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The Heart Is Out Of The Beast - CB250

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by Ewan, Apr 21, 2009.

  1. Hi everyone. I have jumped in and started working on the first stages of my bike rebuild. I am a complete bike newbie (L plater) and have just lowered out the engine. Now I am at the point where i need to start fixing sh!t and put it all back together and, to be honest, I am feeling a little apprehensive.

    Because this is my first engine rebuild ever I will be learning alot as I go along, however, I am trying to get a bit of a shopping list together of what parts I might need for a CB250 motor rebuild. Could you all help me compile a short list of what I will need to rebuild the CB250 motor.

    The spark plugs were all oily and the bike was hardly running so I am anticipated quite a worn engine. Could someone maybe give me a heads up of what I am install for or some things I can look out for?

    The bike is 1979 CB250N Superdream done 86,000 kms.

    Thanks in advance for any advice.
  2. New piston rings (maybe pistons too), spark plugs and head gasket are my bet. You'd probably also want to get a new balancer chain and cam bearings at the milage its done, they're apparently a bit notorious for eating those.

    While you're at it you may want to change the bottom and top end bearings, as this will smooth out the engine as well as increase reliability and expected running life.

    You may need a rebore depending on the condition of them. Scratches equals bad.

    Make sure you take pictures as you go (undoing part, holding part next to where it should go back into, part on bench with numbered tag). This will help you put everything back together in the right place should you mess something up. Its saved me a few times, and is dead easy and quick. A friend or family member who knows what they are doing would also be pretty handy, as would a workshop manual.

    If that sounds daunting, think of the good news; after you're done you'll have a new bike and know it down to a tee!

    Hope all goes well - boingk

    EDIT: You'll also want to get a carby refurbishment kit or two, depending on how many there are on the bike. Take one down to the local parts store or wrecker and see if they have something that will tide you over. Stock jetting to start with, then tune until bike runs well. You may want to just clean the current jets, in which case just get a fine needle for 'digging' and some very fine pipe-cleaners or wool thread (plus needle) for polishing. Failing that, go pro and get a set of jet-drills - tapered hand drills you use to take the jets back to metal and remove decades of unwanted buildup. Jets are clean when you can see the gleam of metal inside them.
  3. I agree. A workshop manual is a *must-have*. Even if you've pulled things apart before and know how things go back together, there's all sorts of specs you'll need from timing (cam and ignition) settings to fluid levels. It's hard to know what you'll need until you know the condition of the parts you have. You may have excessive ring wear, and you may also have badly seated valves. Maybe the clearances hadn't been set for so long the valves weren't closing properly? (can't remember if this motor had shim adjustment or rockers??)

    Anyway. Don't be scared. It's a fairly basic motor-design. Get yourself a good manual, and go for it. It's very rewarding once you fire that puppy up again.

    keep us in the loop.

  4. 10 points for bravery, and extra merit points for understanding that you MAY have bitten off more than you could chew, at least to start with :LOL:.

    now, grasshoper, if you point your browser of choice to here http://www.bikebandit.com/oem-parts you should find microfiche schematics of all the parts you will need. Take note of the numbers, and start asking your way to the local Honda dealer

    {You might also check if there's a Honda CB250N Superdream forum somewhere on the interweb as well...}
  5. ah... go forth my friend..
    bearings, rings, gaskets, hone (maybe re-bore) cam chain + tensioner, carby kit, plugs, filters.
    workshop manual
    and yes.. digital camera to take location pics. clean jars for bolts for certain parts and write on the jar the location.


    save the beer till after it has been running for a few hundred km.
    keep us informed and call out if you need help
  6. Good call on the torque wrench. The workshop manual will call for different torque settings for tightening the various bolts when you reassemble the engine. These are specified so that the bolts will do their job without risking coming undone or being too tight. The torque wrench itself is just an item that tells you how tight you are doing up a given item, and should last you a lifetime if you buy a good one.

    You'll also want a big can of WD40 and a cake of Solvol soap; the former for loosening stuck bolts, lubricating and degreasing, the latter for cleaning your hands effectively of said grease.

    Have you stated doing anything to the bike yet?

    - boingk
  7. and wrap a rag around your head so when you scratch it you wont get grease and oil in your hair..
  8. hair?? surely it will have all been pulled by the time the project's finished :wink:

  9. good one.. will be quite the journey..always fun though
  10. What i find works a treat, is dish-washing detergent... its meant to fight grease and oils... I put some on to dry hands, give a good rub in till its all dry on your hands too... then slowly introduce water as you continue to rub your hands together until clean :grin:
  11. I use laundry powder for the same reason. Seems to be a bit more aggressive than liquid soaps. It can be a bit nasty on those skinned knuckles tho' :roll:
  12. yeah works a treat but not good on engine re-building cuts...worth having a small tub / bottle of hand cleaner for this one and then any left over is always handy. 'fast orange' by permatex is a good one
  13. How is the clutch?

    Because these were such a common learner bike a lot of them have been passed from one learner to the next and have copped a fair bit of clutch wear.
  14. 5L of fuel
    Box of matches

    Will need
    New rings (re-hone standard bore if good)
    Cam Chain
    Cam chain guide (2-off from memory)
    Cam Chain tensioner
    Cam bearings

    May need
    Camshaft (get yours reground)
    Small end bearings

    Bottom end bearings should still be good in this motor, other things die first (see list above).

    Good luck separating the cylinders from the head. Use the leverage points provided.
  15. Thanks so much for all of your encouraging replies. I haven't been back on the thread for a while as I have been trying to make some room in the shed to pull this motor down.

    I am armed with a workshop manual and a torque wrench and hopefully all the other tools I will need to tame this beast.

    The frame is all stripped down and degreased almost ready for paint. The motor will be stripped down next week (hopefully).

    It seems the previous owners had tried everything to get this thing running properly (changed oil/ plugs/ leads/ air filter/ cleaned carbs) but ignored the fact the plugs were getting a thick coat of oil :?

    I am now working through some of the cosmetic problems the O.G. superdream has. :LOL:
  16. Hi Ewan.

    I currently own the same model bike as you. I have put a cb400n engine in it, but I have pulled down the engine twice now (and the 250 and 400 are the same, and have to do it a third time (due to being sold the wrong pistons, but that's another story). They are quite a easy engine to work on once you get the hang of things.

    Two things I will tell you:

    1: If the engine seems mostly ok, but is just blowing a little smoke or seems low on power etc, you may get away with just replacing the following:

    Valve stem seals, pistons, rings, small end bearings as well as a valve grind. you will also need the following: head gasket, base gasket, two oil rings between the barrel and head. Also need a hone or bore done too the barrel.

    If you have done alot of kms (100000 or more) and the cam chain is noisy then that will be replaced and to do that you have to separate the crankcase, in which case you may as well replace the big end bearings, main bearings and you will need a crankcase gasket as well. It will probably cost you about $700 in parts for the full rebuild or about $400 for just the top end. Also check your camshaft for pitting as it may need a regrind.

    You can go as far as you want, but if you can get away with just a basic top end rebuild you should get change out of $400 and it will be a cheap rebuild, plus it's probably not worth doing a full rebuild unless you plan keeping her forever.

    2: If it seems to hard or you are struggling, you are probably going about it the wrong way. I have learnt this the hard way, so don't over tighten or put too much force on a bolt etc. Especially with these old bikes, things can be a bit more brittle, and worn and break easily.

    If i can be of any help please let me know or email me (damo03@hotmail.com) as just today I pulled the head off the 400 engine and had it back together in 5 hours, so I am quite familiar with this engine.

    Good luck mate.

    (by the way, the cam's dont have replaceable bearings so don't worry about that), also your cam chain guide & tensioner may not need replacing, just assess it when you have it apart, the service manual will have all the service limits for tolerances.)

  17. Damo, mate, that is exactly what I needed to hear. Thank you. Don't be surprised if you see a PM from me as I am going to start work on Monday.

    How does the cb400 go? Is it worth doing the swap?

  18. Mate, the cb400n goes heaps better than the 250. It makes cruising at 100 enjoyable (with plenty of speed left) and its much safer in my opinion as it's less critical to have the revs right up to get some power out of it. It makes getting out of dangerous situations much easier and taking off at the lights is much quicker, even than alot newer 250's. It's quite amusing to see them ringing the throttle on their 250s and you can just fang past them.

    If you haven't bought any parts and want to get the most out of your bike in terms of acceleration and enjoyability I would try to get hold of a 400n engine and bolt that in. They are a direct swap over, you will need 400 carbies as well. (I just bought a whole cb400n bike cheap)

    The 400n also has a higher rated speedo and twin disc front end. The cb400n bikes come on ebay a bit but you might have to rebuild that engine when you get it. Mine lasted 6 months before starting to blow smoke, and I will hopefully have time on saturday to finish the rebuild and start the girl.

    One thing I should mention, regardless of wether you rebuild a 250n or 400n is that when you order parts make sure you state the serial number of the engine (the number after cb250n on the engine case) as there are about 3 different types of barrels / pistons / rings etc depending on your serial number. Some bike shops wont ask for this and will just order the first part number that comes up. I have found this out the hard way.

  19. I will continue rebuilding the cb250 and look out for a 400. Cheers. It took me all of Monday to get the caked on grease and road gunk from the bottom of the motor. Yesterday I got prepared to take cylinder head off before work. Today I will get the head off.

    Wish me luck that I don't break anything.