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CM250 engine knock after stall

Discussion in 'Technical and Troubleshooting Torque' started by cactuar, Dec 12, 2011.

  1. Hi, a little while ago (about a month) i bought a honda cm250 might be an 84 i think but am not sure.

    heres a bit of info on the bike
    brand new sprockets and chain
    brand new gearbox
    engine has about 56000 kms on it and i've put about 2k of them on there myself
    when i got the bike it only had 1 muffler on it and the other was remove to make the bike sound louder (i bought 2 new ones to replace the missing one, and match the other side up, as soon as i could. i don't think i did too much damage riding it home, because the exhaust had a balance bar)

    the problem is this, when i was riding at around 80km without about 40km worth of fuel left in the tank (my bike has to rev higher then it should at that speed because of a sprocket that is 8 teeth larger then it should be)
    my bike lost power and died.
    rather then try and restart the bike on a busy high way at night ( was about 10pm and rather cold ) i pulled over and checked the chain, because the rear tyre had momentarilly locked up.

    after i pulled over and checked everything out i started my bike and i got a ticking noise from the engine.
    wasn't there before is now.
    i was wondering if anyone had any idea at all what this could be.
    when i got the bike the guy i got it from told me to use lead additive, which the bike mechanic i get my parts off told me not too because it could cause more issues then not using it due to cleaning the engine too much from carbon build up.

    so in short: honda cm250c 1984, stalled while at speed. gained a new engine knock
    cold night, plenty of fuel in the tank (bike has no taco so i have no idea what the revs were)

    thanks ryan
    ANY help would be greatly appreciated
  2. It does sound suspiciously like you may have had a mild seizure. Back wheel locking, even momentarily is unlikely to be good news. 56000km is well into territory where an engine that's been neglected at some point could have some issues.

    First off, although the rather eccentric exhaust arrangement is not to be recommended, it's unlikely, in itself, to be the cause of the stoppage or the noise. It does, however, say a few rather uncomplimentary things about the technical prowess of the previous owner.

    Secondly, an early 80s Japanese commuter engine should not need lead substitute. Again, though, the presence or absence of LS is unlikely to be the fundamental issue.

    When was the bike last serviced? On these little Hondas it is absolutely vital that the oil is changed at 2500 km intervals (or less) and that the crap is scraped out of their rather primitive oil filtration system as detailed in the manual. If this hasn't been done, engine life is severely compromised.

    If it did seize, you may be lucky in that it was a top-end seizure, which can be sorted without removing the engine and splitting the crankcases. A ticking noise rather than a knock does rather suggest this, although, without hearing it it's difficult to say for sure.

    On the upside, spares for Honda's commuter engines tend to be a lot cheaper and more available than those for their "proper" motorcycles and, IIRC, the CM250 is closely related to the later CB250 which is as common as muck so even a major mechanical disaster is not the end of the world.
  3. Camchain tensioner and camchain is worth looking at too.

    Japan Inc bikes of that vintage almost certainly run fine on unleaded, since the bikes were made for other markets that had unleaded before it came in here, but it shouldn't be hard to find the relevant info for your specific model.
  4. Agree, but I doubt if a loose camchain would lock the back wheel. A snapped one might but then the engine wouldn't restart afterwards. Or ever again, for that matter.
  5. True, but maybe the cam jumped a tooth or two.
  6. I have no idea when the oil was last changed, but when i did get the bike, i topped up the fluids because they we're lower then i was happy with.

    As far as I've been able to find i should be fine running unleaded.

    I looked up what the cam chain does, and i must say i am aprehensive to pull my bike apart to have a look at without first buying a manual. which is what i plan on doing. If you guys have any ideas what other information i can give you that would give you a better idea, please ask me. Thanks to the fact that this is the only vehicle i have access too that i can legally ride on the road i need to keep it in a running state as much as possible.

    just as another note, i have been slowly moving my rear wheel backwards as i stretch in my new chain, which is the biggest headache I've ever had to do.
    That being said i've gotten plently of practice at it so far

  7. OK, given that it's your only vehicle and that keeping it running is a must (I've been there myself so I have considerable sympathy), I would avoid tearing into it right now as long as it runs.

    Under the circumstances, I'd adopt a multi-step program.

    1) Buy a manual, which you appear to have covered.
    2) Change the oil and clean out the strainers (it doesn't have a filter to change AFAIK). The state of the oil that comes out can tell you much about the state of the engine.
    3) Perform the rest of a major service by the book, with particular attention to valve clearances and camchain tensioner adjustment (assuming it is not an automatic tensioner with no adjustment; ancient memory tells me that the small Hondas are not automatic).
    3) Start a hunt for a complete, running, used engine so that if the worst happens the engine can be swapped out. Much simpler and quicker than a major mechanical repair, particularly given limited facilities and novice mechanical skills.
    4) Keep ears open for any change in the noise and senses attuned to detect any drop in power. Either would indicate that the problem is progessing and attention is needed.

    Other than that, as long as it runs I'd continue to ride it until it protests.

    Actually, a couple of thoughts occur that are slightly less cataclysmic than a seizure. Was the weather damp as well as cold when it died? Cool, damp conditions can lead to carburettor icing. This happens because, when fuel evaporates, it pulls heat energy out of the surrounding air, reducing the temperature in the carburettor. This can drop the local temperature below freezing point. If the air is damp, water condenses out of it and freezes onto the interior surfaces of the carb. This can easily block the jets and cut off the fuel supply to the engine. If it happens suddenly enough, the engine could stop sufficiently sharply to momentarily cause a squeak from the back tyre. Once stopped, the ice would melt fairly quickly, allowing a restart. Wouldn't explain the sudden appearance of a tapping noise though.

    Alternatively, although it doesn't usually happen suddenly, a failure in the electrical insulation of the coil or the plug leads could certainly stop your engine (again, more likely in damp conditions) intermittently. High voltage electricity escaping to where it shouldn't go can make a surprisingly loud clicking noise. Run your bike somewhere dark and have a good look around the top end of the engine and under the tank. If there's a short, you should be able to see the sparks. Alternatively, if you've a reasonable pain threshold and no chronic heart conditions, use a bare hand to trace the plug wires back to the coil whilst the bike is running. If there's an electrical leakage you won't miss it :shock:.
  8. i believe the area when the bike stalled didn't have much moisture in the air, but it was only up the road from a running creek, which i did notice made the air considerably colder when i was near it.

    i really appreciate all the help THANKS

    another thing i found out is that the gear ratio that is on my bike is considerably lower then it should be with a 14/38 combo, which means for me to keep my bike at 80kms an hour i have to rev it higher then i like, when i do some more chain adjusting and finally get the slack out of it (from the stretching) i'm likely to get the rear sprocket replaced with a 30 tooth and get my chain cut some more, so it gets back closer to what it should be, which i worked out would get me around 35% higher top end speed, meaning i should be able to do 80 with quite a bit lower revs
  9. Are you sure the ticking is coming from the engine? Is it there stationary or mving? Clutch in and out?
  10. yes the ticking is coming from the engine, as it turned out, my bike had burnt all the oil, and i mean ALL the bike has an oil capacity of about 1.7 litre after filling it back up completely, the noise went down considerably, but i'm now really concerned about the engine damage i might have caused.

    it ticks whether the clutch is in or out, i've even put it on a stand and into gear, and it doesn't alter the ticking at all

    The place i go to get my parts does not have the facilities to rebuild my engine, but has a place they send people too. just for a top end job i'm looking at around 700 bucks.

    Right now i'm in a ride it till it goes bang then buy a new one stage, as i can't find any second hand motors in my area or bikes that i can take the engine from, because the ones i'm looking at are more expensive then my bike, and i might as well just ride those.
  11. Reads like a plan. It will probably go for a long time yet. Won't be worth much on the resale if you are honest withe buyer, but still cheaper than an engine rebuild
  12. when i get a new bike, i'll probably trade it in, as they'll probably be able to use it, for a rebuild or parts at least, as i've put a fair bit of new stuff on the bike already
  13. Small Hondas are remarkably resistant to engine damage from running with no oil. The bottom end of the engine will, generally, survive without major problems. Where serious damage occurs is the bearing surfaces in the cylinder head that the camshaft runs in. This is a death sentence for the head and replacement with a good secondhand one is the only economic solution. There are ways in which a good machinist can reclaim a worn out head but it's sufficiently involved that the labour costs alone would run you enough to buy a better bike.

    However, if your engine is still running and you keep the oil up together (clean oil, clean filters, keep a close eye on the level) it should stay running at least for a while. Maybe long enough to get your licence.

  14. i only need to have it last about another 12 months, because i don't have my car license and i live in sa, i can get my p1's in about a month, and when i go for my p2's about 12 months later, if i don't have any infractions on my license i should be able to get off a restricted license, or so i have been led to believe.

    Just as a follow up note, as soon as i filled the oil back up, the ticking noise became very intermittent, and may only tick for a handfull of seconds every minute, i'm hoping this is because the engine hasn't great oil flow, and that it will quiet out in a few days of riding, i don't ride every day. ( work sucks for me).

    just checked ebay on the cylinder head, well i'll be lucky, they do exist, and are out there.... the bad side is the postage costs more then the item
  15. Oil will help mask engine noise. You should get normal oil pressure back within seconds and the oil should distribute itself into all the places where it ought to be within a minute or so at the longest ( again if not in a few seconds). If you have the correct amount of oil in I can't see it getting quieter.

    Was the oil low before you rode or did the ride burn it all? Or is it leaking somewhere? Oil should not vanish so fast and if it does you have a serious problem. Even if you do no damage it will cost you a small fortune in oil, if it keeps losing it at that rate.
  16. i know what you mean about the oil, the good news is that it is indeed noticeably quieter, the bad news is that when i start it, it blows a very big f-load of smoke. it does get better, but i'm afraid of riding down my street and a cop pulling me over and defecting me, which would screw me completely, as you can't take it to a police station (in sa) to get the defect removed any more, and it would have to go over the pits
  17. Hate to say it but it sounds like your top end is thoroughly shagged. It's either piston rings or valve guide oil seals. Either is a head off job and it might not be worth the effort given the history of the bike.

    You really need to start looking for a replacement engine. You might not find a CM lump but AFAIK your bike shares 99% of its mechanical DNA with the more recent CB250, which are ten a penny. Other NRs more familiar with recent incarnations of small Honda twins may be able to advise in more detail. My own direct experience runs out with the CD200 of c1980.
  18. Given the smoke blows at start up, I'm betting stem seals.

    Compression test time.
  19. i've been looking for an engine, to replace it with, but no such luck so far

    i have noticed that i have lost a considerable amount of power from which it had when i origonally got it, i did get a quote on how much it would cost me to get the top end rebuilt, which was about 700 bucks, how hard would it be for me to remove the engine and replace the seals, gaskets and and valves?
  20. Not very hard at all. The little Hondas are good engines to learn the fine art of fettling Japanese motorcycles. Unlike some of their bigger offerings, they're dead simple and (mostly) made from decent material to logical designs.

    Doing the top end doesn't even entail removing the engine. Head and barrels should come out with engine in frame. You'll need a manual, a decent socket set (preferably 3/8" drive), a torque wrench (cheapo waggly wand type will do at a pinch) and a valve spring compressor. As for parts, budget for new piston rings, both exhaust valves and a gasket set (which should include your valve guide oil seals).

    Good luck.