Honda MT/CR125m frankenbike will not start.

Discussion in '2 smokers' started by RustyStuff, Oct 6, 2020.

  1. RustyStuff

    RustyStuff Long timer Supporter

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    I drug this project out from the back of the shop where it's been ageing for the last 4 years.

    It's a honda MT125 frame and running gear with a bunch of CR125m parts on the motor. Freshly machined '75 CR125m Jug, wiesco piston, CR125m carb, CR125M CDI, CR Pipe, CR shocks, rebuilt the forks, etc.

    I got to ride it once, for about 1 minutes back 4 years ago and then it stopped running and I could never get it to do anything ever again. I spent a ton of time troubleshooting and I was so fed up of working on it that I just covered it up and put it away.
    I
    I've got good compression, I've got spark, I've got fuel. I've moved the timing plate around every which way and it. just. doesn't. do. anything. I've got a sealed crank case with good seals. I've checked the coils on the CDI's resistance, they were fine. The only thing I can think of is the CDI control box has a malfunction of some sort but I've got spark at the plug?

    I'm pulling whats left of my hair out. Other than just shotgunning parts at it i'm at a loss.
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  2. Donkey Hotey

    Donkey Hotey De Jo Momma

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    Another inmate had a similar problem with a CT90 maybe a year ago. I can't remember if it was a bad cap or bad plug but, one of them should have been good (thought it was good / came from running bike) and wasn't. It sparked when it was out of the head and laying on the fins but, once installed, the spark found a nice path to ground somewhere outside the combustion chamber.
    #2
  3. RustyStuff

    RustyStuff Long timer Supporter

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    Huh. Interesting. I know the plugs good because it sparks on any plug I try. I'm using the MT dual plug head, I can switch between them and nothing happens.
    The boot not being any good is not something I had ever considered.

    It's just so weird though.
    #3
  4. Donkey Hotey

    Donkey Hotey De Jo Momma

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    Yeah, the boots can split or crack (if it's plastic). They sometimes end up with a small line of conductive dirt that's really hard to find but, enough that the spark will choose that path if it has to.

    Ditto for plugs. Sometimes they have a small crack that you don't see and the spark is jumping through there.
    #4
  5. RustyStuff

    RustyStuff Long timer Supporter

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    It really sucks that the only options for ignition parts are gambling on more 50 year old CDI stuff or spending like $500 on a new cdi setup from re-mx.

    I think I have about $600 into the bike and all the CR parts.

    I'm tempted to buy another used CDI box but who's to say it's any good either?
    #5
  6. Donkey Hotey

    Donkey Hotey De Jo Momma

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    Since this is a project bike: what was the source of the ignition box, ignition coil and pickup in the engine? Could you have mixed parts from a magneto system and a battery powered one? Or is this one entire ignition kit where everything is known to work together?
    #6
  7. 380Viking

    380Viking Adventurer

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    woodruff key may have sheared
    #7
    spokester and Donkey Hotey like this.
  8. RustyStuff

    RustyStuff Long timer Supporter

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    It was a complete takeoff from a 1975(iirc) honda CR125M I bought on ebay. flywheel rotor, stator, CDI control box and coil.Was supposed to come from a single bike, which I'm pretty sure it did.

    Key's not sheared, I just pulled the rotor/flywheel and reinstalled it 2 nights ago.

    No way parts are mixed up between the MT points system. Too much difference.
    Bike does not run with the point system, I tried it. Even when the engine was still all worn MT125 parts before I rebuilt it, it didn't run all that well.
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  9. Donkey Hotey

    Donkey Hotey De Jo Momma

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    If you pull the plug, put your hand over the carb throat and push the kickstarter through the stroke, do you have good suction on the intake? That eliminates crankcase seals, reed valves, etc.

    If that's all good, give a squirt of starting fluid, carb cleaner or even a syringe of gasoline down the carb throat. With the throttle wide open, try to start it.

    If you're getting suction (air) and fuel (which you just bypassed the carb and entire fuel system), the only thing left is ignition as the culprit.
    #9
  10. RustyStuff

    RustyStuff Long timer Supporter

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    Got suction, it will pull a piece of plastic wrap draped over the carb mount, into the carb. I've gone so far as to dribble fuel down the spark plug hole. Never fires.
    #10
  11. Donkey Hotey

    Donkey Hotey De Jo Momma

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    All that’s left after those is compression. I’ve been fooled by how hard it is to kick in the past. Unless you’re overly familiar, it’s hard to tell 70 PSI from 125. Do you have a gauge?

    Edit: I realize it’s a new top end but maybe a broken ring snagged on a port or something, etc.
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  12. RustyStuff

    RustyStuff Long timer Supporter

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    Rings are good. I had the head off when I put it away, so the first thing I did when I started pokeing at it again was put the head on and torque it . I can check compression again when I get a chance, but it's about as stiff as my KDX200 was with a fresh topend was. Takes a decently stiff kick. There no rings busted off or marks on the cylinder walls.

    yes it has all necessary gaskets, I even annealed the new copper head gasket I had in the kit. Base gasket isn't leaking because I coated it in a skim coat of yamabond when I assembled it and it hasn't been disturbed.

    You can see why this has been driving me nuts. All I can assume is that the CDI unit has failed but I have a pretty decent blue spark with the plug out. Losing spark under compression? You would think it would at least pop or something though?

    I wonder if shortening plug gap would help?
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  13. Donkey Hotey

    Donkey Hotey De Jo Momma

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    It ran for you at one point and suddenly stopped. If there are no mechanical problems (doesn't sound like there are), you're left suspecting spark, happening at the right time. The timing wouldn't have killed it back when it last ran.

    Just because we see a spark when it's out of the chamber, still doesn't mean that it will work in the chamber, under compression. Electricity is lazy. If it was a new plug, it could be infant morality of the new part. Spark plugs are cheap. I'd buy a brand new plug and try that. Or take a similar sized one out of another bike and see if it starts.

    It's also entirely possible that both the original points system and the replacement CDI could have bad plug wires or boots. It happens.
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  14. RustyStuff

    RustyStuff Long timer Supporter

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    I think I have a breakthough. 1- I think my old compression tester was faulty. As I remember it reading over 100 constantly. I've got a new one because I broke the old one a few years ago. It says 75psi.
    That leads to this,
    One difference between now and 4 years ago, is I have a chunk of a surface plate. I pulled the head off and I think I found a spot where the head isn't flat. I lapped it a bit on some 1000 grit on the plate and compression went up to 94ish and the mark on the head gasket after I put it back on leads me to believe a bit more lapping and I might get it to seal better. There's 1 spot that isn't makeing good contact right around the cylinder, it's making poor contact further out on the sealing surface.

    I wonder if when I got it to run that 1 time, that the head (which might not have been very flat to begin with and that wasn't a problem due to the MT headgasket being a very thick copper ring in a composite carrier that could make up the differences in tolerances, VS a thin flat copper sheet for a CR gasket) made just enough contact for that minute that it sort of ran, before it warped enough to lose enough contact, start leaking compression and cease running?
    The head gaskets not blown, the leak is definitely coming from the head.

    I got it to pop once on starting fluid. I think I'll lap the head some more and then coat everything in copper coat spray.

    It's interesting that the compression was so low, it takes a pretty decent kick.
    #14
  15. Strong Bad

    Strong Bad Former World's Foremost Authority Supporter

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    If you have compression, fuel, and spark about the only think that will prevent it from running is the spark at the wrong time. After trying to start it for any length of time, is the plug wet?

    Typically a head leak can be heard.
    #15
  16. RustyStuff

    RustyStuff Long timer Supporter

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    Finally had time to finish lapping the head. Annealed the head gasket again, gave it a smear of coppercote. Compression is 110-111psi now. Better but still low. Don't get it. brand new weisco piston, new rings, fresh bore. I checked it, piston is installed the right way. Rings aren't broken. New ngk plug set to proper gap, I put a new boot on the wire. Got spark, it's not the strongest spark I've ever seen but it's bright blue. fucked with the timing to see if possibly that makes a difference. Made no difference. Got a can of Starting fluid. Fogged that in.
    Not even a sputter after 5 min of kicking. I guess i'll order another cdi box off ebay some time.
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  17. RustyStuff

    RustyStuff Long timer Supporter

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    if you kick it long enough it will foul the plug with fuel.

    You can start with a new dry plug, spray some starting fluid in to the chamber, install plug. Nada.
    #17
  18. Donkey Hotey

    Donkey Hotey De Jo Momma

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    You're probably already doing this but, when you're using starting fluid: wide open throttle while you kick. You've already dumped fuel in and it only needs a ton of air to start.
    #18
  19. RustyStuff

    RustyStuff Long timer Supporter

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    Yup. Tried every combo of throttle.

    Wonder if the spark blows out like a candle under compression.
    #19
  20. Strong Bad

    Strong Bad Former World's Foremost Authority Supporter

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    I've had old school secondary coils fail and refuse to spark under compression. Pale straw colored spark from plug grounded to head.
    #20