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No Spark, won't start. Bad earth?

Discussion in 'Technical and Troubleshooting Torque' started by chicano, Jun 23, 2012.

  1. Hi Guys,

    I have a CB400n that I have been restoring after picking it up from a guy who had it sitting in a paddock for ten years. I've had the bike running and revving nicely after new battery, plugs, carb rebuild etc

    Now for some reason the bike wont start. The starter motor turns the motor over, fuel is getting in etc. When I pull a plug out and hit the starter there is no spark. But if I rest the plug against the engine it sparks.

    Does this mean I have a dodgy earth somewhere? Or is it another problem?

    Any help would be greatly appreciate as it is driving me up the wall.


    PS Sorry, I think I posted in the wrong section by mistake. If an admin could move it that would be greatly appreciated.
  2. Sounds like you have spark, I`d say Back to fuel system
  3. the spark plug will only spark if it is earthed out like you did when you had it on the motor .so the next thing is to check that you have fuel, how old is the fuel?? have you tried to use the choke ?? is the choke working ?. are your spark plugs wet oily ? do the spark plugs smell of fuel after you try to start the bike? it may be fludding the carb and check you have the right gap on your spark plugs .allso with the spark plugs out place your finger over the plug hole and crank it over and see if you have good compression. let us know how you go after checking these things
  4. Moved your thread.

    +1 to tiprat and morbosity
  5. Thanks tiprat,

    Will try all those things and get back to you. Is getting fuel, new fuel, choke is working. So will try the other things.

    Greatly appreciate it!
  6. Ok,

    So today's progress. Got the bike started after playing with the carbs, getting the choke to adjust smoothly etc

    But as soon as the bike starts, it just starts to rev all the way to redline without any throttle applied. Even if you give it no choke it starts beautifully, runs for about 2 seconds and then the revs just build on their own and you have to shut it off.

    I don't have the mufflers on at the moment, so could this lack of back pressure cause this or is it something else?

    Thanks again for all the help
  7. no muffler will not cause this ...ok ..now first thing to do is check that your throttle cable is not hung up this means that the throttle is closeing .does your throttle feel smooth when you move the twist grip, does it snap back when you let go of the twist grip?? pull your air filters of and look at the slides in the carbs .are they all the way down and even if so it might be the floats are not stopping the flow of gas and it is flooding,,, some times the floats get holes in them and they do not float up to stop the feed of gas , may need to adjust the throttle cable ,do you know how to do this ??
  8. mmm, did you also change the oil, etc?

    anyway, I'm wondering what the compression is like after ten year's neglect?
  9. Done an oil change, compression is good.

    Throttle is working well and adjusted, snaps back nicely. But I think tiprat is on to something with the floats and it flooding. Might pull the carbs off and check the floats. Floats themselves were not changed, so am wondering if they have holes in them like you said.

    Will do that tomorrow and get back to you all. Sincerely appreciate the help.
  10. ok, in the few seconds it runs and immediately after, have a sniff of the end of the exhaust; if it smells of raw fuel, you are probably right about float levels....
  11. The throttle cable is caught up somewhere or the throttle stop screw needs adjusting.
  12. That was my first thought ibast, but definitely not the case. Seems to be the fuel bowls are flooding.
  13. Cleaning out the float bowls (float needle seat is critical) and setting the float height will do good, but usually if it's flooding, then the engine will run rough, rather than rev it's tits off.

    but carbie service = good
  14. Given what I know of CB400Ns my first guess would be that the fuel tank is full of rust, flakes of which have filled up your carbs. The carbs may very well have serious internal corrosion of their own.

    Carb strip time I'm afraid.
  15. Will pull the carbs off tomorrow and strip them down and report back. Cheers guys
  16. Pulled the Carbs down and you were right PatB, tank had rusted a little and flakes had got down into the carbs. Fixed tank, fixed up carbs including resetting float level and a good clean.

    Try to start it again and still it revs its tits off. Pull the plugs for a look and nothing too obvious, they were brand new, so chucked the old plugs in and....

    Bike starts and runs smoothly. Check the numbers on the plugs and the dealer had sold me the wrong ones. So they will be going back and hopefully it is sorted.

    Thanks for all the help, but looks like the main cause was the plugs. Should have checked them myself but thought the dealer would not make such a simple mistake.