hard starting GPz900

Maybe a weak/marginal battery? Time for a replacement?

 

Refusal to start when cold is usually an indication of a lean starting
mixture or that your spark plugs are really old and eroded and it takes
more voltage to fire a cold spark plug when the center electrode has
rounded off after many, many, many miles.

I used to take the spark plugs out of my old 1960's Hondas and Yamahas
and heat them up over the gas burners of the kitchen stove and then the
engine would finally start and I was mystifed as to why heating the
plugs helped. I knew nothing of the voltage requirements necessary to
fire a lean mixture and the random nature of fuel/air
mixture being near the spark, should it ever actually occur. :-(

Of course, you don't want to have to remove your spark plugs and heat
them up every morning, and you don't want to have to keep your engine
warm from an outside source, it just invites a fire to start.

If you pull the cap off your spark plug and you see a bright blue spark
at least 1/4 of an inch long jumping to the plug, you should have
enough voltage to start the engine.

It hasn't gotten down to freezing around here yet, but I have already
begun hooking up a trickle charger to my truck's new battery to get
maximum cranking time when I start it up a few times a week.

But I suspect your problem is actually lean mixture. If your idle
mixture screws were adjusted properly, you wouldn't need to touch the
throttle handle at all, you would just put the "choke" lever to the
maximum position and push the starter button.

I'm sure that you know by now that your carburetors don't have real
"choke" plates, they have individual starting enricheners, which are
just little valves in a passage that bypasses the main venturi. When
you move the "choke" lever to the ON position, a plate slides to one
side, opening all four starting plungers. The plungers allow air to go
around the closed butterflies and the air is able to suck fuel right
out of the float bowls.

There is no room in the tiny passages for a tiny little venturi, the
vacuum from the engine is enough to suck gasoline up into the starting
bypass passage just like sucking milk through a straw.

If the throttle butterfly plates are slightly open, there is less
vacuum at the starting enrichener, and no gasoline gets sucked out of
the float bowls and the engine won't start.

So the first thing to do is to turn the master idle knob all the way
counterclockwise so the butterflies are closed all the way to maximize
vacuum at the starting enrichener outlet that's downstream of the
butterflies.

And, I also recommend studying that "choke" linkage that pulls the four
plungers open.
Some Kawasakis have a "fast idle cam" or "chock" that holds the
butterflies open slightly, but only while the "choke" is on. So check
to see if one of those metal parts between the carbs holds the
butterflies open, and you'll definitely know not to turn the throttle
when "choking" the engine.

I suppose by now you have drilled out the plug covering the idle
mixture screw. The mistake newbies make when they try to adjust the
idle mixture screw is that they turn it too far counterclockwise,
expecting the idle speed to increase.

When the idle speed doesn't increase, they may turn the idle mixture
screws so far that the idle speed actually decreases. The exhaust sound
becomes dull and thudding,
and smells rich, and the engine stalls from idling too slow, so they
turn the master idle knob to increase the engine speed.

Now that opens the butterflies too far, uncovering the three transition
ports prematurely. When the engine is warm and the rider blips the
throttle, the idle speed hangs up at 4K to 5K and it takes a long time
to return to normal idle speed.

If the amateur tuner has turned the idle mixture screws excessively far
out, the engine sound at idle is "toot-toot toot-toot toot-toot" and
the excessively rich mixture doesn't clear out until the throttle is
opened a lot. The throttle response is poor and the engine tends to run
too fast at closed throttle, pushing the rider deeper into turns when
he really wants to be slowing down using engine compression.

The correct method of setting the idle mixture screws is to turn the
master idle knob so the engine is idling at the specified RPM, then
turn each idle mixture screw CLOCKWISE about 1/4 of a turn at a time
until the RPM increases, turn the master idle knob down, turn the
screws in, turn the master idle knob down, and continue in equal stages
until the engine begins to run rough from lean mixture, then back each
idle mixture screw out 1/8 or 1/4 of a turn.

It's nice if you have an accurate tachometer that will resolve RPM down
to 50 RPM, or you have vacuum gauges to indicate that each cylinder is
pulling the same amount of vacuum at the specified idle RPM.

And, you'd be in heaven if you had an exhaust gas analyzer you could
plug into each of the four exhaust pipes. You could set your emissions
right on what the EPA specifies. You'd be back to stock and idling very
lean and the engine would be hard to start on cold mornings and would
be slow to warm up.

That's why we started drilling those damned EPA plugs out in the first
place, 25 years ago.

But, if you get your idle mixture screws set right and your master idle
knob set to the specified RPM on the EPA decal, your engine should
start on full choke, without touching the throttle grip.

Well, I would bet that makes the Dynojet folks happy. It solves their
cash flow problem, yannow. Dynojet makes their money off of the
motorcycle owner's self esteem (he wants his machine to be as fast or
faster than the other guy's identical machine) and pride (owning a
sportbike means that you have to be interested in high performance,
that you can't be satisfied with a stock machine and buying a jet kit
gives you something to talk about when you are at the motorcycle
hangout).

But, does the engine start any better with the K&N filters removed?
Have you over-oiled the filters? That's a common problem, riders spray
that K&N oil onto the filters until they are just dripping oil, and
that chokes the engine by restricting airflow and it really is too rich
with over-oiled filters.

So far as the 4-into-1 Muzzy is concerned, it would do NOTHING to make
your mixture too rich at any point in the RPM range, but a 4-into-1
pipe does cause leaness from 5K to 7K, it's the well-known flat spot
where flow reversal takes place as the engine returns an adverse
pressure wave that blows exhaust gasses backwards through the intact
tract during the camshaft overlap period.

Your starter cannot turn the engine 5K to 7K RPM, so the Muzzy won't do
anything as regards starting the engine.

So how many turns out are your idle mixture screws, anyway? Three or
four turns? Try
1.5 turns out on all of them and work towards 1/4 of a turn. The idle
jets on CV carbs are usually HUGE, so the idle mixture screws have to
be turned out only a little bit.

 

This bike is driving me nuts. Its a 1985 GPz900 that up till about 3 months
ago ran fine, started fine and gave me no problems. Now, it will absolutely
NOT start unless its warm (being in Canada in fall and winter that entails
covering it and putting a space heater under the cover) I have cleaned and
adjusted the carbs 7 times now. In desperation I bought a dynojet kit in
case the 4-1 Muzzy or the K&N pod filters were affecting the mixture enough
to prematurely foul my plugs. Plugs have been replaced each time I clean the
carbs (even though they looked fine). Each time I clean and reassemble the
carbs, the bastard fires right up, runs like new, and even idles fine.
(although, it will die if full choke is applied quickly) After it cools, it
AGAIN will not start cold.I have installed an inline fuel filter, and it
looks clean each time I remove the tank/carbs. Each time I remove them, the
carb bowls are full of fuel and they don't piss out while installed.The
intake boots are fine, no vacuum leaks, the engine has lots of compression
and when its finally running it starts and shuts down great.

In desperation I have even tried an old kawy Z trick of wiring a relay up to
provide a full 12+ volts at the coils, no help.

Any help folks??? I'm at my wits end and have been eyeing a gas can, a
length of yarn and a wide open spot in my backyard and calculating the odds
of my neighbors calling the cops while it burns there. Please don't let this
nice old bike suffer a tragic end at my deranged hands!

While I was in at the local Kawy/Duc dealer buying new o-rings and washers,
I spotted a 2003 Z1 NOS for 7g's. That may be enough to push my unstable
self right over the edge.....


TIA!

Tom B

 

There is only one size for the idle mixture screw. The parts diagram
calls the idle mixture screw a "pilot AIR screw" but I believe that's
the wrong nomenclature, since there is a tiny washer and an o-ring on
the screw. Pilot air screws wouldn't need the o-ring and washer.

Problem with the Dynojet main jets is that the sizes don't match Keihin
and Mikuni jet sizes exactly, so you cannot compare them to original
equipment jets.

And the design is different too. The Dynojet main jets are just a
straight drilled hole. The OEM jets have a venturi shape to them. But
main jets don't influence starting.

Yes, 360 degrees is one full turn of the screw. It turns out that the
900 Ninja used fairly small idle jets, they are # 35, and I was talking
to another guy who said that Mikuni had used # 35 pilot jets in his
1200cc Bandit and that he also had to turn the idle mixture screws out
2.5 turns.

The only other things I can think of is that the starter jet that's in
the float bowl or carburetor body is plugged up. The starter jet isn't
shown on the parts diagram, it's probably permanently cast into the
carb body or the float bowl.

And, does turning the petcock to the PRI position help when you start
the engine from cold? It's possible that you just don't have enough
vacuum when you're cranking the engine on a cold day, or maybe the
little rubber diaphragm in the petcock leaks vacuum, or the shut off
valve that the diaphragm is supposed to open is stuck?

 

sorry forgot to mention, also replaced the battery and checked the charging
circuits.

Tom B

 

snipped

I've actually pulled these out and off the carbs twice now, so I could
spritz out the openings with carb cleaner and lots of compressed air. They
move freely and the passages are definately crud free at this point.

I have checked that, with the idle adjustment knob turned all the way out
the butterflies are completely closed.

This bike arrived in my possession sans the plugs, the carbs were supposedly
cleaned and synced "professionally" by the previous owner....perhaps the
wrong sized idle mixture screws?

It starts the same with them on, oiled or un-oiled (have a spare new set)
just runs crappy with them removed. As for the dynojet kit, it was a
desperation thing, I thought the kit would contain a complete range of the
required needles & jets etc instead of just a new jet needle and a selection
of mains

Tried 1.5 originally, as always after a cleaning/adjustment the bike started
fine the first time but popped and spit through the carbs, On subsequent
cleaning and adjustment sessions I have set them at 2, 2.25, 2.5 and 2.75
turns out. The bike idles best with them at 2.75 turns out from fully
seated. Maybe I'm over thinking something? I consider a turn out 1 complete
360 degree revolution of the screw, is that correct?

thanks for the input/advice/help!

regards
Tom B

 

hmmm, I haven't tried starting it in the prime position since my original
carb cleaning session, I never considered the petcock as it appeared to be
working fine.....I will definitely check it.

thanks
Tom B