Hard starting 1985 GPz900R (C model)

Greetings folks. I am having some difficulties starting my old GPz lately.
After it sits long enough to cool off (3-4 hours @ 15 Celsius) it is almost
impossible to start. Thus far I have, changed the plugs (old ones looked
fine) made and installed new plug wires (old ones were a bit corroded where
they screw in to the terminals), cleaned the carbs (they were spotless)
checked the compression (148-152-150-149) installed a new battery, measured
the resistance of the coils (within specs) to no avail, checked the petcock
( works fine), and checked for air leaks at the carb boots (none). It will
kick once immediately upon the first hit of the starter button, with the
choke applied, as though it was going to start, then just crank forever..
The first time it happened I just cranked the heck out of it (sorry starter
but had a buddy impatiently waiting to go for a ride) till I had to boost it
to continue, still nothing, then I did the unthinkable and used some <I know
its bad> quickstart and eventually it caught and started and ran fine.Once
its running it runs like a new bike, although it surges a bit at idle, that
is, the idle sometimes, not always, fluctuates between 1500 and 2000rpm,
other times it idles smoothly at about 1500. The next time it happened it
was a bit cooler out in my garage, about 6 degrees Celsius, so after an
unsuccessful 2 or 3 minutes or so of cranking, I put the cover on the bike,
placed a small heater underneath and let it warm for an hour...damn thing
sparked right up.This bike can't be that cold blooded (can it?). My friend
has a ZL1000, essentially the same engine, his bike sits outside and it
sparks right up every time. Help! I'm pulling my hair out here, any
ideas/comments/suggestions??????

Tom B
1983 GPz550
1984 KZ1105
1985 GPz900

 

Got diaphragm (CV) carburetors?

Your carburetors probably do not have a real choke with a flat plate
that makes the carburetors suck harder when its closed. Instead, they
probably each have a small starting enrichener valve. It's just a
plunger in a hole and when you move the "choke" lever or knob to the
full ON position, you are just letting air bypass around the throttle
butterfly.

The enrichener has its own starter jet in the float bowl and it's
possible for the fuel air mixture to be as rich as 1:1, but there's
still not that much air flowing through the tiny hole that's around
1/4th of an inch in diameter.

If your idle jets have gotten plugged up over a period of time, you or
a previous owner probably noticed the idle RPM was too low and you or
the other guy probably thought it was a simple matter to just turn the
idle RPM up. That was a wrong move. If the idle jets are plugged up and
the throttle butterflies are adjusted too far open, the starting
enrichener cannot suck enough fuel out of the float bowl because there
just isn't enough vacuum available.

Even if the engine should fire up for a second, the moment you twist
the throttle grip,
vacuum totally falls off and nothing gets sucked through the idle jets,
which are plugged anyway.

You want those butterflies closed as far as possible. When owners of
old British singles tried to kickstart their engines, the first thing
they would do is turn the idle RPM screw all the way out and close that
throttle all the way to make the carburetor suck gasoline out of the
float bowl.

So you need to clean those carburetors from the inside and then
readjust the idle RPM to the owner's manual specifications. If you
don't have the owner's manual there is probably a decal under the seat,
on the back side of a side cover, on the air box, or even on the chain
guard.

Your engine runs on the idle jets a lot of the time when you're just
cruising down the highway. You'd be amazed at how little throttle you
actually need to use. The idle jets and idle passages get gummed up
when the motorbike sits around unridden for a few months. Gummed up
jets and passages stop the flow of fuel to the engine when you open the
throttle and you'll hear the exhaust go "piffle-piffle-SNAP!" or even
"fart-BANG!"
as you roll the throttle off.

When the idle jets are plugged up, the engine is hard to start, takes a
long time to warm up, and acts very "cold blooded". It wheezes and dies
when you open the throttle.

Further confirmation of this diagnosis is that you say the engine RPM
"surges" while you're riding. That also points at plugged up idle jets.

The easy solution is to go down to Wal*Mart or any good auto parts
store and buy a 15 ounce can of Berryman's B-12 Choke and Carburetor
Cleaner. Add 3 or 4 ounces to a full tank of gasoline and go for a slow
ride. Watch for cars coming up behind you!

You want to ride slowly so the engine has to suck the mixture through
the idle jets. After several miles of running, the idle RPM will
increase and you will have to turn the master idle knob down. It may be
underneath the carbs or on top. It will probably be between the two
carbs on the right side of the engine as you sit on the motorbike.

Once you have cleaned out your carbs, the engine should start with the
"choke" all the way ON and you shouldn't have to touch the throttle
grip at all. You should be able to put your helmet and gloves on while
the engine warms up, without having to fiddle with the choke to keep
the RPM from getting too high or too low.

If the idle jets and passages are really plugged up and you have to
remove the EPA anti-tamper plugs to spritz out the hole around the idle
mixture screw, I've described the drilling out process and readjustment
of the screws about a bazillion times here in this forum, you can
google up EPA and find it.

 

Thanks for the reply. yup they are Keihin CV's. I checked and recleaned the
idle jets. Damn thing starts now but still dies with no throttle applied
till it gets warm unless I screw the idle adjuster way up, then when warm it
races and surges. Now however, when I flip the choke lever all the way to
the on position, not only will it not start, but when I do so when its
running, it will die. ??????
Of note, I got so stressed trying to get those godforsaken boots back onto
the stock airbox that I gave up and slapped on a set of pod filters I had
laying around. It was running a bit lean so I installed a set of larger
mains (I drilled an old set out very marginally larger than the stock ones
since these carbs have jet needles with no clips to raise or
lower...bad/good/really dumb idea??) and now it runs great when warmed
up,( lost that stutter at 4000rpm I picked up with the 4-1 Muzzy) but does
not seem to have the same initial early throttle response although it still
has lots of power down low. Plugs colour is a nice even tan now after a
chop, but the "choke" inoperativeness question remains.
regards and TIA
Tom B

 

Surging indicates lean mixture.

Did you drill out the EPA anti-tamper plugs so you can spritz out the
idle passages with aerosol carb cleaner? I've describe the process
about 100 times, but basically when you squirt carb cleaner through the
idle mixture screw holes, it should come out the idle mixture port, the
transition ports next to the throttle butterflies, the idle jet and the
pilot air jet. If the carb cleaner doesn't squirt out one of those
holes, cover up the other holes with your fingers and keep quirting
until it does.

All the way ON is up? What you're doing is opening a bypass air valve
in a passage that goes around the closed butterfly. It's an idle
enrichener, not a true choke. It's supposed to suck gasoline straight
out of the float bowls through its own starter jet.

Maybe the starter jet is plugged up? What do the plugs look like when
the engine dies? Are they all sooted up or clean?

Pods will cause the engine to run lean. It's hard for most shade tree
tuners to figure out the round jet sizing system. I've explained that,
too, at least 100 times. A #100 round jet has an orifice that's 1.0
millimeters in diameter. If you install a #120 main jet, it doesn't
pass 20% more fuel, it has like 50% more area in the orifice because
the diameter is 1.2 millimeters. You can work out thearea of the
orifice using the pi times radius squared formula to see that a 120
main jet should be plenty big.

Drilling out the main jets is not a good idea unless you have access to
some very precision drills. The factory can drill those things out very
precisely. The difference between a #100 main jet and the next size (a
#102 main jet) is about 0.0008 inches.

It sounds to me like your idle passages are still all plugged up. Just
as you open the throttle, the butterflies should be uncovering three
transition ports that give the engine the shot of fuel it needs to
accelerate, since the vacuum slides will not have been lifted high
enough to get the tapered part of the needles out of the hole.

Plugs colour is a nice even tan now after a

Plugged up starter jet?

 

There are no plugs covering the idle jets on mine, maybe they have been
changed by a previous owner? I pulled them, took great care to remove the
springs-washers-orings and then squirt cleaner and blow compressed air
through them .

I should have been more specific....The "choke" lever is bar mounted, when
rolled to the fully on position it actuates the enrichener.

Plugs look great now.

I received a fairly complete set of numbered precision bits (finally a
really usefull b-day gift!), I sized the original jets (0.042) and then used
a bit one size larger (0.0425) so oops guess that was a bad idea, good thing
I kept the originals unmolested!

Now heres my newest dilemma....where in the heck is the "starter" jet?????
I've identified, cleaned, and gone over the main jet, the needle jet, the
jet needle, the pilot fuel jet but cannot find a starter jet!

thanks in advance!
regards
Tom B

 

When I had a GPZ900R a few years ago (an A1), I didn't bother with the
old rubbers on the carbs, and replaced them when I had to remove the
carbs to fit a new throttle cable. New rubbers are still tricky to get
on properly, but *much* easier than old hardened ones. It helps if you
have small hands, I was able to get mine inside the airbox.

You could try the Yahoo GPZ900R group.

Leon

 

I looked at the carburetor parts fiche for the 1985 900 Ninja. Is that
what you have? Is your machine water-cooled, with 4 valves per cylinder
and has the cam chain on the right hand end of the engine?

There was a selection of main jets listed, from #128 to #140.

A main jet that is only 0.042 would be a #106, which Keihin doesn't
make, but they do make a #105. That's a very small jet that would work
OK on a smaller, mildly-tuned engine, like maybe a 500cc or 750cc
machine.

I can't imagine your engine running well at full throttle with anything
smaller than a #125 main jet, unless you ride at 5000 feet above sea
level all the time

If you look at a parts diagram and don't see a removable starter jet
listed, that's because it's pressed into the carb body or even into the
float bowl.