motor swap 91 CB 750

Discussion in 'Motorbike Technical Discussion' started by Andy, Sep 20, 2007.

  1. Andy

    Andy Guest


    The knowledgeable folks on this list helped me diagnose a faulty
    starter clutch on my 91 Honda Nighthawk 750 a few months ago. A
    mechanic recently confirmed that this is in fact the problem and that
    fixing it would run me 1500 bucks. So I am looking into swapping the
    whole engine out rather than trying to split the crankcase myself or
    paying more than the bike is worth to have someone do it. A local
    junkyard has apparently located a motor.

    My question is, what is involved in swapping the engine? Would this
    job require a lift or any special tools or techniques that an ordinary
    tinkerer like myself would not have? I need to decide if I am going to
    attempt the job myself, pay someone to do it, or just sell the bike as
    a fixer-upper and forget about it.

    thanks in advance
    Andy, Sep 20, 2007
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  2. Andy

    Mark Olson Guest

    I haven't R&R'ed a CB750 engine but I did put a different engine in my
    ZG1000. There's nothing magical about it, and it's surprising how
    easy it is once you get over how 'major' a job it is (it isn't, really).

    It sure helped me to have a motorcycle lift, much like a transmission
    jack, that I could put under the engine and drop it out of the frame.
    On the ZG, the engine is a stressed member so there's no bottom frame
    tubes to work around. On your bike, assuming that there is a wrap-around
    frame, it might be easier to lay the bike on its side on some blankets
    and drop the engine out the side rather than trying to hoist it out
    sideways with the bike upright.

    If cost is an issue you might find it to be cheaper to buy an entire
    wrecked bike with a good engine, and selling the remaining bits
    on ebay, you might even end up making money on the deal.
    Mark Olson, Sep 20, 2007
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  3. Heh. Told you so!
    Getting a 750cc engine out of a frame is generally a two-person job,
    because of the weight of the thing.

    Getting it back in is worse. An old dodge is to lay the engine on its
    side, on a stout box, and lower the frame, also on its side, over it,
    but with a bike that's in one lump (as opposed to just a bare frame)
    that's often just as awkward - you need to remove all the body work,
    battery, and anything that might get damaged or dinged.

    Basically, the job can be done with ordinary tools, but it *is* a lot of
    hassle. Just getting the carbs off and back again may make you scream.

    And, of course, you have no idea whether the other engine has the same
    ailment - it's a common one on those lumps, as I think I said at the

    My advice would be to get rid of it and start again.
    The Older Gentleman, Sep 20, 2007
  4. Andy

    fweddybear Guest

    I've removed a 750 from its frame to do repair work....but that was on a
    79.....I had to take out a piece of the was just a couple of
    bolts and then the engine would come out the right side....and yes, putting
    it back in was a bit trickier, but all in all, it was a simple job to
    do....not much wiring involved, but then again, its not a 91 either....but
    still shouldn't be that hard...

    fweddybear, Sep 20, 2007
  5. Andy

    Ian Singer Guest

    All I know is that with a Suzuki 650 engine I can just about lift it one
    inch off the ground without help. Walking with it was almost above my
    limits, but I had to get it out of the car and on to the ground so no
    choice. To lift and put in a frame and line it up I would need help.

    But I am small and weak.

    Ian Singer

    See my homepage at
    hosted on
    All genealogy is stored in TMG from
    Charts and searching using TNG from
    I am near Toronto Canada, can I tell where you are from your reply?
    Ian Singer, Sep 20, 2007
  6. Removing and replacing an engine may seem like open heart surgery,
    but, if you take your time and make notes and keep all the different
    hardware segregated in separate boxes, you won't have a dead patient
    on your operating table when you're finished.

    Got some good quality metric combination box/open end wrenches and a
    few good quality allen wrenches?

    6mm, 8 mm, 9mm, 10mm, 11mm, 12mm, 13mm, 14mm and 17mm are commonly
    used box/open wrenches. Honda tends to use smaller bolts with uneven
    size heads.

    5mm, 6mm, and 8mm allen wrenches will come in handy. Hardened steel
    allen wrenches are black, not chromed.

    A good quality #2 phillips screwdriver will be necessary.

    Gather several cardboard boxes to store parts in as you remove them.
    You can put the nuts and bolts into plastic containers.

    You can make a sturdy engine work box out of scrap lumber with the aid
    of a saw and a hammer and some nails. It's nice to have some old
    pieces of carpet to lay on the cement floor, if you're going to be
    laying the engine on the cement instead.

    The seat is the first thing that has to be removed and the last thing
    to be reinstalled. Stash it out of the way.

    Carefully remove the plastic side covers, and store them in a box.

    Disconnect the battery and its vent hose. You'll probably want to
    remove the battery and set it aside if you're going to lay the bike on
    its side to remove the engine.

    If you have an automatic vacuum operated petcock, you won't need to
    drain the gas tank unless the petcock leaks.

    Disconnect the fuel line and vacuum hose and any air vent hose and
    electrical wires coming from the gas tank.

    There are probably two bolts holding the gas tank down at the rear.
    Slide the tank backwards and set it aside in a cardboard box so it
    won't get scratched.

    The spark plug cables have to be disconnected and tied up out of the
    way and the crankcase ventilation hose has to be disconnected.

    The clamps holding the carburetors to their rubber hoses have to be
    loosened up so the carbs can be worked loose from the rubber hoses.

    You might have to remove the airbox lid and remove the air filter and
    set it aside so you can slide some of the rubber carburetor hoses back
    into the airbox to make room to remove the carbs.

    If you have to remove the airbox, some air boxes are a challenging
    puzzle to wiggle out of the frame. Stash the airbox in a cardboard box
    with whatever hoses and hardware was associated with it.

    You might have to spray the carburetor mounting hoses with aerosol
    carburetor cleaner to soften them enough to work the carbs loose.

    Some people say that hot water works to soften the hoses. You can
    probably connect a garden hose to your water heater if you decide to
    go the wet route.

    You will have to disconnect the choke cable if it goes up to the
    handlebar. If the "choke" is just a knob on the side of the carbs,
    leave it alone.

    You will have to de-adjust the throttle cable barrels to get enough
    slack to disconnect the cables from the carbs.

    If there is a gawd damned charcoal canister on top of the engine
    crankcase at the rear, that thing will have to be disconnected from
    the carbs and stashed in its own box. There is probably a decal under
    the seat or on top of the air box to show how to reconnect the rubber
    hose octopus associated with the canister.

    Look for color coded tape on the hoses or painted dots of different

    Once you have worked the carbs loose, pull the whole bank of four
    carbs out to the right hand side of the engine and turn the throttle
    quadrant between the #3 and #4 carbs so you can disconnect the ends of
    the cables from the keyhole slots they are in.

    Pay attention to how the throttle cables connect to the carbs, you're
    going to have to reconnect them and rig out the throttle cable tension
    after you reinstall the carbs.

    Set the carbs aside in a box with all the associated clamps and hoses
    that you've removed.

    You'll need to disconnect the starter cable from the starter and the
    ground cable that goes from the frame to the crankcase.

    You will have to remove the exhaust system and you can get into
    problems when you loosen the bolts holding the exhaust pipes to the

    If you strip out a bolt hole or break an exhaust pipe bolt, you'll be

    I recommend spraying the bolts with penetrating oil and and smacking
    the heads of the bolts lightly with a hammer and drift to set up a
    vibration to get the oil to penetrate if you don't have an air impact
    wrench available.

    Hang the exhaust pipes and mufflers up so they are out of your way.
    Watch for exhaust gaskets stuck up in the exhaust ports, you'll want
    to re-use them if they aren't damaged.

    The clutch cable has to be disconnected if it's a cable type, or, if
    the clutch is hydraulic, you can remove the slave cylinder from the
    side cover and tie it up out of the way.

    You'll have to remove the front sprocket cover to get at the front

    You'll have to loosen the rear axle with the tool in the motorcycle
    tool kit and de-adjust the drive chain tensioners so you can at least
    slip the drive chain forward off the countershaft sprocket.

    If you have an endless chain, you will probably just leave it on the
    swing arm, but if you have a clip type master link, you might want to
    remove the master link and the chain and stash the chain in a box.

    You'll have to disconnect the alternator stator connector and the
    pulser coil connector.

    If I haven't forgotten anything, you are now ready to loosen up and
    remove the engine mount bolts preparatory to laying the bike on its
    side and maneuvering the engine onto its right hand side, sitting on a
    sturdy wooden box.

    That's right, Andy, you've torn your Honda to flinders and you're not
    even half done.

    With the engine out of the frame, you can pick up the chassis and roll
    it away from the engine and then maneuver the old engine onto some
    kind of low dolly so you can roll it away to make room for your "new"

    I use a hack saw to cut the basket off an old shopping cart to make
    engine dollies.

    Good luck, and don't worry, be happy.
    Buteo lineatus, Sep 20, 2007
  7. Andy

    paul c Guest

    By coincidence, two days ago I took the engine out of a friend's 1982
    cb750. (I'm not sure how this differs from the 91 Nighthawk, the '82
    has four cylinders.) Used an electric impact wrench to make quick work
    of the hangers, but I guess I could have taken them out by hand. Carbs
    were already off. Took off the tank, airbox and coils because he wanted
    them off. I found the frame of this model is split on the bottom right
    side which made lifting the engine out fairly easy without hurting any
    delicate parts. I think it weighs about 150 lbs. Used a very small
    jack and some pieces of wood to brace it while removing the hangers, I
    think there were four altogether. Both of us lifted it out but i think
    I could have done it by myself, kneeling low and then sliding it on to a
    dolly. I was tempted to try laying the right side of the bike onto the
    dolly since I'd already removed the rear wheel and chain (no axle in the
    rear wheel because I'd used it as rotor puller and friend then misplaced
    it!), so at that point the frame and front end didn't weigh much.

    Took me about an hour and a half but I'm a slow poke. I'd never done
    this before but now I imagine I could swap engines on a complete bike in
    a day, assuming nobody has muddled up the wiring. But that doesn't
    include setting up those carbs which took me some hours over many days
    on another cb750 just to get it cold-start quickly and idle fairly
    smoothly. This included re-tapping some of the float bowl and other
    screws, I guess due to the carbs having been dis-assembled many times
    over the years. I'd say if you can do a leak-down test on the
    replacement engine and test the clutch and gears by hand and that stuff
    all seems good and if the wiring harness seems intact and compatible,
    the only question would be how well your carbs work with the replacement

    I wonder how long it would take a pro' mechanic to replace the starter
    clutch? Don't have a parts explosion so I'm not even sure if it can be
    got at without removing the engine.
    paul c, Sep 21, 2007
  8. Starter clutch is splined onto #7 which is the alternator drive shaft.
    There's no way to get the shaft out without major disassembly of the

    Alternator drive shaft is driven by a hyvo chain and there is a
    belleville spring-loaded
    cam-type damper on the alternator shaft
    Buteo lineatus, Sep 21, 2007
  9. Andy

    Andy Guest

    Thanks for the replies...sounds like I could manage the job over a
    week or so, and I'd certainly learn a lot in the process.

    I'm a bit confused about the carbs. Do they need to be essentially
    disassembled, or just disconnected? Would the replacement engine
    likely come with or without carbs attached?

    I've got a line on a engine with only 6k miles, which sounds pretty
    good...problem is they want 800 bucks for it, which is a bit more than
    I was prepared to spend. Does that sound like a reasonable price to
    you all? Also, is it normal for a used engine to come with some kind
    of warranty?

    Andy, Sep 21, 2007
  10. Yes, but that was the old CB750 engine/frame, and they had a removeable
    frame rail, as did the CB900 (and others, actually). The Nighthawk
    doesn't, afaik, which renders this advice useless.
    The Older Gentleman, Sep 21, 2007
  11. Oh for Christ's sake, the Nighthawk doesn't have this feature.

    We did this before, some time ago, and the OP says he's been quoted 1500

    Wake up.
    The Older Gentleman, Sep 21, 2007
  12. Disconnected. They come off in a bank of four.
    For a 6k miles engine, yes indeed.
    Some places do, some don't. If they're saying it's a 6k miles engine,
    I'd want a warranty.
    The Older Gentleman, Sep 21, 2007
  13. Andy

    fweddybear Guest


    I was afraid of that being the how do you get the engine out
    of the frame?? It sounds like there must be enough room to slide it in and
    out from the side.....

    fweddybear, Sep 21, 2007
  14. Andy

    fweddybear Guest

    I would check around at other engines and see what kinda price they want the price of 800 with others and determine if you think its
    out of line....have you checked with any bike bone yards?? That might be a
    good place to get a decent engine at a fair price....

    fweddybear, Sep 21, 2007
  15. That is exactly how you do it - and it's a PITA.
    chateau.murray, Sep 21, 2007
  16. On motorcycles that have steel tubing double diamond frames, you can
    usually unbolt the front motor mounts and remove them, but the rear
    motor mount usually doesn't unbolt, so you have to lift up the back of
    the motor and rotate the motor slightly and then push the motor out to
    the side. There are often steering head brace tubes directly over the
    camshaft cover which restrict how high an inline-4 engine can be
    lifted in the front.

    On motorcycles that have no lower frame crade, you remove all but the
    lower engine mount bolt and pivot the engine down to your engine dolly
    before removing the last bolt.

    Modern design trend increasingly uses the engine as a stressed member
    of the frame, with the swing arm pivot in the rear of the crankcase
    and some designs even bolt the steering head to the top of the front

    Suck designs require the whole motorcycle to be torn apart to get the
    chassis parts off the engine, rather than get the engine out of the
    Buteo lineatus, Sep 21, 2007
  17. Andy

    Mark Olson Guest

    "It certainly does suck"
    Mark Olson, Sep 21, 2007
  18. Andy

    paul c Guest

    so just how many hours was that?
    paul c, Sep 21, 2007
  19. Refer to previous posts on the subject.

    But count on a dozen at least, unless you've done so many of them that
    you can now perform the task in your sleep.

    Remove engine - which means, obviously removing exhaust, bodywork, all
    umbilicals, carbs, footpegs, gear lever, brake pedal, etc etc.

    Strip engine down to crankshaft.

    Replace chain.

    Rebuild engine from crankshaft upwards.

    Replace engine.

    Don't forget to add cost of complete gasket and oil seal set as well as
    cost of knackered chain.

    These engines are notorious for this problem, which first surfaced in
    the Honda CBX750 (the Nighthawk uses the same engine).
    The Older Gentleman, Sep 22, 2007
  20. Andy

    Andy Guest

    Ok, so I have got the original motor out and have another one ready to
    install! I only stripped allen two bolts in process so I am pretty
    satisfied with how that went. I have everything in boxes and baggies
    labeled and ready to put back on.

    Last night three of us spent over an hour trying to maneuver the new
    engine into place in the frame and could not get it in. It always
    seems to get hung up somewhere, and it's too heavy to pull any
    complicated maneuvers to get around the hangups Is there some trick I
    am unaware of that would help? It's like trying to put in a 200 pound
    puzzle piece. I'm considering an engine hoist, but they seem to be
    more designed for dropping an automotive engine into an engine bay/
    hood, so I'm not sure if that would help or get in the way.

    I know the motor is identical except for the age and milage, just to
    get that out of the way.

    Andy, Oct 1, 2007
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