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Louie, Technician
Category: Motorcycle
Satisfied Customers: 12571
Experience:  25+ yrs. experience
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I have a 79 kz400 i get one or two rides out of it until my

Customer Question

i have a 79 kz400 i get one or two rides out of it until my spark plugs are no good, they get really black and i have to swap them out, i figure its a getting too much gas how could i fix this issue
Submitted: 1 year ago.
Category: Motorcycle
Expert:  Louie replied 1 year ago.

Hi Daniel and welcome to JustAnswer,

Thank you for the question and your patience. Should you still need assistance for information purposes only...

Initially, proposed preliminary tries (if have not been done):

  • re-check the burn color of the spark plug's inner ceramic tip;
  • - if black/sooty, then rich and/or vacuum leak issue @ the intake/after the carb;
  • - if oily, then an internal engine problem and consider having a compression check or better yet, a leak down test;
  • • check the battery voltage;
  • - at least 12.7V when idling;
  • - but not to exceed 14.5V ~ 15.0V when revving
  • - if overly high, try/run with the regulator temporarily disconnected.
Customer: replied 1 year ago.
Thanks louie ive already diagnosed this as a gas/intake problem im just wondering how i would go about fixing it is it simply matter of jetting my carbs?
Expert:  Louie replied 1 year ago.

If the idle is relatively high and/or can not be adjusted/lowered, then most likely a vacuum leak; i.e. at the intake manifold coupling/carb holder possibly cracked and/or loose clamp.

On the other hand, if the idle is normal, then for a KZ400 Standard either:

  • jet needle is set too high (most likely);
  • primary/main jet is bigger than #70 (likely);
  • air filter too restricted (least likely).