header pipes on it before you can start it and run it for longer than a couple of seconds (kinda hard on your exhaust valves otherwise) once you've got compression, air, fuel, and spark, either the motor fires or your problems begin.

    somewhere in there you're going to need oil in the motor too.

    Go here to download service &/or parts manuals if you haven't already -> http://www.carlsalter.com/honda-service-manuals.asp
    Last edited by TrialsRider; 02-27-2016 at 07:43 PM.

  6. #5
    Junior Member UniHalf's Avatar
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    Should I focus on getting the thing to run with shitty carbs first, or should the carbs be cleaned first?

  7. #6
    Senior Member hillsy's Avatar
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    Have you got a frame for this thing? Pretty hard to get an engine running when there's nothing holding it up/in/whatever...

    Oh, and the ignitor is pretty much essential as well.

  8. #7
    Senior Member TrialsRider's Avatar
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    Quote Originally Posted by UniHalf View Post
    Should I focus on getting the thing to run with shitty carbs first, or should the carbs be cleaned first?
    Almost afraid to ask; do you know how to clean a carburetor?
    again it kinda depends if the engine was operational when it was dismantled. If yes, I'm assuming the carbs don't have any fuel in them to go bad, if no, they probably need cleaning.
    Probably pretty safe bet that any problems will be in the bottom half of the carburetors and it only takes 4 philips screws to remove a float bowl, so you might want to remove the number one carb bowl and look inside. (photos of that would be good) #1 will be the one on the side stand side.

    already told him he was going to bolt it into something hillsy. I assume he's building a stationary gas burner at the moment.

  9. #8
    Senior Member kenessex's Avatar
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    So, that motor is out of an 83 CB750SC and will fit in all of the 79-83 CB750/900/1100 frames except the 900/1000C as they were shaft drive pigs. I would get this: https://columbus.craigslist.org/mcy/5447653141.html

    Offer him $400. This has all the parts you would need to get your motor running and then you would end up with a spare motor and some parts. Then do some work to the bike and have some fun riding around this summer for about $1000 unless you paid too much for the motor.
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  10. #9
    Junior Member UniHalf's Avatar
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    That's not a bad idea, and probably something close to what I'll end up doing.

    As for the carbs, the ones I have are not quite complete. Are they interchangeable with other nighthawk carbs, or, are there better carbs or there that I could buy and rebuild?

    Also don't know how to clean a carb. I assumed YouTube would be my friend here.

  11. #10
    Junior Member UniHalf's Avatar
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    Another thing: the guy who sold it to me had it hooked up to a battery and turning over, but I can't for the life of me remember how it was hooked up. Ideas?

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