93 gsx750f idle

Decided to try to resucitate my Katana. Last time I tried to start it
I came to the conclusion that the engine is toast. That was a couple
years ago.

I got it to start last weekend but the idle is all over the place. If
I set it to 1k then open the throttle a bit the idle picks up to like
3000. If I set the idle back down it dies.

The last thing I did to it before I parked it back when dirt was young
was dismantle, clean, mantle and synchronize the Mikuni's. I rode it a
couple days and it exhibited the exact same behavior then as it does
now. About the 3rd day it decided to not start so I threw in the
towel.

Based on the behavior above, is it time to try cleaning carbs again?

(Tangential Wierdness: I was getting real intermittent spark on all
plugs (only about 1 in 10 tries would any of them try to fire) so
after testing signal generator and coils I opened up the ignitor to
see if there was any obvious ugliness in there. There wasn't so I put
it back together and it decided to start deliviering regular spark.)

It has a knock in the engine at low rpms that goes away at 2.5k rpm.
The head sounds really noisy too. When I went throught the carbs I
also set valves and adjusted the cam chain tensioner.

So...Any clues on the idle moving all over the place?

Also, any tests on the engine to try and isolate ugly noises?

Im going to check compression this weekend. I couldn't hear any
knocking on the bottom end w/ a stethescope. Matter o' fact the
bottom end sounded very quiet. When the engine is cold, cranking it
will turn the oil light out. When it's hot the oil light will flicker
during cranking. I drained the oil and it didn't look bad (no flakes
or glitter) but that oil probably only has about 25 miles on it. I
have yet to pull the pan to see if there's any metal there.

Just looking for input on idle and any other general input.

TIA!

Rusty

 

When you dismantled the carburetors, did you drill out the EPA anti-
tamper plugs, or has somebody else done that deed, in hopes of
eliminating the notorious lean condition at idle?

Removing the EPA plug by drilling a pilot hole and then screwing a
small sheet metal screw into the pilot hole and pulling the screw and
plug out together is a Good Thing To Do to any engine equipped with
diaphragm carburetors, as they run on the idle jets most of the time.

But most Joe Shade Tree mechanics don't know how to adjust the idle
mixture screws. They turn them too far counterclockwise, expecting the
idle speed to continue to increase. But the idle speed slows down when
the idle mixture gets too rich, so they turn the master idle knob up
and they experience the symptoms you describe.

Take a mirror and look up underneath the carburetors, in front of the
float bowls. If you see a shiny aluminum plug, that's the EPA anti-
tamper plug, undisturbed. If you see the head of a brass slot head
screw, the plugs have been drilled out and the
perpetrator has turned the idle mixture screws out too far and has
compensated
for an excessively rich idle mixture by turning the master idle knob
up too far.

This would uncover the three transition ports just downstream of the
throttle butterflies. The transition ports are not controled by the
idle screws. When you try to get the engine to idle down, it's getting
too much fuel and it idles too fast.

When you get the throttle closed, the engine stalls.

And, it seems illogical, but the same thing happens if nobody has
drilled out the EPA anti-tamper plugs. The single idle mixture port
gets plugged up from the evaporation of gasoline, leaving the port
full of gum and varnish.

So, the owner discovers that the engine is hard to start and won't
idle. So he turns up the master idle knob and he's running off the
transition ports which are for acceleration, not idle. The engine
either idles too fast, or it stalls.

Well, rather than to remove the carbs and try to clean them, I
recommend inspecting to see if the anti-tamper plugs have been
removed. Drain down the float bowls into an old rag to catch the dirty
gasoline. Then mix some clear carburetor cleaner like Berryman B-12 or
STP or GumOut with gasoline, and squirt that mixturedown the fuel hose
into the float bowls and let it sit for half an hour.

You can start the engine and run it on the carburetor cleaner/gasoline
mixture and if you can get the engine to run slowly enough, adjust all
the idle mixture screws to around 1 full turn open and turn the master
idle knob down as low as it will go.

You want to arrive at the lowest smooth idle speed, by turning the
idle mixture screws
clockwise to lean up the mixture, and turning the master idle knob
counterclockwise to close the butterflies.

Probably just a bad electrical connection somewhere, or a kill switch
or ignition switch problem.

The cam chain and valves make a lot of noise in any Suzuki SACS engine
because there is nothing to deaden the sound. Also, the fairing blocks
wind noise while you're riding and a horrible growling sound comes up
from the engine.

Idle mixture screw versus master idle knob setting. Try to achieve a
smooth idle at the lowest possible setting of the master idle knob.

Put your stethoscope on the round cover on the left side of the engine
and listen. You might be having starter clutch problems. The starter
clutch idler gear has a bronze bushing that gets oil pressure from the
crankshaft. If the bushing is worn out, the idler gear will knock.

A worn out idler gear bushing is nothing to laugh at or neglect. It
could cause the starter clutch to engage while you are riding down the
highway. The starter could be backdriven at 40,000 RPM, destroying the
starter and locking up the rear wheel.

 

The plugs were still in and I didn't touch them last time. I'll take
another stab and see if I can get them really cleaned out this time.
As opposed to just "kinda" cleaned out.

I'd tested the kill/start circuit and all seemed normal. Also cleaned
and reseated all plugs on the harness. It wasn't till I took the ECM
apart that it started delivering spark with any reliability.

The starter DOES make some ugly noise (thunking sound) when it
disengages, particularly if the battery is low. I'll inspect that as
well.

MANY thanks for the input!

 

Suzuki SACS engines are mechanically noisy, there is no getting around
the fact that the engine castings are extremely thin and mechanical
noise comes right through the thin aluminum.

But a starter that makes a "thunking" sound is suspicious.

You must remember this: the starter clutch automatically ENGAGES every
time you
stop the engine. There are three cylindrical rollers on ramps inside
the starter clutch so they can roll away from the stub end of the
idler gear as the engine starts running on its own.

Then, when you stop the engine, three little springs push the rollers
back down onto the stub end of the idler gear.

The normal sound of the rollers falling is a "whiz-ching" as the
starter clutch re-engages, in order to be ready for the NEXT time you
start the engine.

When you stop the engine, the starter clutch is trying to make the
starter turn, but it can't, so it helps stop the crankshaft instead.

If you're hearing a loud thunking sound, put the motorbike onto a
swingarm stand if there is no centerstand so you won't have drain the
oil.

Pull that round left side cover off and grab the idler gear and see if
you can rock it around and lift it up and push it down. If you can
move the idler gear around more than just slightly, the bronze bushing
is worn out and riding the motorcycle is life-threateningly dangerous.

Your mention of a flickering oil light when trying to start the engine
when it's hot may be meaningless, but low oil pressure could cause a
lack of lubrication to the critical
bearings, camshafts and that pernicious starter clutch bronze bushing

There is a large Allen headed plug on the right hand side of the
engine, underneath the ignition timing cover. You need a special
adapter to attach an oil pressure gauge to the hole that the plug came
out. A $crewzuki $tealer$hip would have the adapter and the pressure
gauge.

With the engine warmed up and running at 3000 RPM, you should read 43
to 85 PSI.

If the oil pressure is lower than that, you might have problems with
the oil pressure relief valve inside the oil pan. The relief valve
normally operates when the oil temperature is low and the viscosity is
high. It operates when there is a difference of pressure 28 PSI
between the entrance and exit of the oil cooler.

Back to the idler gear. Does it move around a lot? A worn out bronze
bushing would cause that.

Put the cover back on and throw your Kan O' Tuna into the back of a
pickup truck and take it over to a $crewzuki $tealer$hip and ask the
mechanic if he will please come outside and remove the starter rotor
for you.

That's what I did to avoid unloading my GS-1100 at Glendale Suzuki...

The starter clutch on older GS750/1000/1100 models used to bolt to the
backside of the alternator rotor. But Suzuki moved the alternator up
under the carburetors on the GSXR and Kan O' Tuna models.

Strangely, they still call the round piece that bolts onto the end of
the crankshaft a "rotor" even though it has nothing to do with
anything electrical.

Suzuki shops have a special tool that holds the rotor while it is
unbolted with a 1/2 inch breaker bar. Then a special cylindrical tool
that has internal threads screws onto the rotor and a hardened steel
bolt pushes against the end of the crankshaft and the rotor comes free
with a "ching!"

I bought that special tool to work on my GS1100. It cost about $50.00
and that was 20 years ago.

 

Whew. No up/down play on the starter gears. Both the large crank
gear and the idler gear have about 1/8" play in and out (parallel to
the axis of the shaft)

I'll try and test the oil pressure this weekend.

 

I have the 1985/86/87 GSXR-750 shop manual. The Kan O' Tuna engine is
pretty much the same. No specifications are given for radial play of
the idler gear.

But there will normally be some axial play of the large idler gear
parallel to the axis of the crankshaft.

The idler gear is actually the large gear that the crankshaft spins
inside of while it remains motionless, or IDLE.

The manual is wrong in calling the smaller gear that has two sets of
teeth on it an "idler gear". That gear is actually the starter
reduction gear. It's sitting on a rod that isn't spinning. So it
couldn't reasonably be called an "idler gear".

Good luck on finding an adapter for the hole.

 

The saga continues.

Fabbed up an oil pressure guage w/ $10 worth of brass fittings and a
die and an old pressure guage I had. It runs within specs. 60-65 at
idle and 70-80 at speed. Haven't yet pulled the pan to see if
there's any ugliness sitting in the bottom of it.

I found the #1 carb was having issues while checking compression. I
checked it one day and it was 60ish. I'd left it hooked up (in
disgust) and just for kicks checked it the followinng day and it was
180. I thought "WTF?" and checked #2. While cranking it gas start
splashing out of #1. Not good.

I figured I'd go through the carbs again and see if I can make it run
any better. Since it hasn't run much the last few years I thought
that if I could get it to idle well I would point a fan at the cooler
and just let it run a good while and try getting some top end cleaner
like seafoam in it to see if that didn't help the compression.

Turns out the carbs I have (I got them from the JY a while back) did
have the EPA plugs removed. When I cleaned em up last the book said
don't touch the idle screws so I didn't but this time on the #1 I
screwed it in to count the # of turns then removed it. As I blew carb
cleaner through it a bunch of yucky gunk came out so Im betting 2nd
time will be a charm at least for the carbs.

Now I gotta find parts for it. Seems bike bandit carries the parts I
need. (I wish they made 'kits' for them. I found lots of Mikuni
rebuild kits for carbs on Jetskis and whatnot but none for
motorcycles).

Two questions:

a) Are the needles supposed to have a worn off spot about 2/3rds the
way up from the narrow end? The one I pulled out of #1 looks like a
ramped groove was machined on one side. This seems "not right" since
if it's only on one side it would seem to be polarized but there's
nothing to prevent it from turning.

fairly legible pic here http://noob.kicks-ass.net/albums/Mikuni/carb_needle.jpg

b) On the slider there was some residue like white lithium grease.
Should these be greased or was this just normal gunk/residue?

TIA - Rusty

 

I found some closeups of a jet kit and it appears my needles are
definitely worn out. Guess I'll put in a stage 1 jet kit. I found
one website that tested Ivan, Factory Pro and DynoJet and said the
Ivan kit provided the best all around performance across the widest
power band.

Do diaphrams typically get replaced during a rebuild? Mine look
fine. There are a few other rubber parts that could be replaced
(aside from the o-rings.) The plunger seal and bowl gasket etc.

Still looking for tips on assembly. The factory manual has very
little on getting it all back together. Anything other than just
cleaning the heck out of it and putting it all back together?
( Luckily it did mention base-lining the mixture at 2.5 turns out. I
would have started at 2 like on a holley . )

 

Good. I'm glad to hear that there's somebody out there who isn't
afraid to make up his own tools and test apparatus.

Did gasoline come out the spark plug hole?

If you left the petcock in PRI, the float valves probably leaked. Is
the oil diluted by gasoline now?

If so, it's time for an oil change.

Constant vacuum carburetors run on the idle jets and idle circuits
about 85% of the time unless you're a track day rider. So the carbs
have to be kept squeaky clean.

Look at www.oldbikebarn.com and see what they have.

Bummer. The jet needle has vibrated against the needle jet and both
are ruined. The needle jet can be easily pushed out of the carb body.

If you look closely at the needle jet, you'll see the hole is worn to
an oval shape. The needle jet will always drool excess gasoline at low
throttle openings if you don't replace it.

It's condensed oil vapors mixed with water condensation. Clean the
slides off with gasoline or carb cleaner.

Clean all the oil condensation out of the airbox. Replace the paper
air filter if it is oil soaked.

The square box on top of the cam cover is an oil/air separator. The
metal mesh inside is supposed to condense oil vapors and allow the oil
to drip back to the cylinder head.

If you run the piss out of the engine, the wire mesh gets hot, and oil
droplets get into the airbox and go out the crankcase breather hose to
the airbox.

Look under the airbox for a rubber tube that is like an appendix.
Owners are supposed to know they should remove the tube and drain the
appendix on a regular basis. But there is usually nothing about the
subject in a shop manual or owners manual.

 

**** the Dynojet kit. Save your money if you're not a track day rider
or a racer. The best thing about Dynojet kits are that they have made
the guys at Dynojet rich from selling aftermarket jets and needles
that are less precision made than the Mikuni or Keihin parts.

If I was a racer, I would buy from Factory Pro. Mark Salvisberg is a
lot freer with information than Dynojet.

Most riders never get onto the main jet 95% of the time. 75% of the
fuel flow up to 80% throttle comes through the idle jets, so get to
know and love your idle jets.

The original factory jetting gives you the best performance and best
mileage across the widest range. All you have to do is know how to
adjust the idle mixture screws and how the idle jet numbering system
works.

The hole in a # 30 idle jet is 0.30 millimeters in diameter. The hole
in a # 40 idles jet is 0.40 millimeters in diameter.

If you work out the area = pi X radius squared formula, you will see
that the # 40 idle jet's area is about 175% of the # 30's area.

So there is a wide difference in the number of turns
required on the idle screws. The idle screws are only regulating ONE
of the four holes in the carburetor throat.

I will repeat that statement for emphasis. The idle screws are only
regulating one of the FOUR holes in the carburetor throat.

This means that there is a wide range of recommended starting
settings. Like a carb with # 30 idle jets might need the idle screws
to be opened as much as 3.0 to 3.5 full turns, while the carb with the
# 40 idle jets may require less than 1/4 of a turn from bottomed as a
starting point.

The amateur mechanic gets into trouble adjusting his idle screws
because he doesn't understand that the other three holes are
controlled by the throttle butterflies.

The amateur motorcycle mechanic keeps opening his idle screws more and
more, thinking that the idle speed should keep increasing.

When the idle mixture gets too rich, the exhaust note becomes dull and
thudding, and the idle RPM
decreases. The amateur mechanic notices that his
engine is prone to stalling, so he turns the master idle knob to
increase the idle RPM.

This uncovers the three transition ports too soon and the engine won't
idle down when the throttle is blipped. The transition ports are for
accelerating once the motorcycle is moving.

Maybe the amateur mechanic turns the idle RPM back down, and the
engine is prone to stalling again.

The amateur mechanic becomes all confused because he doesn't
understand the basic process of idle screw adjustment, which is to get
the fastest stable idle speed at the lowest throttle opening.

The idea is to turn the idle mixture screws IN, and if the idle speed
increases, turn the master idle knob down. Then repeat the procedure.

When the idle mixture becomes so lean the engine stalls, turn the idle
screws back out 1/8th of a turn.

Only replace the diaphragms if they are torn. There is a place in
England that makes aftermarket diaphragms. Original equipment
diaphragms come with new slides. They will cost about $100 apiece.

Well, they are Mikuni carbs, not Holleys. The main reason that the
recommended base-line is 2.5 turns is because the carbs have a
smallish idle jet.

The bigger the idle jets, the fewer turns out the idle screws will be.
My FZR-1000 has #40 idle jets, so the screws are all set at 1/4 of a
turn open and that's plenty rich for easy starting and good throttle
response.

You can find out what the stock jetting is if you look at the
carburetor parts diagrams on www.bikebandit.com

 

Sir, that is a wealth of information! MANY THANKS. It will take me a
while to digest it all. I'm a slow learner.

Seemed an easier and cheaper route than trying to find the factory
doohickey.

It did. The cylinder was full of gas.

I'll check the oil. The petcock isn't on as I've got it on a homemade
fuel IV bottle.

Too bad the Clymer manual said "Dont touch"! Im betting that was 90%
of my running problems (low compression aside).

Will do. In my searces that site never came up.

So the mains run rich and the idles run lean due to gunk? Not a good
combo!

The airbox and filter are squeaky clean.

My old Nighthawk had this and I did drain it often. The Katana just
has a hose on the airbox that's open. Is it supposed to be a sealed
fitting?

I would like to stick w/ the factory OEM as it's been my experience
that futzing w/ th design on something leads to increased
maintenance. Particularly so if the system being futzed with is
something that has very small tolerances like a carb. The only reason
I thought of going w/ an aftermarket kit was cause it came in a kit.
I'll continue the search!

Check! What would be a good baseline setting on the master idle?

Will do. Thanks again so much for all the info!

 

Then that would indicate a stuck float valve that allowed the
carburetor float bowl to overfill. The additives in fresh gasoline
will help unstick the float valve.

If that doesn't work, spray some aerosol carburetor cleaner down the
fuel hose and let it work for half an hour. If that doesn't unstick
the float valve, you will have to disassemble the carbs for further
cleaning.

I had the tiniest speck of gum on the float valve seat keep the float
valve from sealing properly on one motorcycle. I could barely see it
with a magnifying glass.

I used a piece of emery paper on the end of a stick to lap the float
valve seat.

The EPA mandated lean idle mixtures in the 1980's because cars sit in
traffic idling about half the time. The carburetor manufacturers
jetted all the carbs really lean and then sealed the idle mixture
screws.

1990's motorcycles were cold-blooded until they warmed up and were
hard to start and slow to warm up.

If the idle passages got gummed up from storing the motorcycle, the
starting problems only got worse.

Then the white residue on the slides is probably evaporated alcohol
and water. Most people assume that the air flow through a carburetor
only goes one way, but, during the valve overlap period when the
exhaust valve is open at the same time as the intake valve, there will
be reverse flow through the carburetor.

Any water that got into the float bowls and mixed with the alcohol in
the gasoline gets blown backwards out of the carburetors. When the
alcohol and water evaporate, you get a strange white film on the
slides.

Yes. If the appendix is missing, just plug the hose with a 1/4" or
5/16" bolt so dirty air won't get sucked up the hose into the airbox
on the wrong side of the air cleaner.

The airbox is an acoustic cavity called a "Helmholtz Resonator". The
air inside the box resonates at a certain frequency that is related to
the volume of the air box and the diameter and length of the intake
tubes.

Having the air vent hose unplugged lowers the resonant frequency of
the airbox slightly, but the fact that the vent hose can suck dirty
air is more important than the business about resonant frequency being
lowered slightly.

There should be a white EPA sticker underneath the seat that tells you
to set the idle RPM around 1100 RPM.

If your engine won't idle at the specified RPM, tweak the idle screws
and adjust the master idle knob accordingly.

The only reason to deviate from the specified RPM would be if your
valve train, cam chain, and clutch basket were
too noisy at the recommended RPM.

I have seen race engines with aftermarket cams that were unhappy
idling at less than 1700 RPM, but that didn't matter for a motorcycle
that was never ridden in street traffic.

It turns out that the size of the idle jet is not specified on the
Suzuki parts diagram. Maybe it's in the shop manual.

Another way to find out the idle jet size is to look at the tiny
numbers stamped on the jet itself.

If somebody has boogered the end of the jet trying to remove it, the
numbers cannot be read.

Suzuki idle jets are cross-drilled for adding emulsion air to the idle
mixture, so they should be removed when cleaning the carbs.

Never try to remove an idle jet if you don't have a small slot
screwdriver that fits the jet perfectly.

Yamaha idle jets are not cross-drilled, they don't have to be removed.