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've got a blue spark when tested outside the motor.
    #21
  2. RustyStuff

    RustyStuff Long timer Supporter

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    I'm going to steal the 6v coil off my ATC90 engine I've got sitting around and pair it up to the 6v MT125 magneto to see if that does anything while I decide what to do. I think the coil for the magneto wasn't all that good as the plug wire is badly corroded.

    I messed around more. Pretty much 100% sure the CR125M CDI unit has a bad capacitor in it. I'm tempted to try and cut it open to check it out but I have a feeling that I would destroy it before I could get all of the epoxy potting out. But I don't think I have much to lose. I compared the spark from the POS atc 90 engine and the CR125M cdi unit. The ADI unit spark is nice and blue but when compared to a known working engine it's not very large, considering that the ATC 90 has a low voltage points system in it, rotten wiring, corroded points and came from a dumpster, that's no bueno .

    The biggest problem with the MT125 magneto is that there is no advance mechanism. Timing is fixed. So it makes crap power unlike the CDI that had an electronic advance system. I have no idea why honda hated this bike ( the MT125) as much but to make a "13"hp 125 with fixed timing out of a 20hp 125 race motor leads one to believe they hated it with a burning passion. Iirc magazine reviews found it made 3.7hp at the rear wheel stock. I can believe it, based on how gutless it was when I got it running in bone stock condition. It was about as quick as my puegoet 102 50cc moped but with more top end.
    #22
    Donkey Hotey likes this.
  3. RustyStuff

    RustyStuff Long timer Supporter

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    I've had an idea. I'm working though adapting a DIY chevy HEI module based ignition system to my CB650 to replace the failed factory electronic ignition. I wonder if I could use the MT125 points setup to trigger a HEI and coil. Would necessitate running a small 12v battery and charging it after riding as it would be total loss but I don't see why it couldn't work? Wouldn't have an advance mechanism but I guess I could time it to run best at high rpm and accept it not running well at low rpm. Would certainly give a hot enough spark to fire and I have all the parts I would need sitting on a shelf.

    Considering it's a no plate track only bike it might be a workable solution.
    #23
  4. Donkey Hotey

    Donkey Hotey De Jo Momma

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    If you went back to points, you could run any coil you wanted, no? Or were you saying you'd reinstall the points plate and use an HEI trigger to fire it as well?
    #24
  5. RustyStuff

    RustyStuff Long timer Supporter

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    Trigger the HEI with the points. The stock honda points magneto for this bike isn't very strong.
    I've got a new 6v universal points system coil in my shopping cart on ebay but I might hold off just to see if this works.

    I found some interesting info, a guy claimed on thumpertalk that he got this to work on a CR80 useing the trigger coil on a CR80 cdi to trigger the HEI. If that works it would give me the ability to run the much lighter flywheel for the CDI vs the heavy points one. I've got an external HEI coil and 2 modules from old distributors I can play with. It would still be fixed timing but there wouldn't be any doubt about how much spark energy it would have, guys on the CB650 forum were reporting being able to run plug gaps near 80 thou before the wires or caps didn't have enough dielectric strength to stop arcing.
    #25
  6. RustyStuff

    RustyStuff Long timer Supporter

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    So the HEI system running off the CDI stator totally works. Wired up the honda trigger coil to the module inputs, wire the coil up like normal to the module, hook power and ground to it in the normal way off a battery and it fires fat purple sparks.

    Bikes still a POS that won't fire. You can smell it trying to fire ( burned fuel smell coming out the exhaust) but no bang.
    #26
  7. Donkey Hotey

    Donkey Hotey De Jo Momma

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    Do you have an inductive timing light? Could you put a couple of tape marks where you could kick and point the light at them at the same time? Are we absolutely positive this thing is firing somewhere near top dead center?
    #27
  8. RustyStuff

    RustyStuff Long timer Supporter

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    I do. I'd have to get someone to hold it while I kick as they are on opposite sides of the bike. The timing mark on the case, stator plate and rotor all align properly with the piston hitting tdc though.
    #28
  9. RustyStuff

    RustyStuff Long timer Supporter

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    So a minor breakthrough. With the HEI i can just turn the rotor with my finger and watch the plug to see when it sparks. For reasons I can't understand with the CDI stator plate in it's proper orientation it's 180 out of time. It was firing on the bottom. The timeing marks on the rotor line up with TDC as indicated with a indicator.... So I flipped the stator over. It fires on top now. Got a few pops and fireballs out of the exhaust port but it didn't run on start spray. I'm going to tear the carb down tomorrow. I guess I'll extend the wires on the stator plate so I can have it come though the proper grommets.
    #29