Won't start... out of ideas

Discussion in '2-Stroke Engines' started by Yvan, Aug 6, 2014.

  1. Yvan

    Yvan New Member

    I've reassembled my engine after having to do major surgery.

    Lots of big fat blue spark.

    Good compression when turning over the engine. No leaks or hisses.

    Carb has fuel, full bowl, not flooding (unless I hold down float bypass button), solid seal -- no vacuum leak.

    I've tried every combination of full choke, half choke, no choke, no throttle, some throttle, WOT

    No sputters or hint of combustion whatsoever.

    What else is there?
    The rings are fine or I wouldn't get good compression.
    There aren't any valves.
    Clutch isn't an issue, because I'm trying to start it at the crankshaft (yes, tried the old fashioned way too).
    I've tried another carb -- both have worked fine in the past.


  2. butre

    butre Well-Known Member

    is your magnet on right? magnet on backwards usually won't start. your crankcase compression is good too, right?
  3. jaguar

    jaguar Well-Known Member

    I second butre's suggestion of checking the crank seals.
    that's also very true about the magneto.
    once you get the engine too wet it just won't start. You may even have a pool of fuel in the crank.
    take the plug out and push start it. but the ignition has to be dead to keep from damaging the high voltage coil. disconnect one of the wires going to the CDI first. that will dry out the engine a bit.
    then put the plug back in and hold the throttle wide open and push start it and have a friend spray in starter fluid into the filter while the engine is turning over. best to do this on a downhill. once it starts sputtering then return the throttle to a minimal open position and see if it keeps trying to run. then see if its better or worse with the choke on. (make sure you are certain which way the lever moves to choke it)
    Let us know what happens
  4. Yvan

    Yvan New Member

    Thanks for the tips. I don't think it is the magnet, because the engine has run fine in the past, and I haven't touched the ignition system.

    I removed the head over the weekend and everything looked OK, but I guess there is no way of telling if there is fuel in the crank without splitting the case, so I'll try drying it out like you suggest and then trying starter fluid.
  5. Yvan

    Yvan New Member

    Still no luck :confused:
    • I spun it quite a bit with the plug out and the carb off to dry it out.
    • Then I let it sit up-side down for a few hours.
    • I checked the magneto. It is a 1 o'clock at TDC.
    • I regapped the plug. Gobs of spark.
    • I removed the head/cylinder and shot lots of compressed air in the bottom of the crank.
    • I tried starting fluid.
    Not even a sputter.

    It's like gas stopped being flammable. Oh, I tried fresh gas too.

    I noticed that the spark plug boot leaks a little. If I hold it, I get little shocks that feel like sliding over a carpeted floor and touching metal. I'll fix that, but it doesn't explain anything.

    I have to say, I don't believe peoples' descriptions of spark plugs that fire outside of the cylinder but not in it. I mean, physics doesn't change in the dark; but something is weird.
  6. jaguar

    jaguar Well-Known Member

    with pressure it takes more voltage to spark the gap which is why it can spark outside of the engine but not in it.
    Disconnect the stator coil wires from the CDI and measure the AC voltage when pushing the bike. It should be at least 30 volts. If not then replace it.
  7. Yvan

    Yvan New Member

    Hmmm. I'm getting about 26VAC under load; 75VAC no load.
  8. Yvan

    Yvan New Member

    How does one measure/evaluate crankcase compression?
  9. Yvan

    Yvan New Member

    What gap seems to work well with the stock plugs?
  10. crassius

    crassius Well-Known Member

    one can judge the integrity of the case by examination of seals and all the seams of the case - anything crooked or seeping oil may be causing a loss of compression

    gap at .028in is usually good

    I'm beginning to suspect that "having to do major surgery" may be the cause - do you have a repair guy in the area?
  11. butre

    butre Well-Known Member

    I always gap at .035
  12. jaguar

    jaguar Well-Known Member

    with a reed valve intake most of the pressure in the crank is positive but with a piston port intake there is alternating positive and negative pressure in the crank making it harder to test. You can try spraying the outer part of each seal with soapy water while slowly turning over the engine and see if you get any soapy bubbles coming out from the seal area.
  13. Yvan

    Yvan New Member

    I did the soapy bubble test when I reassembled after I took the head off. Air-tight.

    My gap was wider -- probably .040 but I tried 0.7mm (~0.235) and it didn't make a difference.

    As far as the major surgery went, it was all clutch-related; nothing that affected the crank-side. Head disassembly confirmed that everything on the crank-side is where it should be and rings seal well.
  14. jaguar

    jaguar Well-Known Member

    Did the bike starting problem begin after working on it? If so then think hard about what error you may have done.

    You said that now it has good compression but you didn't state a number which means you are guesstimating based on how it feels when you push it. Seat of the pants judgements are only good from experts. Get a true compression gauge and see what the pressure is.
  15. Yvan

    Yvan New Member

    Thanks jaguar. I'll definitely bring the compression tester to the shop tomorrow to find out. Even if there was poor compression, I would expect some combustion to take place.
  16. butre

    butre Well-Known Member

    I've had engines you could turn over by hand fire up just fine, just with hardly any power. there doesn't have to be a lot of compression to fire up, just some.
  17. jaguar

    jaguar Well-Known Member

    yeah but that depends on timing and jetting
  18. HeadSmess

    HeadSmess Well-Known Member

    tried a new spark plug yet?

    test that its producing spark when held against the HEAD itself, too. all part of the circuit.

    easy way to test crank case compression...pull the spark plug, turn her over. there should be a distinct "pop" as the transfer ports open. bit of oil down the hole helps it seal and makes for a better pop. should be able to feel the secondary compression too, even if it is only slight.
  19. Yvan

    Yvan New Member

    I bought some new spark plugs. NGK in a few sizes and temperatures. No luck.

    Compression is ~100-110 PSI.

    I've ordered spares of everything since I've had no luck yet.
  20. Yvan

    Yvan New Member

    Got my parts in 1 day, thanks to MotorizedBicycle.ca (that's twice they have impressed me).

    The first thing I did is replace the coil. It runs like new.

    I suspect that the issue is the ]COLOR="#FF0000"]wire[/COLOR], not the coil itself. I mentioned early on that I could get a very mild shock by holding the boot. Yet the spark was nice and bright blue. This indicates to me that the coil was producing sufficient voltage, but the wire had too much impedance to deliver the required current needed for spark under compression.

    I'll measure the impedance and post the numbers of the two coils for future readers of this thread.