Prev: Different Headlight for 86 Honda VF700C
Next: Rebuilding koni motorcycle shocks
From: Potage St. Germaine on 22 Mar 2007 00:09
On Mar 21, 6:28?pm, tyle...(a)gmail.com wrote:
> Made some progress tonight. I appear to be running rich if I read the
> above narrative correctly. When I first start it cold, it revs to
> 4,000 for a brief moment then idles at the correct RPM (1,200) though
> with a bit of a lumpiness. Then as the engine warms up, it idles at
> 4,000. Doing anything with the "choke" kills it.

Yes, your idle mixture is far too rich, so you are getting extra
gasoline through the transition ports that are prematurely uncovered
by the throttle butterflies which are too far open because the idle
mixture is too rich.

> The idle mixture screws were set to 5.0 and 5.5 turns. It got too dark
> to continue, but tomorrow I shall try screwing them in one turn each
> and see where that gets me.

Who drilled out the anti-tamper plugs? The culprit probably screwed
the idle mixture screws out too far, not knowing what he was doing.

The rule of thumb is that if you have to open the idle mixture screws
more than 3.0 to 3.5 turns, your idle jets are too small.

I'm not saying you should run out and buy $10 worth of idle jets to
make your idle screw settings meet the rule of thumb.

I don't know what idle jets you have in there because the
partsfish.com diagram doesn't specify what they are.

But I would start at 2.5 full turns open and work with the screws,
tightening them equally and turning the master idle knob
counterclockwise until I got the fastest smooth idle with the least
amount of throttle opening.

That may be anywhere from 2.5 to 1/4 of a turn open, depending on what
size the idle jets are.

> I'm left hoping
> that my mystery hole is just a leftover from the manufacturing process
> to create the passage for the idle mixture screw or the three little
> transition ports.

You could be right. Mikuni has made changes to carbs before, without
bothering to tell us customers about what they did.

The knowledge gradually gets trickled down from the factory
technicians on the race teams to the hot rodders and then to the
magazines. But nobody tells the commuter motorcycle owner what's up
with his carbs.


From: Potage St. Germaine on 23 Mar 2007 11:47
On Mar 23, 5:01?am, sb2...(a)yahoo.com wrote:


> I have a 79 xs750. My bike often sits for months and I have the same
> problem you describe every time I try to start it. I've found that it
> is caused by the fuel pickup tube being blocked. this passage
> is open because this is the idle fuel circuit pickup.

Absolutely WRONG! The brass tube is the "straw" that the starting
enrichener sucks gasoline through. It has absolutely NOTHING whatever
to do with the idle mixture. The pressed in jet that you see in the
bottom of the brass tube is the starter jet. It doesn't appear on the
carburetor parts diagram because it is not ordinarily replaced. You'd
have to buy a whole carburetor body to get a new starter jet.

> There are
> three other tiny holes just b4 the butterfly valve also. These are
> fuel passages for the choke circuit. This is why the bike will only
> idle with the choke pulled. When you engage the choke, fuel passes
> through these holes and enriches the mixture.

Again, you are absolutely WRONG. The "choke" uses a separate passage
that goes around the butterfly. The starter outlet port is around
1/8th or 1/4 inch in diameter. There is NO WAY you could mistake that
single port for the three holes.

The three tiny holes by the butterfly are the transition mixture
ports. They give the engine a much needed shot of fuel air mixture as
vacuum drops off when you first crack the throttle to accelerate from
a stop.

They get their fuel from the idle jet that is concealed by a rubber
plug. The idle jet has to draw its fuel through a diagonal
passage that goes to the main jet.

http://i19.tinypic.com/3324bci.gif is a little confusing to look at.
There is no connection between the starting enrichener and the idle
mixture circuit, but the drawing makes it appear that there is.

> When the bike warms up
> and you close the choke, fuel is supposed to pass through the single
> hole that is the idle fuel circuit.

Again, you are WRONG. The single hole is NOT the idle fuel circuit, it
is the idle mixture PORT.

> This circuit should always be open
> (not gunked up). The idle mixture screws are part of this circuit also

You are throwing terms around that you do not understand.

The idle circuit is a branched passageway with three intake jets and
four outlet ports

The idle jet gets fuel through the main jet. The idle mixture circuit
gets gasoline through the idle jet.

The idle mixture circuit gets air through the pilot air jet.

The air and fuel mix in a passageway that branches out to feed the
idle mixture port and the three transition ports.

Only the idle mixture port is adjustable, the transition ports are
controlled by movement of the trailing edge of the throttle butterfly.

The idle mixture screw acts as a sort of "trimmer" on the idle mixture
circuit. Throttle butterfly position is critical to the operation of
the idle mixture circuit.

The mistake that amateur tuners make is believing that the engine RPM
will continue to increase as they turn the idle mixture screw
counterclockwise.

When they turn it too far, the idle mixture becomes too rich and the
idle RPM falls. They compound their mistake by turning the master idle
knob clockwise to raise idle RPM.

This uncovers the three transition ports and then the engine gets far
too much fuel at idle. It responds by idling far too fast, around 4000
to 5000 RPM.

The amateur tuner may turn the master idle knob back down, but then he
is stuck with the problem of low RPM idle and stalling.

If amateur tuners just understood how the idle mixture circuit worked,
they would know that they had to turn the idle mixture screws a little
clockwise to lean up the idle mixture.



From: tylernt on 26 Mar 2007 20:36
Argh, I am losing this battle. Screwed the idle mixture screws down to
3.5, 3.0, 2.5, and now 2.0 but I still have to give it some throttle
(or a huge amount of master idle ajustment) in order to get it to
idle. I think it's backfiring through the carbs at 2.5 and 2.0 but at
3.0 it sounded like afterfiring in the pipes (not really sure, I am a
total noob at this). Is it really so sensitive that I'd have to do
something like 2.75? Is there a good way to distinguish lean
backfiring vs rich afterfiring?

I'm at my wits end here...

From: Potage St. Germaine on 26 Mar 2007 21:44
On Mar 26, 4:36?pm, tyle...(a)gmail.com wrote:
> Argh, I am losing this battle. Screwed the idle mixture screws down to
> 3.5, 3.0, 2.5, and now 2.0 but I still have to give it some throttle
> (or a huge amount of master idle ajustment) in order to get it to
> idle.

It sounds like your carbs are either still dirty inside, or the float
level is too low. If you have to use a lot of throttle, you create
more vacuum and make the engine suck harder, and perhaps it's getting
fuel from the needle jet, instead of the idle jet.

When you spritzed the passages out with carb cleaner, did you see a
nice stream of liquid come out each of the three transition ports and
a stream come out of the single idle mixture port? Did you remove the
idle jet and squirt carb cleaner through it to get a stream of liquid
come out the four idle ports?

If the carbs are that clean, the engine should idle down on a small
amount of throttle opening.

> I think it's backfiring through the carbs at 2.5 and 2.0 but at
> 3.0 it sounded like afterfiring in the pipes (not really sure, I am a
> total noob at this). Is it really so sensitive that I'd have to do
> something like 2.75?

Some people talk about adjusting their idle screws down to 1/16th of a
turn one away or another to get the mixture perfect.

I generally do idle mixture adjustments 1/4 turn at a time. I'm happy
when I get good throttle response, the idle RPM doesn't hang up too
high when the engine is warm and the engine starts on the "choke".

I don't have an exhaust gas analyzer to adjust my carbs to factory CO,
NOX and HC emissions standards, but EGA's don't work right on engines
with 4-into-1 exhaust systems, they only manage to read an average of
the four carbs on a 4 cylinder machine.

The idle mixture screws on my FZR1000 had an incredible difference in
adjustment. One was 1/8th of a turn open, two of the screws were about
3/8ths of a turn open, but the last screw was 1-1/4 turns open!

And that was the factory setting! I don't know how the hell Mikuni or
Yamaha adjusts idle mixture at whichever factory do it, but they sure
missed the setting on my Yamaha...

Another expen$ive way to get the idle mixture set right is to drill a
hole into each exhaust pipe and weld on a threaded spigot so a
thermocouple can be screwed into the hole. When the exhaust gas
temperature is around 1150 degrees, the fuel air mixture is perfect,
as I recall from reading private airplane manuals.

But, you don't need a lot of fancy and expensive equipment to adjust
idle mixture and synchronize a twin cylinder engine.

BMW riders have said that they just make a simple manometer out of a
piece of aquarium air hose with some ATF in it.

They hook the two ends of the tube to vacuum ports on the carbs, and
adjust the idle RPM screws so the ATF is the same level on both sides
of the tube when the engine is running.

Don't twist the throttle while doing this. You'll suck the ATF into
the engine and maybe oil the spark plugs, or even hydraulic lock the
cylinder. That's bad. Rods bend when the piston suddenly stops.

Other riders have said that you can use a simple well manometer to
adjust idle mixture before synchronizing the carbs.

One end of the plastic aquarium air hose is hooked to the vacuum port
on the carb, and the other end is sitting in a jar of ATF.

You try to get the idle mixture screws and master idle knob to the
point where the engine idles at the specified idle RPM with the least
amount of throttle opening and the idle screws open the smallest
amount that results in a smooth idle.

> Is there a good way to distinguish lean
> backfiring vs rich afterfiring?

It's the pitch of the sound. Lean backfiring is a SNAP! and rich
backfiring is a BOOM! An engine will often lean backfire out the
carburetors

> I'm at my wits end here.

Been there, done that, went through all kinds of frustration to learn
simple facts about how engines worked.


First  |  Prev  | 
Pages: 1 2 3
Prev: Different Headlight for 86 Honda VF700C
Next: Rebuilding koni motorcycle shocks