'87 CBR600: Lo-speed miss

'87 CBR600, 20k easy mi.

I've never been a torque-bigot, never owned a big Harley twin, but when
I bought my first (and only) crotchy-rocket back in '88, the 1st thing
I tested for was lo-end torque. The bike pulled reasonably well and
smoothly from around 1200 rpm. That and the song it sang from 8k-12k
rpm made me a happy camper.

It continued like that for many years.

For the last 5 years or so, it's had a lo-speed miss. Chokes and burbles
from about 1200-1500 rpm or so. Like its firing on 2 or 3 cyls. Very
annoying in traffic.

I pulled the Keihin CV carbs and cleaned them meticulously May '06.

The easy answer is crud in lower end of carbs. Doesn't seem to fit the facts.

There is a section in my Garbage-Clymer manual on "Pilot Screw Adjustment".
I haven't tried any of it.

There's no evidence that the motor is running either lean or rich.

Any ideas?

Thanx,
Puddin'

" ... and the bees made honey in the lion's head."
- from "If I Had My Way", Blind Willie Johnson

 

Plugs and/or wires.

Wild idea: Battery (or charging system) about ready to shoot craps and not
providing enough voltage to the ignition system at low rpm's.

Somehow I didn't get from your message whether your carb work made the
situation better or worse or no change ??

 

It can't be very annoying if you've been putting up with it for five
years.

 

But it runs great from about 1800 rpm to 10+k. Anything above around
1800 rpm feels fine.

No change.

P

" ... and the bees made honey in the lion's head."
- from "If I Had My Way", Blind Willie Johnson

 

Your idle ports and passages are probably plugged up with gum and
varnish from evaporating gasoline. If you can squirt some Berryman B12
through the pilot air jet (it's probably in the air inlet bell of the
carburetor) and the B12 comes out through the three transition ports
downstream of the butterflies and the single idle mixture outlet port
that is about another inch downstream of the transition ports, your
idle passages are clean enough to adjust the idle mixture screw open
another 1/4 of a turn.

Setting the idle mixture screws too rich may slow the engine idle
down, and then, when you turn the master idle speed knob up, the
transition ports will be uncovered prematurely and the engine will
idle too fast after you blip the throttle.

 

BUT can you fricking READ ???
Or is it that you don't have a clue about how charging systems work?
Clue: Even when working right, the alternator provides little or no current
at low rpm's.
If the battery is also near death, electrical systems can fail.......at low
rpm's.
I did say it was a LONG shot.

Pay attention to Krusty's post about Berryman's.
Most likely that IS your problem.

 

Bless po' me, I sho'ly woulda thought that if it starts OK, starter motor
doesn't drag, then there would be sufficient charge to fire the coils at
lo rpm's.

P

" ... and the bees made honey in the lion's head."
- from "If I Had My Way", Blind Willie Johnson

 

I think it's back by the manifold boot ...

There's a fair amount of detail in what you say. Is it really
specific to Keihin CV's on an '87 CBR600? Or is it kinda generic
and hopefully applicable ... ?

Thx,
P

" ... and the bees made honey in the lion's head."
- from "If I Had My Way", Blind Willie Johnson

 

Sounds perfectly logical but might not be true.

It takes a lot of current to turn a starter but not necessary a lot of
voltage. A failing battery might be able to turn the starter enough, if the
bike starts easily, and then not have enough voltage to fire an electronic
ignition.

Being an '87, I don't know if it even HAS an electronic ignition.

So, please forgive the interruption. Sorry I brought it up.

You ask for ideas and you get all kinds; some good, some not so.

 

That would be the idle MIXTURE screw. The idle mixture screw controls
fuel and air mixed in a passageway inside the carb. The fuel comes
from the idle JET and the air comes through the pilot AIR jet.

If you squirt B12 through the pilot AIR jet, it has to come out the
idle jet, the three transition ports donstream of the throttle
butterflies, and the single idle mixture port that is controlled by
the idle mixture SCREW.

There are two possible locations for the pilot AIR jet. The usual
place is in the inlet bell of the carb, on the right hand side if the
enrichener valves are on the left side. The other place is underneath
the rubber diaphragm that raises and lowers the vacuum slide. Since
the parts fish for Carburetor Components doesn't have a part number
for the pilot AIR jet, it's probably a brass bit that is pressed or
cast into the aluminum body of the carb.

This information is generic to Keihin, Mikuni, and Bing CV carbs.

Troubleshooting a carb tuning problem usually isn't so much a matter
of knowing specific information about certain models, but is more
about understanding the basics of how CV carbs work.

 

If a motorcycle doesn't have mechanical ignition points, it has an
electronic ignition system.

 

It does. Trust me.

 

.. wrote:
....

Just wondering, does that have to do with it being simpler to make/cast
a straight passage, one that is diagonal to the body?

 

Is true!

To my knowledge, all the CBR's have electronic ignition.

This one does fo' sho': I had to troubleshoot it years ago.

P

" ... and the bees made honey in the lion's head."
- from "If I Had My Way", Blind Willie Johnson

 

The tech writers probably never expected the problems that riders are
experiencing with gasahol evaporating and plugging up the ports and
passages.

Try and imagine the idle mixture passages as an "H", laying on its
side. Gasoline is sucked in through the pilot (idle) jet, and air is
sucked in through the pilot air jet (wherever it may be, it's in there
somewhere). Gas and air mix in the crossbar of the "H" and fuel/air
mixture comes out the two legs of the "H", downstream of each
butterfly.

Some later CBR600 models have the pilot air jet hidden behind a
removable inlet bell. The parts fiche at partsfish.com doesn't break
it down that far.

Since the early 1980's, idle mixture screws have been hidden under an
EPA anti-tamper plug. If you see a round plug
downstream of the throttle butterflies, that's the EPA plug. It may be
on top of the carbs or underneath.

The EPA anti-tamper plugs can be removed by carefully drilling a small
pilot hole in each plug, then threading a small sheet metal screw into
the pilot hole. When you pull the sheetmetal screw out with a pair of
pliers, the anti-tamper plug comes out with it.

When you remove the idle mixture screw, turn it all the way clockwise,
until it just stops. Count the number of full turns and fractions of
turns and write the number down. It may be anywhere from 1/4 of a turn
to 3-1/2 turns, depending upon the idle jet size.

Then you can remove the screws, segregating each screw, spring,
washer, and tiny rubber o-ring in a container marked to correspond to
the carb the parts came out of.

When reinstalling the idle mixture screws, turn them all the way
clockwise until they just stop turning, then back them out the same
number of turns you wrote down.

Opening each screw about 1/4 turn more will make the engine easier to
start in cold weather, it will warm up quicker, and the engine will
have better throttle response.

All of the manufacturers of CV carbs are working on the same basic
idea, which is to take direct control of the needle and main jet away
from the rider. A CV carburetor is a sort of "automatic carburetor",
intended to keep the atmosphere we breathe a little cleaner.

Standardization of home repair manuals is beyond my authority ;-(

Idle mixture screw, pilot screw, and fuel-air mixture screw are all
names for the same thing, just as idle jet and pilot jet are the same
thing.

 

The old gas had no additives that would cause such problems?

OK (as for as it goes).

I wonder why?

The 87-89 Clymer manual sez:
... a limiter cap has been installed over each pilot jet.
Pull the limiter cap off of each pilot screw.
and it depicts the cap back by the rubber intake boot.

I thought this was air/fuel (not idle) mixture. I need to pull these?

The one up by the vacuum piston?

turn it all the way clockwise,

The 87-89 Clymer manual depicts a "pilot jet" on the bottom of the carb,
inside the float chamber. I ran tons of Berryhill thru this when I
had the carbs off just over a year ago.

Sho'ly, sho'ly. Works about as well as doing emissions inspections
on personal use vehicles but not commercial trucks etc.

"Home" repair manuals? You mean like the (Honda) CBR600F_'89-90_Service_Manual.pdf
at http://stephygee.com/ ???

Re carbs, my manuals are an unholy hideous mess. It's the same with computer
components and Gawd-knows what-all else.

"They operate on a higher ("technical") plane, speak their own language, and
will not be held responsible for the behavior of their contrivances except
under the conditions which they choose to test and document, which are few
and cryptic."

As I mentioned, the phillips screws holding the flanges on the tops of the
carbs *do* *not* wanna unscrew. I tried a huge phillips driver and an impact
wrench. Know any tricks for breaking mutant Nipponese mot'cycle fasteners?

Thx,
P

" ... and the bees made honey in the lion's head."
- from "If I Had My Way", Blind Willie Johnson

 

Puddin' Man wrote:
....

That really puzzles me, assuming you're talking about the cover that
goes above the diaphragm, those are pretty small screws and I'd have
thought a good quality impact DRIVER and a good Phillips bit (probably a
number 2 size, ie. the biggest size that will go all the way in) with
maybe some thread loosener and appropriate positioning in some soft vice
jaws plus one or two good whacks with at least a 16 ounce hammer would
do the trick no problem. Did you squirt lots of thread loosener, give
it all a few taps with the rubber or plastic handle of a screwdriver and
let it sit for a while before trying the impact driver?


I've seen people use heat then an aerosol 'freeze' spray but I wouldn't
try it on one of my own carbs. Perhaps some mild propane heat for a few
seconds and possibly a quick spray from one of those aerosol cans with
the pressurized 'dry ice' stuff might budge them, but too much heat
might spoil the rubber inside or maybe even metal warp. If those screws
won't come out because of severe rust or because somebody has already
mangled the heads, then you'll need to replace them anyway, in which
case they might as well just be drilled out and the thread chased.


Really, I'm not telling you what to do, I'm as much asking the question
to others here who have more experience than I do!

 

I never had so much trouble with gum and varnish as I have had in the
last few years since
gasahol became more common.

However, the temperature in the area that I live in now gets up to 100
degrees every day from Memorial Day to mid-September, and that
evaporates the gasahol rapidly if I don't ride every day.

Same thing.

If you don't get any response by adjusting the air/fuel (idle mixture)
screws, the single idle mixture port controlled by that screw is
plugged up.

The idle mixture screw may be on top of the carb, forward of the
diaphragm chamber, or underneath the carb, depending upon the
particular carb you have.

The parts fish didn't list any idle mixture screw at all, but that
just means that Honda doesn't want home mechanics to know about the
screw.

If they don't know it's there, they won't tamper with the idle mixture
settings.

Yes, that jet is also called the "idle jet" or the "slow jet" by
various tech writers.

I will put the phillips driver that came in the tool kit into the head
of the screw, and smack it with a hammer, while turning it with the t-
handle that usually comes in the tool kit.

Japanese "phillips" screwdrivers often have a very pointy tip,
compared to American "phillips" screwdrivers that have a rounded tip.
This pointy tip helps with Japanese phillips screws, but not with
American phillips screws.

 

A couple screws for each carb, inside the "air cleaner base".

those are pretty small screws and I'd have

My impact driver is a cheapo.

I used liquid-wrench, tapped and waited.

No corrosion, and they'd never been removed. Just the factory install.

No problem. Appreciate the effort.

P

" ... and the bees made honey in the lion's head."
- from "If I Had My Way", Blind Willie Johnson

 

Assuming there's a model # on the carbs (given that it's absent from
both the idiot Clymer and idiot Honda manuals), would that help? Or
is it not possible to ID specific details of a Keihin by model #?

I've no idea what "parts fish" you refer to.

Honda no doubt figgered if the "idle mixture" really needed adjustment,
the owner should junk the bike ($20 ??). I may be in agreement before
my carb problems are resolved.

Just checked. The phillips in the tool kit (slips into a cheap plastic
handle) has a flat tip, like US phillips.

Do you know what "SW, AR Type" are in the Honda manual?

Thx,
P

" ... and the bees made honey in the lion's head."
- from "If I Had My Way", Blind Willie Johnson