Bike sometimes refuses to start when warm, has trouble when cold

Hey guys -- 1984 Kawasaki KZ700, 8900 miles.

The bike usually starts when its cold but it has trouble fast idling;
its erratic and will die if I try revving it at all. After 15 or so
seconds its rideable on the half-choke setting.

A couple times though, the bike will start briefly when warm but quit
and then refuses to start. I tried all three choke positions - on, half,
and off - tried opening the throttle and cranking, let it sit for 5
minutes, still didn't work. After another 10 or so minutes it finally
started.

What could cause this? it happened once about a month ago and happened a
gain yesterday. Could it be bad gas? water in the gas? can bikes get
vapor lock or anything? flooded? I know it had fuel, I have a [clean]
fuel filter and it definitely had gas in it. I even tried it on PRI and ON.

Any suggestions? only thing I know that might be wrong with the bike is
that one cylinder seems to have a bad valve stem seal (it blows some
smoke on start up).
Thanks!

 

When the idle passages and ports are clean and the idle mixture screws
are set correctly, and the idle speed is set correctly, the engine will
start on full choke without you having to touch the throttle.

These carburetor problems always begin the same way. The idle jets and
idle passages gradually get dirty and the owner notices that the engine
stalls when he rolls the throttle off.

So he adjusts the idle speed higher instead of cleaning out the carbs
with a fuel system cleaner.

Then the starting problems begin. The engine can't suck fuel air
mixture through the "choke" valve because the throttle butterflies are
open too far.

And, maybe the throttle butterflies are so far open the transition
ports are uncovered and the idle speed hangs up when you blip the
throttle.

Your idle speed may be set right on the ragged edge of the above
described condition of excess throttle opening and improperly adjusted
idle mixture screws and dirty passages.

There may even be some dirt moving back and forth in the passages.

You said you would work on the carb problem after finals. So, what's up?

 

I did work on the carb problems. I completely tore down the carbs for a
second time and cleaned all of the passages. It turned out one of the
idle jets was plugged up with rust, and that there was a lot of other
crud in there. The bike ran tremendously better after that cleaning.

I put some B-12 in this tank of gas in case there is some stuff building
up again.

 

Adding a fuel filter would prevent rust from clogging things up. A Wix
33011 is a good one. Any particulate that gets past that is so small that
it is like throwning a 1 inch diameter rock down a 10" pipe.

pierce

 

If there's rust or if the original tank coating is coming loose, you
need to treat the tank not just add filters. If you let it continue to
shed crud, it'll be an endless source of problems.

Similarly, if there's water in the tank you want to know why.
If you think you got rid of it once, why is it back ? Drain the
sucker dry. recycle all but the last fraction of an inch into
your car's tank and start with clean fuel in the bike. If the
problem crops up again, switch gas stations.

I've had good luck with POR-15 as a treatment for lining
problems and pinhole leaks. They sell a motorcycle specific
kit that worked well for me.

 

I actually have the tank coated with Kreem, I just did the application
not long ago. Before the application the filter kept getting completely
plugged with rust chips - now the filter seems clean.

I didn't think of water in the fuel. It's been established that my cap
is leaking. I'll have to check into that.

Thanks for the advice.