No spark? Checked CDI, plug and magneto.

Discussion in 'Electrical' started by Ollie, Feb 26, 2016.

  1. Ollie

    Ollie Member

    Hey y'all, does anyone know what's up with my engine? Recently it made a strange sound then stopped working. When I try to start it, it seems like it has no spark. I have another working MB so I've been able to swap out components to check if they're faulty. So far, the CDI, spark plug, magneto, carb, pipe and throttle are all fully operational.
    However, the engine won't start. When I try to jump-start it, it seems to be cycling air so I assume the piston is working properly. My wife reckons it could be a loss of compression from somewhere that's causing it not to spark, but I'm wary of tearing apart the engine until I know what I'm looking for.
    Any ideas?


  2. skyash

    skyash Active Member

    If you put the plug on the out side of the motor and turn the motor over do you see spark? Think I have had a week spark and it doesn't work. Or no fuel? If you herd a noise mybe the rings broke I think it won't start if thay do go. If there is a big air leak it won't start. So maybe spray some wd40 on the motor when you are trying to start it not every where you don't want to much so you can't see where its leaking.
  3. danlandberg

    danlandberg Member

    Air leak or broken rings will not affect spark. With plug out and grounded against the head or frame, pedal with cluch lever out (engaged) to turn the engine over as fast as possible. Watching for spark. I have a center kick stand, so it's pretty easy to do. If no spark, it's probably the magneto. (I've had more trouble with them than CDI's) My NO spark problem was the magnet It self. (One lost strength) √ all components with a multi meter. There are several threads on here that have the procedure.
  4. HeadSmess

    HeadSmess Well-Known Member

    unless the wife works on race cars or something in her spare time, ignore her. and given her opinion...she doesnt :D

    No (read it, its NO) loss of compression due to ANYTHING will cause a loss of spark.

    well, you have checked basically everything and still no spark?

    strange noise + no spark, sorta sounds to me like you may have lost/snapped, etc the woodruf key that holds the magnet in place on the crankshaft.

    if its just free to do what it wants and isnt locked to the crankshaft...sure, youll get no spark.

    you should notice if anythings up when you remove/swap the magneto.

    the other suggestion made above...could have lost its magnetism...unusual, but not necessarily impossible. ie, check it.

    you gotta turn them over pretty fast... usually i just hold the rear wheel off the ground and give the pedal a good kick...

    there is the possibility that the head isnt earthing itself to the rest of the engine.
    Last edited: Feb 28, 2016
  5. crassius

    crassius Well-Known Member

    hold spark plug in your hand while turning the motor - if you feel nothing, you have no spark & can work from there

    if you feel the voltage, then you need to check fuel and compression (unless timing somehow changed)
  6. danlandberg

    danlandberg Member

  7. danlandberg

    danlandberg Member

    Plug in open air (out of head) this is a no compression situation. Compression has nothing to do with a NO spark problem. No compression gives a NO start problem.
    If all ignition components are in good working order, you WILL have spark, compression or not.
  8. danlandberg

    danlandberg Member

    Strange noise, then NO spark, sounds like the magnet came loose. If the magnet grounds on the magneto, you will not have spark. √ the woodruff key that holds magnet on crank in its proper timing position.
  9. Ollie

    Ollie Member

    Thanks for all the suggestions guys! The engine is no longer on the bike, as I used it's working magneto to get my other dead engine running (water ingress killed it) and installed that instead.

    I had previously looked for a spark like you suggested and didn't see one, but maybe I wasn't turning it over fast enough. I'm not sure how I can check it again without putting it back on a bike (which I don't have the patience for), so I'll open it up and have a look at the piston rings.

    It's my oldest engine and noticably more powerful than the others, so I'd love to get it going again.
  10. HeadSmess

    HeadSmess Well-Known Member

    yeah yeah, my keyboard tends to miss the odd letter here and there...i meant "No loss of compression, due to anything, will cause a loss of spark..."

    sheesh, one letter...cant you figure out what i meant?

    richard... ;)
    Last edited: Feb 28, 2016
    Timbone likes this.
  11. HeadSmess

    HeadSmess Well-Known Member

    remove the plug and wack the good old electric drill on the crankshaft ;)
  12. HeadSmess

    HeadSmess Well-Known Member

    tick the woodruff key?:oops:

    square root the woodruff key?:rolleyes:

  13. danlandberg

    danlandberg Member

    I know what you meant. Your still WRONG! compression or lack of, has nothing to do with spark or lack of. And the square root symbol is as close to "check" as this phones keyboard allowed.
    And you don't need to call people names.
    That's being a "Richard".
  14. HeadSmess

    HeadSmess Well-Known Member

    um...maybe someone else can confirm this...

    thats exactly what i said, wasnt it?

    "no loss of compression will cause a loss of spark"

    i didnt know there WAS a symbol meaning "check"? so why not just spell it?

    ha ha...richard... ot me there...



    Timbone likes this.
  15. Ollie

    Ollie Member

    Well, I broke down the engine using Al.Fisherman's handy guide and found the culprit:

    Attached Files:

  16. Ollie

    Ollie Member

    Looks like a piece of piston ring broke off and scored a line down the piston, causing a loss of compression. I'm going to order a new piston and rings. There's also this thing, that looks like it's been damaged. Maybe it was a piece of this that caused the damage?

    Attached Files:

  17. crassius

    crassius Well-Known Member

    that is the upper rod bearing and that means getting metal pieces in the bottom end even if the rod is still good

    fixing and checking that is more work than most folks want to do, with a chance that it will blow out pretty quickly once put together again
  18. Ollie

    Ollie Member

    Now that I have a working bike with my other engine, I'm prepared to fully tear this apart and replace whatever I need to to get it fixed. I'm guessing I should disassemble the bottom of the engine and check for any pieces of the upper rod bearing?

    Why is there a chance of it blowing out again? Is there anything else I can replace or modify to stop that happening?
  19. crassius

    crassius Well-Known Member

    metal of cases can be warped a bit by a hard stop, crank can be knocked out of true during the process of opening/closing cases if the bearings get tight - not to say it isn't an interesting project for the hobbyist, but can be hard to get right

    new mains might help in case some small bit of debris is hiding behind the bearing cage
  20. Ollie

    Ollie Member

    It didn't have a hard stop, more like a fizzling-out, but I'm a bit wary of doing it now. I appreciate the warning, I might just replace the piston, rings and bearing, then wait until the next thing goes wrong :)

    What do you mean by 'mains'?