Is rejetting/shimming really a must after fitting just a slip on can?

[RBN]

I've owned a ton of bikes over the years and never had to rejet after fitting a straight-through end can. But I have read in several places here that on the EX250 the end can alone makes a big difference to jetting. Which in my experience is quite unusual.

However here is the difference and the reason I am asking. All of the ones I have read here were looking for a performance increase which I am not interested in at all, I ONLY want the bike to run reasonably well without being horribly lean. And if I can avoid the hassle of a rejet I would gladly do so.

Has anyone changed just the end can without having to rejet/shim?

I'm running the larger standard 108/38 jets (some ran 105's stock) with matching N16I needles in both sides and live pretty much at sea level.


Oh and also does anyone know the factory spec pilot screw turns out for carbs with the OEM jets mentioned? As I seem to be reading a lot of conflicting numbers anywhere from 1.5 to 2.75 which is quite a difference, the manual I have suggests 1.75 but lists the jet size as 105.

[Ghostt]

My 2¢,

IMHO Slip-ons don't require a jet change, you can play with shimming the needles, it's easy enough to do, and undo.

If your gonna play with the main jets, I strongly suggest going slowly, small steps up, track your MPG. The point is if you a single change at a time, it's easy to know if it worked or not, and you can undo it.

It's more important to make sure the carburetors are in proper working order, clean, float heights set, etc.. Also just as important if not more is valve adjustments, if the valves are off, tuning is pointless.

Finally here's my write-up on the pilot screws, they need to be fine tuned for best results.

Your initial setting should be 3 full turns out from the bottom, remember just slightly tighten them, don't go all Conan on them.



Then warm it up, slowly turn 1 side pilot screw in until the rpms drop, then back out till RPM's peak (and no further)* Twist throttle...see how it reacts...tweak to taste.
Repeat on other side... test ride. You're looking for best throttle response and transition from just cracked to 1/4 throttle opening or so.*

Thats a quickie, 10 cent tutorial on setting pilot screws.**

Finally unless you have access to an exhaust gas analyzer, playing with jetting is a guessing game.

Playing with carburetors, and jets is a skill, art, and requires a lot of patience.

[allanoue]

Slip ons don't need rejetting.

[jkv45]

Even a completely stock engine can benefit from carb tuning.

Start with the way it runs at WOT, and move down from there. If it runs clean and pulls strong on WOT your Main Jet is correct. Next look at the way it runs in the 1/2-throttle range. The combination of the needle size and height and the size of the Main Jet control that range.

The Pilot Jet controls the general idle mixture and slightly above.

There are no perfect settings for the idle mixture screws. It will depend on other factors like elevation, temp, type of gas, etc. In general, 2.5 turns out gets you close. If you are significantly in or out more than that you need to change the Pilot jet to get in a better adjustment range.

[LittleRedNinjette]

Yes they do. Been there done that 3 times now.

A properly tuned bike is a happy bike.

[allanoue]

Originally Posted by LittleRedNinjette

Yes they do. Been there done that 3 times now.

A properly tuned bike is a happy bike.

*sigh*

You still won't accept your gains from rejetting may have been the same with a stock can.

[Ghostt]

I love jetting posts.

[jkv45]

Originally Posted by Ghostt

I love jetting posts.

More or less than oil threads?

[RacinNinja]

Short answers:

No, it's not required. Install the slip on and make noise.

BUT.....it's worth the time and effort.

[soonerbillz]

Here is the bottom line...


"Here is the secret of the CV carburetor: up to 3/4 throttle, the engine
is getting most of its mixture from the pilot jets, not the jet
needle/needle jet combined orifice.

Compared to the pilot jets in an older slide valve carburetor, the CV
carb has a huge orifice. Whereas a slide valve carb might have a #17
pilot jet ( the hole is 0.17 mm in diameter ), a CV carb might have a
#30 to #45 pilot jet.

We all learned to calculate the area of a circle in junior high school.
The #45 pilot jet has SEVEN times the area as the #17 pilot jet.

So the CV carb's pilot jet flows most of the fuel into the carb when
the throttle isn't wide open and the slide isn't lifted far enough to
pull the jet needle out of the needle jet.

You can really enhance your off idle throttle response and improve the
midrange power by finding the pilot mixture screw and tweaking it about
half to one full turn counterclockwise.

The pilot mixture screw is probably hidden under a blind plug, forward
of the diaphragm cap on top of the carb, or underneath the carb,
forward of the float bowl.

If you cannot see a slot screw on the side or top of the carb or underneath it,
the EPA plugs are still intact. You have to pilot drill the plug, screw a
small sheetmetal screw into the pilot hole and then pull the screw and
plug out with a pair of pliers.

Now you can adjust the pilot mixture screw by turning it counterclockwise a bit.
Note that CV carb idle mixture screws work the opposite way. Clockwise is leaner,counterclockwise is richer.

Riders get into trouble when they expect the idle RPM to increase the
more they turn the screw counterclockwise. When they've gone too far,
the exhaust sound is dull and thudding and maybe the the idle RPM is
too low.

So they adjust the master idle knob to increase the idle RPM. Then,
when they twist the throttle grip open and closed, the engine RPM hangs
up at 5000 RPM and they wonder what the heck is going on?

The amateur tuner doesn't know about the three transition ports right
by the throttle butterfly's bottom edge. The transition ports are
getting fuel from the pilot jet too, but they aren't controlled by the
pilot mixture screw, they are controlled by the bottom edge of the
butterfly.

The amateur tuner tweaks the pilot screws CW and CCW and can't figure
out which way they should be turned because he doesn't understand that
the transition ports are getting fuel that BYPASSES the pilot screws...

Pilot fuel is sucked out of the float bowl through the idle jet. It
goes through a branched passage to a pattern of transition ports that
are covered by the edge of the throttle butterfly when it's closed. Air
from the fixed air jet is added to the same passage. Fuel/air mixing
occurs in the passage. As the throttle butterfly is opened *slightly*
the vacuum in the carb's bore can suck fuel/air mixture out of the
transition ports and the engine speed can increase smoothly. There is
another branch of the same passage that has an outlet about 1 inch
downstream of the transtion ports. Vacuum is weaker there. The pilot
screw adjusts fuel/air flowing out of that port...

When the idle RPM hangs up at 4000, 5000,even 6000 RPM, that tells the
experienced tuner that the pilot mixture screw is open too far. So, he
has to turn it back the other way to get the engine to idle smoothly at
the smallest amount of throttle opening and the least number of turns
out of the pilot screw.

You can do so much with pilot jets and adjusting the pilot mixture
screw, The strategy for adjusting the pilot mixture on a CV carb is to get the
engine to run smoothly at the specified RPM with the smallest amount of
throttle opening possible. The amateur tuner fools around with the
pilot mixture screws, not understanding that the engine is drawing fuel
air mixture from the transition ports that may be slightly uncovered by
the throttle butterfly, according to the setting of the master idle
knob and the synchronizing screws...

Now, one of the tests for pilot mixture setting is that you open the
throttle quickly, and close it. The engine speed should increase, but
it should return to the original setting. If it hangs up, the pilot
mixture is too rich, because the engine is ALSO getting fuel from the
pilot outlet port AND the transition ports.

> What about main jet size?

Let me educate you on the Mikuni/Keihin jet sizing system. It's just
like the pilot jet sizing system.

The orifice size is based upon a #100 mainjet having a hole that's
exactly 1.000 millimeters in diameter.

The area of this orifice increases as to pi times the square of the
radius, so you can see that going from a #100 main jet to a #120 main
jet will result in an orifice with an area that's 44% larger and will
flow far too much fuel whenever the engine has enough vacuum to suck
fuel out of the float bowls.

The engine will be drowned by excess fuel if you go up too many jet
sizes at once.

Going from a #100 mainjet to a #110 mainjet is not just one size jump,
it's FOUR sizes larger, #102, #105, #107, then #110.

Even the #110 mainjet has 21% more area than the #100 main jet. What
would make an intelligent rider think that his new exhaust pipe by
itself was going to raise his horsepower by 21%, requiring 21% more
fuel?

Constant vacuum carburetors rarely run on the main jet. You could even
probably get away with running the stock main jet or one that's just
maybe two sizes larger than the original jets.


> Will a needle shim be enough?. Should definitely be in the right direction.


Nope. That's an urban myth amongst amateur tuners that don't undertand
how CV carbs work. CV carbs have short fat needle without much taper,
and whacking open the throttle just doesn't lift the needle out of the
hole enough to matter. The grooves on jet needles with clips help
control when the tapered part of the needle begins to have effect, but
you'll get more off-idle acceleration effect out of turning the pilot
mixture screw just exactly the right amount.

You run at full throttle so rarely on the street or the highway, it
just doesn't make sense to mess around with main jets and needle
shimming. It makes a lot more sense to learn about the pilot jets and
how to set them correctly.

You may ask, "But why do all the tuning manuals say to start tuning by
finding the correct sized main jet?"

It's not about ultimate power, as shown on a dynamometer. You
can get a higher horsepower reading out of an engine that's just warm
enough to carburate cleanly than you'll get out of a hot engine.

Heat builds up during a long race and it builds up faster on a high
speed course like Daytona.

Racers who really *race* need a main jet that's big enough to pass
extra fuel so the engine won't overheat. The racers are willing to
waste fuel to keep the engine cool.

But the typical rider who buys a Dynojet or other kit thinks that he's buying "instant power"
and an instant solution to the EPA mandated lean burn jetting from the factory.

Sorry, it doesn't work that way. The rider who succeeds in getting what
he wants out of his engine is the rider who understands the technology
of that engine and why that particular technology was chosen for it."

[jkv45]

^ There are a few things in there that I don't exactly agree with...

[RacinNinja]

Originally Posted by jkv45

^ There are a few things in there that I don't exactly agree with...

You ain't the only one.

[Ghostt]

Originally Posted by jkv45

More or less than oil threads?

There both equally amusing.

[RBN]

Wow, did not expect such a lot of response after posting what is kind of just another jetting question on the face of it. Thanks to everyone who commented.

Ok so having read through these various comments I decided to go ahead and shove it all together and then see how it performs and take it from there.

And that's about where my simple question became more complicated,........

I thought what I had here was a matching set of parts from the same bike. But I don't think it is (unless the parts were shared more than I thought)

As I am assembling it I realise what I have bought here is in fact a set of 1998 ZZR250 carbs with the 108/38 jets and N16I needles as I stated above, a zzr 250 airbox. A ZZR250 CDI (part number ends in 1229), and a later model 2008 motor and 2008(?) stainless 2 into 1 exhaust header (no cat).

A call to the previous owner has confirmed that the engine has had the rotor and pick up swapped to match the CDI.

But I'd wager the jetting is way off. As I think the post 2007 bikes ran a 98 main.

How quickly a 5 minute job can turn into a messy headache. Right off I go to search the forum for anyone else who as fitted this motor to this set of carbs etc......

[JohnnyBravo]

Frankenbike... Always interesting

[RacinNinja]

@ducatiman

[Ghostt]

This just got interesting, pieces are pieces, parts is parts.

Link to original page on YouTube.

[SLOWn60]

@juliusmichaelhonrada needs to read this:

Originally Posted by soonerbillz

Here is the bottom line...


"Here is the secret of the CV carburetor: up to 3/4 throttle, the engine
is getting most of its mixture from the pilot jets, not the jet
needle/needle jet combined orifice.

Compared to the pilot jets in an older slide valve carburetor, the CV
carb has a huge orifice. Whereas a slide valve carb might have a #17
pilot jet ( the hole is 0.17 mm in diameter ), a CV carb might have a
#30 to #45 pilot jet.

We all learned to calculate the area of a circle in junior high school.
The #45 pilot jet has SEVEN times the area as the #17 pilot jet.

So the CV carb's pilot jet flows most of the fuel into the carb when
the throttle isn't wide open and the slide isn't lifted far enough to
pull the jet needle out of the needle jet.

You can really enhance your off idle throttle response and improve the
midrange power by finding the pilot mixture screw and tweaking it about
half to one full turn counterclockwise.

The pilot mixture screw is probably hidden under a blind plug, forward
of the diaphragm cap on top of the carb, or underneath the carb,
forward of the float bowl.

If you cannot see a slot screw on the side or top of the carb or underneath it,
the EPA plugs are still intact. You have to pilot drill the plug, screw a
small sheetmetal screw into the pilot hole and then pull the screw and
plug out with a pair of pliers.

Now you can adjust the pilot mixture screw by turning it counterclockwise a bit.
Note that CV carb idle mixture screws work the opposite way. Clockwise is leaner,counterclockwise is richer.

Riders get into trouble when they expect the idle RPM to increase the
more they turn the screw counterclockwise. When they've gone too far,
the exhaust sound is dull and thudding and maybe the the idle RPM is
too low.

So they adjust the master idle knob to increase the idle RPM. Then,
when they twist the throttle grip open and closed, the engine RPM hangs
up at 5000 RPM and they wonder what the heck is going on?

The amateur tuner doesn't know about the three transition ports right
by the throttle butterfly's bottom edge. The transition ports are
getting fuel from the pilot jet too, but they aren't controlled by the
pilot mixture screw, they are controlled by the bottom edge of the
butterfly.

The amateur tuner tweaks the pilot screws CW and CCW and can't figure
out which way they should be turned because he doesn't understand that
the transition ports are getting fuel that BYPASSES the pilot screws...

Pilot fuel is sucked out of the float bowl through the idle jet. It
goes through a branched passage to a pattern of transition ports that
are covered by the edge of the throttle butterfly when it's closed. Air
from the fixed air jet is added to the same passage. Fuel/air mixing
occurs in the passage. As the throttle butterfly is opened *slightly*
the vacuum in the carb's bore can suck fuel/air mixture out of the
transition ports and the engine speed can increase smoothly. There is
another branch of the same passage that has an outlet about 1 inch
downstream of the transtion ports. Vacuum is weaker there. The pilot
screw adjusts fuel/air flowing out of that port...

When the idle RPM hangs up at 4000, 5000,even 6000 RPM, that tells the
experienced tuner that the pilot mixture screw is open too far. So, he
has to turn it back the other way to get the engine to idle smoothly at
the smallest amount of throttle opening and the least number of turns
out of the pilot screw.

You can do so much with pilot jets and adjusting the pilot mixture
screw, The strategy for adjusting the pilot mixture on a CV carb is to get the
engine to run smoothly at the specified RPM with the smallest amount of
throttle opening possible. The amateur tuner fools around with the
pilot mixture screws, not understanding that the engine is drawing fuel
air mixture from the transition ports that may be slightly uncovered by
the throttle butterfly, according to the setting of the master idle
knob and the synchronizing screws...

Now, one of the tests for pilot mixture setting is that you open the
throttle quickly, and close it. The engine speed should increase, but
it should return to the original setting. If it hangs up, the pilot
mixture is too rich, because the engine is ALSO getting fuel from the
pilot outlet port AND the transition ports.

> What about main jet size?

Let me educate you on the Mikuni/Keihin jet sizing system. It's just
like the pilot jet sizing system.

The orifice size is based upon a #100 mainjet having a hole that's
exactly 1.000 millimeters in diameter.

The area of this orifice increases as to pi times the square of the
radius, so you can see that going from a #100 main jet to a #120 main
jet will result in an orifice with an area that's 44% larger and will
flow far too much fuel whenever the engine has enough vacuum to suck
fuel out of the float bowls.

The engine will be drowned by excess fuel if you go up too many jet
sizes at once.

Going from a #100 mainjet to a #110 mainjet is not just one size jump,
it's FOUR sizes larger, #102, #105, #107, then #110.

Even the #110 mainjet has 21% more area than the #100 main jet. What
would make an intelligent rider think that his new exhaust pipe by
itself was going to raise his horsepower by 21%, requiring 21% more
fuel?

Constant vacuum carburetors rarely run on the main jet. You could even
probably get away with running the stock main jet or one that's just
maybe two sizes larger than the original jets.


> Will a needle shim be enough?. Should definitely be in the right direction.


Nope. That's an urban myth amongst amateur tuners that don't undertand
how CV carbs work. CV carbs have short fat needle without much taper,
and whacking open the throttle just doesn't lift the needle out of the
hole enough to matter. The grooves on jet needles with clips help
control when the tapered part of the needle begins to have effect, but
you'll get more off-idle acceleration effect out of turning the pilot
mixture screw just exactly the right amount.

You run at full throttle so rarely on the street or the highway, it
just doesn't make sense to mess around with main jets and needle
shimming. It makes a lot more sense to learn about the pilot jets and
how to set them correctly.

You may ask, "But why do all the tuning manuals say to start tuning by
finding the correct sized main jet?"

It's not about ultimate power, as shown on a dynamometer. You
can get a higher horsepower reading out of an engine that's just warm
enough to carburate cleanly than you'll get out of a hot engine.

Heat builds up during a long race and it builds up faster on a high
speed course like Daytona.

Racers who really *race* need a main jet that's big enough to pass
extra fuel so the engine won't overheat. The racers are willing to
waste fuel to keep the engine cool.

But the typical rider who buys a Dynojet or other kit thinks that he's buying "instant power"
and an instant solution to the EPA mandated lean burn jetting from the factory.

Sorry, it doesn't work that way. The rider who succeeds in getting what
he wants out of his engine is the rider who understands the technology
of that engine and why that particular technology was chosen for it."

[RBN]

Last nights jetting blog(?) for anyone who is interested. (is this worthy of it's own thread?)

Not 100% finished yet but we are making progress.

Start specs:

Carbs: Standard CVK30 108 main 38 pilot N16I needles, 2.5 turns out pilot screw.
Airbox: ZZR250
CDI: ZZR250
Engine: 2008 fuel injected converted to carbs with ZZR Rotor, Pickup and Windings.
Exhaust: 2008 stainless downpipes, cat removed, straight through end can fitted.

Elevation: About sea level.
Fuel: 95 unleaded petrol.
Weather conditions: Splendid.
My breakfast this morning: Scrambled egg on Toast and 2 cups of coffee.
Coffee cup size: Medium.
Demeanour: Apathetic and pessimistic, aka British.

------------------------------------------------------------------------
Test ride 1:

Took the bike out for a ride, idles and starts fine both warm and cold. Has a flat spot/stumble at about 1/4 throttle which gets worse as bike warms up but can easily be ridden through. Hard roll off causes light popping from exhaust and occasional loud backfire. Sounds kind of rough at WOT but seems to pull fine for the most part but no powerband. Got up to 70mph but didn't want to go much over

Remembered I had not checked the air filter sponge which was almost totally dry.

-------------------------------------------------------------------------

Test ride 2:

Cleaned and oiled the filter as per the manual and set off again.

Bike seems slightly smoother (could be placebo) but remains largely unchanged by doing the air filter with a couple of exceptions. The light popping from the exhaust has completely gone, replaced instead by a single, predictable, colossal backfire (echos off nearby buildings) on very hard roll off.

Flat spot seems to occur over a shorter throttle range but is still very much there, top end cruising speed seems better.

------------------------------------------------------------------------

Test ride 3:

Pilot adjustments: Warmed bike and set about trying different pilot settings. The first flat spot/stumble is at around 1/4 throttle, but I really can't tell exactly so I'm not sure if it is pilot jet or needle, so lets start with the easier one.

1.75 turns no major effect*
2 turns out no major effect*
2.75 turns no major effect*

*on flat spot or top end, i think idle seems/sounds happiest at about 2 turns out.

------------------------------------------------------------------------

Test ride 3

Well as pilot jet adjustment did pretty much nothing, lets try the needle. Returning the pilot screw to original 2.5 turns (accompanied by a quick ride to make sure it is all back to original).

1 m3 washer under each needle: Flat spot worse, seems (may be my imagination) to pull better at about 2/3 open throttle. As bike warms up flat spot gets worse.

2 M3 washers under each needle: Completely unridable, starts and idles fine warm and cold, flat spot now 10x worse and bike will hardly accelerate at all, jolting, coughing and really unhappy.

Ok so that to me confirms that the needle is too rich, and it's a N16I so I can't drop the needle. That's frustrating as I think dropping it a notch or two would be perfect.

----------------------------------------------------------------------

Test ride 4

Everything returned to standard, snorkel removed as I can't think of another way to make it leaner and don't want remove the filter completely just yet.

Best improvement so far regarding the flat spot which has been dramatically reduced. Acceleration above 1/2 turn sounds very edgy/raspy (man how hard is it to describe sounds!?) which I don't like much. Can't recall how these sound? Do they get more raspy at high rpm? No backfire or popping on hard roll off.

------------------------------------------------------------------------

More later today............

[jkv45]

Try a leaner Main Jet.

[InvisiBill]

http://factorypro.com/tech/tech_tuni...m_engines.html has good guidelines for dialing in the different ranges of the carbs.

Originally Posted by RBN

Demeanour: Apathetic and pessimistic, aka British.

TIL I might be British.

[bruce71198]

Originally Posted by soonerbillz

Here is the bottom line...


"Here is the secret of the CV carburetor: up to 3/4 throttle, the engine
is getting most of its mixture from the pilot jets, not the jet
needle/needle jet combined orifice.

Compared to the pilot jets in an older slide valve carburetor, the CV
carb has a huge orifice. Whereas a slide valve carb might have a #17
pilot jet ( the hole is 0.17 mm in diameter ), a CV carb might have a
#30 to #45 pilot jet.

We all learned to calculate the area of a circle in junior high school.
The #45 pilot jet has SEVEN times the area as the #17 pilot jet.

So the CV carb's pilot jet flows most of the fuel into the carb when
the throttle isn't wide open and the slide isn't lifted far enough to
pull the jet needle out of the needle jet.

You can really enhance your off idle throttle response and improve the
midrange power by finding the pilot mixture screw and tweaking it about
half to one full turn counterclockwise.

The pilot mixture screw is probably hidden under a blind plug, forward
of the diaphragm cap on top of the carb, or underneath the carb,
forward of the float bowl.

If you cannot see a slot screw on the side or top of the carb or underneath it,
the EPA plugs are still intact. You have to pilot drill the plug, screw a
small sheetmetal screw into the pilot hole and then pull the screw and
plug out with a pair of pliers.

Now you can adjust the pilot mixture screw by turning it counterclockwise a bit.
Note that CV carb idle mixture screws work the opposite way. Clockwise is leaner,counterclockwise is richer.

Riders get into trouble when they expect the idle RPM to increase the
more they turn the screw counterclockwise. When they've gone too far,
the exhaust sound is dull and thudding and maybe the the idle RPM is
too low.

So they adjust the master idle knob to increase the idle RPM. Then,
when they twist the throttle grip open and closed, the engine RPM hangs
up at 5000 RPM and they wonder what the heck is going on?

The amateur tuner doesn't know about the three transition ports right
by the throttle butterfly's bottom edge. The transition ports are
getting fuel from the pilot jet too, but they aren't controlled by the
pilot mixture screw, they are controlled by the bottom edge of the
butterfly.

The amateur tuner tweaks the pilot screws CW and CCW and can't figure
out which way they should be turned because he doesn't understand that
the transition ports are getting fuel that BYPASSES the pilot screws...

Pilot fuel is sucked out of the float bowl through the idle jet. It
goes through a branched passage to a pattern of transition ports that
are covered by the edge of the throttle butterfly when it's closed. Air
from the fixed air jet is added to the same passage. Fuel/air mixing
occurs in the passage. As the throttle butterfly is opened *slightly*
the vacuum in the carb's bore can suck fuel/air mixture out of the
transition ports and the engine speed can increase smoothly. There is
another branch of the same passage that has an outlet about 1 inch
downstream of the transtion ports. Vacuum is weaker there. The pilot
screw adjusts fuel/air flowing out of that port...

When the idle RPM hangs up at 4000, 5000,even 6000 RPM, that tells the
experienced tuner that the pilot mixture screw is open too far. So, he
has to turn it back the other way to get the engine to idle smoothly at
the smallest amount of throttle opening and the least number of turns
out of the pilot screw.

You can do so much with pilot jets and adjusting the pilot mixture
screw, The strategy for adjusting the pilot mixture on a CV carb is to get the
engine to run smoothly at the specified RPM with the smallest amount of
throttle opening possible. The amateur tuner fools around with the
pilot mixture screws, not understanding that the engine is drawing fuel
air mixture from the transition ports that may be slightly uncovered by
the throttle butterfly, according to the setting of the master idle
knob and the synchronizing screws...

Now, one of the tests for pilot mixture setting is that you open the
throttle quickly, and close it. The engine speed should increase, but
it should return to the original setting. If it hangs up, the pilot
mixture is too rich, because the engine is ALSO getting fuel from the
pilot outlet port AND the transition ports.

> What about main jet size?

Let me educate you on the Mikuni/Keihin jet sizing system. It's just
like the pilot jet sizing system.

The orifice size is based upon a #100 mainjet having a hole that's
exactly 1.000 millimeters in diameter.

The area of this orifice increases as to pi times the square of the
radius, so you can see that going from a #100 main jet to a #120 main
jet will result in an orifice with an area that's 44% larger and will
flow far too much fuel whenever the engine has enough vacuum to suck
fuel out of the float bowls.

The engine will be drowned by excess fuel if you go up too many jet
sizes at once.

Going from a #100 mainjet to a #110 mainjet is not just one size jump,
it's FOUR sizes larger, #102, #105, #107, then #110.

Even the #110 mainjet has 21% more area than the #100 main jet. What
would make an intelligent rider think that his new exhaust pipe by
itself was going to raise his horsepower by 21%, requiring 21% more
fuel?

Constant vacuum carburetors rarely run on the main jet. You could even
probably get away with running the stock main jet or one that's just
maybe two sizes larger than the original jets.


> Will a needle shim be enough?. Should definitely be in the right direction.


Nope. That's an urban myth amongst amateur tuners that don't undertand
how CV carbs work. CV carbs have short fat needle without much taper,
and whacking open the throttle just doesn't lift the needle out of the
hole enough to matter. The grooves on jet needles with clips help
control when the tapered part of the needle begins to have effect, but
you'll get more off-idle acceleration effect out of turning the pilot
mixture screw just exactly the right amount.

You run at full throttle so rarely on the street or the highway, it
just doesn't make sense to mess around with main jets and needle
shimming. It makes a lot more sense to learn about the pilot jets and
how to set them correctly.

You may ask, "But why do all the tuning manuals say to start tuning by
finding the correct sized main jet?"

It's not about ultimate power, as shown on a dynamometer. You
can get a higher horsepower reading out of an engine that's just warm
enough to carburate cleanly than you'll get out of a hot engine.

Heat builds up during a long race and it builds up faster on a high
speed course like Daytona.

Racers who really *race* need a main jet that's big enough to pass
extra fuel so the engine won't overheat. The racers are willing to
waste fuel to keep the engine cool.

But the typical rider who buys a Dynojet or other kit thinks that he's buying "instant power"
and an instant solution to the EPA mandated lean burn jetting from the factory.

Sorry, it doesn't work that way. The rider who succeeds in getting what
he wants out of his engine is the rider who understands the technology
of that engine and why that particular technology was chosen for it."

Things that make you go HMMMM

[JohnnyBravo]

Sounds like your getting there

[ducatiman]

I'm currently working on carbs (not 250 ninja) with main jets *5 sizes* over stock.
Not uncommon to discover carbs containing jet kits with way oversize mains, adjustable needles with shims, and oversized pilot jets...unnecessarily overfueling EVERY circuit.

[RBN]

Blimey what a debacle!

So, I have finally (yes it took this long) brought this whole thing to a conclusion. Ok so I basically went ahead and spent what felt like an eternity trying different jets/settings/screw adjustments and filter alterations all to no avail. I would think I had it just right then it would be running rough again and I would convince myself that the engine hadn't been warm enough or whatever to give the correct mixture and off we would go again.

BUT two things stayed consistent 1, it would always start from cold with no choke regardless of any settings/jets etc and 2, For some odd reason if I stood with the bike idling for a while (like at a railway crossing) occasionally it would die and be reluctant to restart. As the latter is something I pretty much never have to do it didn't happen often enough to cause any alarm bells to ring.

And then one day whilst messing around the carbs again I noticed that the pilot air jet on one carb looks slightly wet, which IME means the fuel level is sitting high. And usually precedes the carbs starting to overflow, only on this occasion it didn't it only ever looked slightly damp and that was it.

I stripped the carbs inspected everything only to find it all spotlessly clean, both float were in good order and no cause could be found. Reassembling the carbs and connecting the fuel lead straight back to the exact same problem. Albeit intermittent.

Several more disassembly's later and I'm sat on the sofa still unable to find a damn thing amiss, with one of the float needles in my hand fiddling with it mindlessly and i suddenly notice that the clip that holds it to the float is stuck solid. IME these should be a loose fit on the needle allowing it to move about and seat properly.

I took it off, stretched it a tiny amount refitted and checked to make sure it was a loose fit and once again reassembled the whole mess.

And the bogging/rich problem was gone!



With that solved I set off for Spain! Which is about 1600 miles from my house here in Nottingham.

All was well except the fan was coming on a lot and the bike sounded a bit.........edgy. I can't describe the sound but it's how a bike sounds when it's leaner than it wants to be. Combined with the apparently higher temp (my bike has no actual gauge) I decided that it was indeed now lean.

SO we had basically cured one problem (too rich) only to discover another (too lean).

After the headache of the rich problem I had no mood for another fuel related nonsense.

Firstly I checked the air filter, which to my annoyance was appropriately oiled and properly fitted. Then I did the deodorant test and found that the carb rubbers were leaking a bit of air. Just a little but certainly enough to cause problems. I tightened the hose clips and the problem seemed to go away, only to steadily creep back again after a few hundred miles until it was just as bad, if not worse than before. And now no amount of tightening seem to work 100%.

So by now we are near the mountains in the south of france and with years of experience behind me we checked into a campsite, hoping that they could help me acquire some bathroom silicone. With this I could cover the gaps that were letting air in, continue the trip and simply reapply when it worked loose or came off (done this several times before and it works well).

Unfortunately the manifolds had a different idea.........



Yep, that happened. Never seen that before with manifolds that were in such good condition, still soft and pliable as the day the were made. But none the less they had completely split, irreparably.

This was a major blow, unable to buy any and looking like our trip had ended in disaster. However we managed to find a guy on ebay who could send us some 38mm marine fuel hose, which I cut and installed in place of the manifolds. Impervious to all fuel types as well as heat. PERFECT! The bike fired up and we set off, now too late to make the bike rally we set sail instead for Barcelona.

And all was well..........for about 700 miles. Then as we got to Toulouse on the way home from Barcelona the bike started running rich again. Exactly as it had before when there was the problem with the clip.

"Ok not to worry" I assured my lovely lady "we'll head for a campsite and I'll sort the carbs again, it's just that daft needle jamming again"

Only it wasn't........



Yeah that's the inside of the fuel hose, which apparently was impervious against all things except whatever it is the good people of France put in their (bright blue) petrol. Combine the deterioration caused by the fuel with suction from engine braking and this happens.

So long story short I bodged the bits of leftover hose together with some steel tube we found into some makeshift manifolds and rode the entire way home from Toulouse to Nottingham with no engine braking.

So it made for a horrible trip, but in every clode I try to find a silver lining. And this time I guess that it's that despite all I put it through, despite cutting off it's air supply, running it lean as hell for hundreds of miles, despite a coolant hose coming off and draining all but about a pint of coolant whilst riding in 40 degrees Celsius temperatures (yeah that's another story). This damned fine motor just kept going.

[RBN]

Oh I nearly forgot to mention, once home I has able to install some new manifolds, and with the needle jet fixed the bike now runs like a dream.

[Triple Jim]

Good point about the float needles needing free movement. I've had similar trouble when the brass finger that pushes the needle got a dent worn in it, and didn't want to slide on the needle as it moved.

[JohnnyBravo]

that is how you tell a story!!! I wanna ride to Spain now

[JohnnyBravo]

Anybody know how to convert a 300 into a submarine?

[Petrolh34d]

Originally Posted by JohnnyBravo

Anybody know how to convert a 300 into a submarine?

Yeah. Put a scuba suit on it