Why is there 1 carb per cylinder when a car has one carb for multiple cylinders?

Nah, I agree with you just as much. See other thread. But, as an
illustration, that huge tome addressed to the guy with the dicky
CG125....

 

Oh yes indeed )

 

Like I said before, I work from the presumption that anybody asking a
question to rec.motorcycles.tech knows very little about motorcycles,
but may have some misconceptions caused by his/her experience with
cars. So I take extra effort to start with the basics.

Too bad if that bothers you.

 

What kind of motorcycle is it? Is it an inline-4? Does it have a
4-into-1 pipe? Or does it have a stock exhaust system with balance
tubes between the pairs of exhaust pipes? Do you only ride in straight
lines, with no slow corners?

 

Not to argue with you, but i also have a bike with pod filters and no
(noticeable) flat spots in the midrange. it's a honda cb350 twin
with stock
(CV?) keihin carbs and k&n pods.
of course, the stock air filters are just paper elements with a metal
casing. and the exhaust consists of two seperate headers leading to two
seperate mufflers. no cross-over of exhaust gases at all.

But I've never had it on a dyno, so perhaps my perception is off.

-Sean

 

No, it's fine, but if you were to start with the basics on a CG125 (and
bikes don't come any more basic), it would be a good idea to refer to
slide carbs rather than CVs, which it doesn't use (and never has).
Otherwise you wind up with something totally irrelevant and which has
the potential to confuse a newbie to the point where he starts acting
on incorrect advice.

It also has the side-effect of showing that you have zero familiarity
with the bike under discussion....

 

OK, so you have a twin cylinder machine with separate pipes and that is
a major clue
as to why you don't have the dreaded inline four flat spot.

The original poster has a motorcycle with an inline four cylinder
engine.

New riders who own inline fours often wonder why they have all those
carburetors, and, wouldn't it be simpler to just have ONE carburetor on
a simple log type manifold,
like a car has. That's why he asked about motorcycles that had single
carbs.

As I explained previously, George Kerker and other motorcycle
hotrodders decided to try 4-into-1 exhaust headers on the most popular
inline fours to take advantage of the high RPM cylinder over-filling
effect caused by another cylinder which is exhausting burnt mixture.

This effect can only work if the cylinder that is filling has valve
overlap. The intake valve opens early and the exhaust valve closes late
and a cylinder that is on the exhaust stroke helps suck fresh mixture
into the charging cylinder.

Problem is that the beneficial pressure waves in the exhaust system can
be out of phase with what you hope to accomplish. The speed of sound in
a gas is based upon its temperature and the exhaust system has a
certain tuned length and the pressure waves return to the cylinder at
regular intervals that can be determined by mathematical formulae.

The waves have to be "in synch" with the motions of the pistons. A
4-into-1 pipe returns a beneficial sucking wave at high RPM but it also
sends an adverse positive pressure wave back through the cylinder and
out the open intake valve and through the carburetor at some RPM.

The carb is a dumb device, it has no idea of which way the air is going
through it. So it picks up a shot of gasoline out of the float bowl and
throws it into the open air behind the carb, or into the air filter or
into the air box, depending on what you have.

Then the airflow reverses direction, the carburetor picks up a second
shot of gasoline.

The amateur tuner feels this problem as a flat spot between 5000 and
7000 to 8000 RPM, and imagine he might be able to do *something* with
jets, but it's a mystery. Nothing he does with jets works.

The various aftermarket exhaust system manufacturers offered pipes with
extra chambers to add volume, they offered 4-into-2-into-1 pipes and
claimed that they were the solution to the adverse pressure
wave-induced flat spot.

Some inventors even suggested variable length 4-into-1 pipes with
computer-controlled servo motors to vary the lengthof the system in
order to get rid of the flat spot.

But the system that works is Yamaha's ExUP, the computer-controlled
servo-operated exhaust throttling system. All of the Japan Inc.
manufacturers have adopted the ExUP system, with various acronyms that
seem to indicate that they invented it themselves.

 

Um, just a question:

How come the manufacturers manage to produce in-line fours with
four-into-one pipes, without flat spots? I remember a very respected
tuner saying that the one huge mistake to make was that the factory
designer knew nothing about exhaust systems...

That said, a lot of aftermarket four-into-ones have been utterly
dreadful.

Actually, no, they haven't, because Yamaha's patent lawyers would have
their arses if they did.

They have variations on a theme, agreed, but Yamaha's EXUP is indeed the
original and best. As was its YPVS system on its two-strokes. Yes, other
makers built power valves, but none was as elegant and effective as
Yamaha's, and nobody copied it for exactly the same reason they haven't
copied the EXUP. Patents.

"Yamaha's EXUP system that functions to eliminate torque valleys and
provide excellent power development characteristics has been made more
compact on this model while retaining the same basic operating
principle. Unlike the former system that included a separate valve for
each of the four exhaust pipes, the system on the new YZF-R1 places just
two EXUP valves at the points where the four exhaust pipes merge into
two, thus creating a more compact system that controls the pulse based
on calculations of the 1st and 2nd cylinders' exhaust and the 3rd and
4th cylinders' exhaust."

(From Yamaha's website)

And it's not just fitted to four-cylinder engines, either. *Please* stop
relying on your own perceived wisdom.

 

You can use zero overlap valve timing.

Snip a bunch of cut and paste...

Why? In a desert of one-liners and other egotistical post, my own
perceived wisdom is what I have to offer people who ask honest
questions.

If you will pardon me now, I shall go and assist the happy campers at
the lake...

 

I've posted this before in this forum, but for the record it's a 1980 CB750K

stock, 4 into 4.

Only when I'm on the freeway (every day)

Seriously, I think your putting way too much reliance on any Hemholtz
resonance in the airbox fixing a flat spot. I have no doubt that
the airbox changes things, but I don't think it's as important as jetting
and general maintainence of the motorcycle - at least, not on a larger
displacement bike. Perhaps a 50-100 cc engine it would matter,
or perhaps if your racing at high speed (not some offroad dirt bike
thing) you would see a difference.

On my bike, the pod filters were on it when I bought the bike. I did
get the airbox from the prior owner, unfortunately it's missing one of
the manifold rubber connectors so it's just sitting until I can find a
replacement that won't break the bank.

Incidentally, there WAS a flat spot at 6000 RPM when I bought my
bike, in fact the seller commented on it. I thought it was carb settings
but I haven't yet got around to doing anything about the carbs. Do you
know what fixed it? I'll tell you. Replacing the chain.

When I bought my bike the chain was an old o-ring chain stretched to
the limit, most rings had fallen out of it, it was rusted, the front and
rear
sprockets were both pretty worn. I ran about 400 miles on that mess then
replaced everything, a new steel front sprocket, new aluminum rear sprocket,
and new O-ring chain. I did NOT change number of teeth on either sprocket,
and took pains to make sure that the sprockets had the same number of
teeth as the factory stock sprockets (the existing ones did, and the new
ones
do) In fact, seat-of-the-pants performance of the bike increased after
doing this.

My assumption is that the old chain and sprockets were so incredibly
worn and also adjusted too tight, that they somehow introduced enough
drag at speed as to create the flat spot. Beyond that I have no other
explanation, and I really don't like that explanation anyway, because
if the chain could have actually taken that much horsepower, why didn't it
break, or melt into slag.

Ted

 

Fine. As long as they get accurate answers....

 

Well, I don't try to remember what every user in this NG rides or owns
or claims to have ridden or owned. The main idea of
rec.motorcycles.tech is that a newbie asks a question and we respond to
*that* question, avoiding turning the question into a debate.

OK, that's what it is. You don't notice the flat spot when riding
straight up in a straight line because you can accelerate rapidly
through the RPM zone where reverse flow occurs.

Racers routinely removed airboxes from their motorcycles in the early
1980's and replaced them with K&N's or foam Pods. The early 80's was an
era when drag racing
fascinated grass roots riders who weren't all that keen on going around
corners.

But, when the production racers like GSXR's became available the focus
changed to going around tight corners at part throttle and accelerating
out of the corners.

The racers began putting their airboxes back onto their bikes and
pretty soon they were also ducting cold air into the boxes and putting
ram air scoops on the fairings.

The ram air system may only give about 1/3rd of a pound of pressure,
but the extra density of the cooler air, combined with the resonant
frequency of the airbox helps to flatten out the midrange torque
valley.

 

I had to X-post this to ukrm, where there are plenty of People WK.

 

In message

People Who Kayak ?

I will thumb through my ancient dog eared 'Tuning for Speed' to see if
Saint Phil has anything to say on the subject, but as it was written in
the days of 7Rs (that's PROPER ONES CHAMP) and Vincents he may be silent
on the matter.

--

+-----------------------------------------------------------------+
| Pete Fisher at Home: |
| Voxan Roadster Moto Guzzi Mille GT/Squire RS3 Gilera Nordwest |
| Gilera GFR Moto Morini 2C/375 |
+-----------------------------------------------------------------+

 

Individual foam filters are only of any use in a custom mode - where you
don't or can't get a suitable airbox into the space available. In that
case they provide a cheap and easy fix with suitable jetting. Some
numbnuts run with only trumpet intakes and stone guards, but they're,
quite frankly, off their heads.

The manufacturers' airbox is a thing of beauty, designed to flow
sufficient air at all parts of the rev range and avoid such problems as
mentioned above, such as flat spots, etc. If you had to pay for an
airbox solution and somebody was selling the same, you could bet your
arse that airboxes would be the next cool thing.

You get them for free with most bikes and they do the job they're
supposed to do. People who throw airboxes away are idiots.

 

Maybe, but I used to have all the dyno graphs from the work on my
XJ900F, done at Dynojet's place in Warrington. They borrowed my bike
to develop their stage 1 and stage 3 kits for the XJ, and did 30 - 40
dyno runs in all.

At stage 3 it had individual K&Ns and a Laser race 4-2 exhaust plus
the appropriate Dynojet kit.

The torque curves were beautifully flat, with no holes at all, and the
power was up by 12% on stock over most of the range and rather more
where it had filled in the mid-range dip.

 

Yebbut, midrange dips were designed into engines to get through noise
regs... weren't they?

 

So I believe. The race cans made that irrelevant.

 

<suitably chastised>
Oops. Tired and emotional at time of posting. All misty eyed after
blowing the dust of TFS and opening it to find all my old gear ratio
calcs from the two stroke single class road racing days.

--

+-----------------------------------------------------------------+
| Pete Fisher at Home: |
| Voxan Roadster Moto Guzzi Mille GT/Squire RS3 Gilera Nordwest |
| Gilera GFR Moto Morini 2C/375 |
+-----------------------------------------------------------------+

 

Using the patented Mavis Beacon "Hunt&Peck" Technique, Champ

And this negates the "off their heads" assertion how, exactly?

--
Wicked Uncle Nigel - Podium Placed Ducati Race Engineer

WS* GHPOTHUF#24 APOSTLE#14 DLC#1 COFF#20 BOTAFOT#150 HYPO#0(KoTL) IbW#41
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