classic squid tech

that unfortunately I need help with.

Good friend bought a new ZRX1200R this spring. Decided a couple of
months ago that it wasn't "loud" enuff, bought a used slip-on muffler
and installed it.

Just rode it last night. Starts and idles fine. Blat blat blat-blat
blat-blat blat out the exhaust while mildly accelerating (up to 1/4
throttle). Seems to either clean up (or the blats run together)as rpm's
build. Bottom end torque about 1/2 of what it was with the stock pipe.
Doesn't miss or hesitate, takes throttle well, just doesn't "pull" like
it did-accompanied by the blatting exhaust.

Drop the hammer and the exhaust note sounds like a turbine engine
(which I am quite familiar with). With the new muff, can effortless
lift the front wheel when the r's are up. I had ridden it a couple
times with the stock muff, and this was NOT the case.

Looked in partsfish, am personally unfamiliar with what looks to me
like a diaphram-type carb. I can work my way through a generic slide
carb (and have). Was able to identify the idle bleed/plug in the pic.

He's asking me if he's "hurting" the bike to ride it this way, and if
there would be a relatively easy way to "clean up" the low end.

I really don't want to wrench on his brand-new scooter, but would like
to be able to help steer him in the right direction.

Thanks;

TC

 

What year?

That's the classic loss of mid-range torque from reducing "back
pressure".
Maybe a semi-tunable canister like a Supertrapp is in order. But he'll
probably discover that when the low end torque feels right, the exhaust
note isn't as loud as the other can...

Carburetor? Late model super duper motorbikes all have FUEL INJECTION!
It may be Power Commander time for that machine if you want to re-MAP
the ECU...

Running with an open exhaust system can burn the valves because of
fuel/air mixture leaness. More air can flow through the engine, but the
ECU may not be able to richen the mixture up enough...

 

2003 (might be an 04)

that's kinda what I thought...

Found a picture that looked just like what was on the bike, Keihin CVK
on the side of the body, you may be indeed correct, but it sure looks
like a carb (x4) to me. It's more a retro super duper motorbike.

Problem is in this neck of the woods, there aren't any reputable (or
knowledgeable) tuners. My gut is a WOT chop-and-stop will show that the
top end is close. Be nice to have access to super duper electronic
goodies that tell the rest of the story.

Thanks for the info;

TC

 

wrote in @f14g2000cwb.googlegroups.com:

Easy? How about straight forward?

He needs to, at least, put a jet kit in the bike.

See: http://www.ivansperformanceproducts.com/zrx1200.htm

I put one of Ivan's kits in my Bandit 1200. It was a straight-forward
process. I had the bike Dyno'd later and the results were pretty good.
"Only" about 110 peak horsepower, but a wide-ass power band, and good
mid-range torque.

I'd say that'd be about the easiest way to go.

 

OK, that motorbike does in fact have CV carbs. The pilot mixture screw
is underneath the carbs, forward of the float bowls. You have to drill
out the
92066 EPA anti-tamper plug to access the pilot mixture screws. You
could open it half a turn or so. Won't do much for the midrange flat
spot, but will make the off-idle acceleration a little better.

Mid range flat spot. If you look at dyno run sheets, you'll usually see
a double humped torque curve. Screw the power curve, it's just a
calculated curve based on the torque curve and the RPM. Big top end
power numbers are meaningless if you have a falt spot or a dip in the
torque curve.

The torque curve just jumps right up from zero in the first 2000 RPM,
you get air velocity increasing through the intake ports and the torque
jumps up and it rises to a peak about 5000 RPM and a well tuned engine
with an airbox and the factory mufflers may just have a flat spot
between 5K and 7K...

But, if you have a 4-into-1 pipe and you diddle with the canister, to
may get the the dreaded flat spot or even a dip in the torque curve
like you've observed from seat of the pants experience.

After about 7K, the engine recovers from the dip and accelerates away
like crazy, as you also noticed. Back in the late 1970's motorcycle
magazine rider were just starting to notice this phenomenon. The guy
who wrote about the GS750 Suzuki claimed that it had a "delightful two
stage power delivery", that the first hump in the torque curve was nice
for riding around town and that the second hump was great for blitzing
down a wide open road.

By the mid-1980's, the motorcycle magazine riders were admitting that
they had a serious flat spot problem. Now that riders are wanting
nostalgia bikes like that big Kwacker your friend has, the dreaded falt
spot is part of the package.

The jet needles don't seem to have any slots for adjustment, you'd
probably have to buy a bunch of small 3mm inside diameter washers and
stack them underneath the needle cap to raise the needles to get the
engine to come onto the taper with less movement of the vacuum slide.

The pilot jet is a #35. You could go to a #40 or #42. That will help
the cruise mode out. The engine is running on the pilot jet while
cruising down the highway anyway, it's not running on the main jet.

The main jets are RIDICULOUSLY small for an engine of this size. The
parts diagram on www.buykawasaki.com shows that available jets are #88,
#90, #92, #95, #98 and #100. On a 1200cc engine you could use #125 jets
at WOT.

Whether the needle jet would pass enough fuel at WOT with the needle
pulled all the way up by the vacuum slides is a different story. You
might have to do some research on needle jet orifice diameters and jet
needle diameters at various points to get a needle jet/jet needle combo
that is larger than the area of a #125 main jet.

You'll probably find all that info on the www.sudco.com website, they
have
..pdf files on all the carb parts.

Or, maybe some speed tuners are tweaking this particular model and can
steer you the right way. I hesitate to tell you to buy a Dynojet kit
and buy into all their secret propietary information. Owners will buy
DJ kits, thinking that it's a bolt on solution to their carburetion
problem that's driving them nuts.

But Dynojet did all the dyno testing and gathered all the secret
information and they give you little mysterious instruction sheets that
say to do this test or that test and if the carburetion fails this or
that test to install this jet or that jet and they never tell you how
the carburetor works or what the basic problem is.

See, what you have is a retrobike with a 4-into-1 exhaust system. 4
into 1 pipes return an adverse positive pressure wave back up the
exhaust pipe at 5000 to 7000 RPM. The wave goes all the way through the
exhaust valve AND the intake valve while they are open during the valve
overlap period.

This pushes air backwards through the carburetors. The carbs are stupid
devices, they don't know which way the air is going, so they suck up a
shot of gas out of the float bowls. Then the air starts going the right
way, and guess what the carbs do? Right. They suck up a second shot of
gasoline!

The mixture goes from too lean to too rich and back and forth as you
putz around in the range of the flat spot.

There's really not much you can do for the midrange flat spot by
tinkering with jets, because the air is flowing the WRONG way. Hot
rodders used to look under the hoods of their cars that had no air
cleaners. As they revved the engine up to impress each other with the
sound of power, they'd notice the fog of fuel/air mixture standing
OUTSIDE the carburetor.

That's why they called it "stand off". It was really a standoff between
the exhaust system that was doing the WRONG thing and the camshafts,
which were *trying* to do the right thing, i.e., get fresh fuel/air
mixture into the cylinders.

The Dynojet customer service rep told me that what I was supposed to do
was
accelerate quickly through the flat spot and get through it on
momentum. I couldn't do that, as I was canyon riding in tight twisties
where I needed to use the mid range of the machine!

I think a WOT run would reveal that the mainjet is ridiculously small
for
high speed runs. You can hear the sour, groaning sound in the exhaust
when the mixture is too lean. The spark plug insulator tip may show
black specks like pepper. That's from pinging because the mixture is
too lean. The next step is when you see little silvery balls of
aluminum on the insulator tip. That's the piston melting from lean
mixture.

Then there's the airbox. Engines like their airboxes. The airbox
resonates at a certain frequency which enhances the power delivery and
helps overcome the mismatch between the 4-into-1 pipe and the cam
timing. The airbox keeps a slight pressure inside of it, it's not just
a holder for the air filter element.

Dragracers who run their engines at WOT can remove the airbox and
install K&N separate filters and they never run into the loss of
midrange torque
because the shoot right through the midrange on WOT...

We used to fool around with removing the airbox lid to get more air
into the engine. That was like installing separate filters, the airbox
resonance was defeated. Guys used to drill a lot of small holes in the
airbox too. Swiss cheesing the airbox defeated its resonance too.

One thing that didn't work well was removing the snorkel (Kawasaki
calls it a "silencer" on the model your friend has). My Suszuki really
*needed* that snaorkel. With the snorkel removed, the engine started
lean surging in the mid range.

 

When the exhaust valves burn out, buy it off him realllly cheap. Fix it,
put the stock muffler on it and enjoy.

I have no patience for those who just want to make noise.

- Nate >>

 

Me either-maybe you missed the "pulls the front end" comment.

Perhaps I should have mentioned that having ridden performance
oriented motorcyles for over 30 years (while having never really
maintained/modified one), he was amazed that throwing on an el cheapo
can made that much of a difference. So was I, truthfully.

He was actually a little disappointed with the performance
before-hand, and was trying to make himself feel better by making it a
little "louder".

Personally, I would appreciate a little better idea of what is going
to "burn out" the exhaust valves.

Regards;

TC

 

Simple. Your engine does not burn a stoichmetric mixture or fuel and
air, i.e., there is not exactly enough fuel to combine with the oxygen
passing though it, there is extra fuel (with the stock jetting) and the
extra fuel keeps the mixture rich and the temperature inside the
combustion chamber remains below the temperature that aluminum melts at
and the valves don't get so hot they momentarily weld themselves to the
valve seats.

Now, if you keep the jets exactly the same size, but you radically
increase the airflow through the engine by uncorking the exhaust system
there is an increased amount of oxygen available to make the flame
temperature higher, the pistons can melt, the valves can weld
themselves to the valve seats, and of course the camshafts immediately
open the valves, tearing the tiny welds apart and some steel off the
valves and seats goes flying out the exhaust pipe.

After several thousand miles of this overheating, the valves will look
like they've been chewed on by mice. The way to avoid valve burning is
to install larger jets, as I mentioned previously, richening the
mixture back to what it was when the bike was stock, or by restricting
the exhaust pipe outlet like it was orignally with a muffler that
doesn't flow as much air...

That's how Supertrapp is able to sell an aftermarket muffler that
requires no rejetting. The purchaser of a Supertrapp has the pleasure
of modifying his exhaust system without increasing the airflow (and the
noise).

Then, if he rides the motorbike harder, he can be impressed by all the
power that was always there, he just never felt he wanted to use it ;-)