trouble starting when warm

I recently bought a 86' ninja zx600. When it is cold I can get it to
start with the electric ignition, but after it warms up it won't fire
over. If I push it when it is warm and won't start electronicly, it
will fire right over. any help? could this be a valve problem

 

Could be, yes. Could be a coil breaking down when hot, as well.

Try it with some new plugs first (cheap solution, and one that works
surprisingly frequently).

 

It wouldn't hurt to check the valve clearances. If they have tightened
up, the clearance
necessary to allow the valves to seal in compression may be gone.

But, while you have the gas tank off to check the valve clearances,
look carefully at the carburetors to see if the EPA anti-tamper plugs
have been drilled out.

If you can see the slot-headed idle mixture screws, you know that some
shade tree mechanic has been tampering with the mixture screws. Shade
tree mechanics usually don't have a clue about the idle mixture system,
they always turn the screws too far counterclockwise and the engine RPM
slows down, so they adjust the master idle knob to make the engine idle
faster.

This opens the throttle butterflies slightly and that reduces engine
vacuum downstream of the butterflies and defeats the "choke" system.

Your carburetors do not have a real choke, there is no choke plate.
Instead there is a starting enrichener system that is a little valve in
a bypass passage. When you turn the handlebar-mounted "choke" lever to
the ON position (or pull it sideways if its a "choke" knob) airflow
through the bypass passage sucks gasoline straight out of the float
bowls.

But, if some shade tree mechanic has screwed up the idle mixture screws
and turned up the master idle knob to compensate for his screw up, the
starting enrichener is defeated.

You can go to www.partsfish.com or www.bikebandit.com to look at
carburetor parts diagrams. The idle mixture screws are probably
underneath the carburetors, forward of the float bowls, or they may be
on top, downstream of the butterflies and forward of the diaphragm
chamber caps. If you can see slot headed screws there, Joe Shadetree
has screwed up your idle mixture.

OTOH, if you see a round cap in that area, nobody has ever messed with
the idle mixture screws. However, that is not necessarily a good thing.
The idle mixture passages and ports may be gummed up and partly
plugged.

I recommend mixing about three ounces of Berryman's B-12 Chemtool Choke
and Carburetor Cleaner with a full tank of gasoline and going for a
ride.

B-12 is very available in the USA. You can get it in liquid form or an
aerosol can for about $3.00. B-12 contains xylene, actetone and methyl
alcohol and quickly cleans out the tiny idle passages and ports. if you
can't find B-12, STP or Gum Out carburetor cleaner will work, but B-12
is the only one I know of that you can get in liquid form.

After you have ridden slowly for some miles with B-12 in your tank, you
may notice that the engine idles a lot faster, that's normal if the
idle passages were plugged up and somebody turned the idle speed up to
compensate for dirty passages, so you will have to turn down the idle
RPM knob.

Check the diagrams on the sites I mention to see where the knob is, or
look in your owners manual. The knob is probably between the # 3 and #
4 carbs, the # 4 being the one on the right, unless the knob is on a
cable. In that case, the knob will be on the left side of the frame.

 

What the others seem to ignore is that your bike starts just
fine if you push start it.
To me this indicates one or both of two conditions - a weak
battery or a rich idle mixture, possibly made worse by bad
spark plugs.

Are your plugs very sooty after low speed riding and idling?
Check your air filter and try increasing the throttle opening
(the idle adjuster) to let more air in if your idle RPM is on
the low side. See FB's post for more details.

With a good battery, sparks can be strong even on dirty
plugs and when the starter motor is running and might
manage to ignite even a very rich mixture, that's why I
mentioned the battery.
N.B. New plugs will soon be bad plugs if your idle mixture
is too rich.

As one of my bikes have no electric starter, no battery and
is damn hard to push start I know the importance of a clean
air filter and a good spark plug. Kicking an engine into life is
no fun after the first minutes.