Exhaust definitions.......

[loudest143]

So in my quest to find a cost effective, free flow solution for my 150cc gy6 scooter, I've realized one very clear thing...

I cannot find anywhere the difference in sizes between exhausts for different engine cc's. Work with me here..... If a scooter muffler for a 47cc scooter has the same exhaust midpipe size, why is it JUST for a 47cc scooter? How about the superbike mufflers for Hayabusas that have the same bolt pattern and size as my stock 150cc exhaust midpipe.... Will it work on my scooter?

Why is it that I haven't found a chart, or table or equation that tells me that for this cc engine it should have this size pipe, and this size exhaust with this amount of flow. It just seems all so vague to me. I'm doing okay with my current modified stock muffler, even though I need a weld job done..... I'm just interested in REAL, factual text here.

Here's the thing.... What's the difference between an MRP supercoolshinypipe, and the JCWhitney universal run of the mill exhaust that fits any motorcycle or scooter?

How much difference can there be, really? It's expanded metal and packing. Is it really rocket science?


Loudest143
*loudpipessavelives

[tp555]

Basically if you had 5 different mufflers each rated for different size engines the difference would be the size of the internal pipe.The 150cc scoot muffler has an exit pipe of about 3/4 of an inch od.
They match the cc's to a size of pipe that gives the performance they want.So if you put a muffler with 1 3/4" pipe on your scoot it would be loud and have poor performance.The motor needs some back pressure to keep the exhaust gas velocities high for scavinging effects.Further tuning can be done by using slightly larger dia pipe then the optimum for higher end power or slightly smaller pipe for low end power.I used a 1" pipe on my custom exhaust and it works great.So if you find an exhaust that bolts up good tells us.If the displacement is close to your scoot you can try it out and let us know how it turns out.Does the busa really have the same bolt pattern?Is it cheap? I 'd try that out just for fun if it was cheap enough.I'll take a used one.
regards.

[loudest143]

tp555 said, ".....if you put a muffler with 1 3/4" pipe on your scoot it would be loud and have poor performance.The motor needs some back pressure to keep the exhaust gas velocities high for scavinging effects.Further tuning can be done by using slightly larger dia pipe then the optimum for higher end power or slightly smaller pipe for low end power.I used a 1" pipe on my custom exhaust and it works great."

I agree and understand all of your points. I am trying to figure out how you arrived at your 1" pipe size. Did you guess, or read that someone else did it that way? Did you build the custom pipe yourself? I know that some in our forum have made it themselves. All fine and good situations, I'm trying to find the definitive chart or parameters that any racing scooter builder might use to say, 'my bike is this cc, and I want this amount of back pressure, so I will use this size pipe and this size muffler because that's the defined and documented best combination.' You can't tell me that racing teams, and MRP designers just guess and fool with it til is sounds and runs the best. Too much science involved for that to be the case. I wrote an email to FMF Racing Exhaust, to ask this question, and got no response. (big suprise, right?)

Is this information that secretive? I can find out from numerous sites how to properly rebuild/rejet my carb, with data that reflects proper jet size, yet nothing SEEMS to be out there regarding muffler tuning.

I'm not looking to create a monster racing scooter mind you. I just want top performance out of my scooter. Funny thing is, with derestricted air filter, iridium plug, and gutted muffler, it's still running rich. Figure that one out.

Loudest143
*Midasize it!

[earlwb]

A heavy scarf worn around the neck for warmth, oops sorry you meant....

Muffler
A device used to attenuate sound while also allowing fluid (usually gas) to flow through it; also known as silencer in British usage. Mufflers are extensively used to reduce the intake and exhaust noise from pumps, fans, compressors, and internal combustion engines. Although active noise control techniques are emerging, most mufflers continue to use passive silencing methods. Passive mufflers are categorized as reactive or dissipative based on their primary method of attenuation. Reactive mufflers reflect sound back toward the noise source, and dissipative mufflers use porous materials to absorb the sound.

Reactive mufflers reflect acoustic waves at locations where a duct expands, contracts, or branches. Often a combination of reactive elements such as expansion chambers, resonators, and flow reversals is used. Reactive mufflers can be designed to provide better low-frequency attenuation than a dissipative muffler of similar size. Also, reactive mufflers can be used in harsh environments that dissipative or active mufflers might not withstand. In most cases, reactive mufflers are best suited for low-to-moderate frequencies, where acoustic wavelengths are larger than any cross dimension of the muffler. At these frequencies, mufflers can exhibit resonance or broadband attenuation behavior. See also Resonance (acoustics and mechanics).

Dissipative mufflers use absorptive materials that dissipate the acoustic energy into heat. A variety of porous media can be used for absorption, with fibrous materials such as fiberglass being common. The linings and baffles can be flat, contoured, constructed from layers of different materials, or mixed and matched for a particular application. Absorptive materials may face challenges due to harsh conditions such as high temperatures and potential clogging from particulate-laden flows. Dissipative mufflers are best suited for moderate-to-high frequencies, since absorption is less effective at low frequencies. At frequencies where the absorptive materials are effective, the attenuation is broadband, and the passbands exhibited by reactive mufflers are reduced or eliminated. Compared to reactive mufflers of similar size, dissipative mufflers can have higher attenuation (except at resonances for the reactive muffler) and lower pressure drop. At higher frequencies, where the acoustic wavelength is smaller than the duct width, the attenuation of a dissipative muffler may decrease considerably. See also Sound absorption.

Active mufflers attenuate unwanted noise by adding sound to counteract it. The disturbances add algebraically, resulting in a cancellation of the unwanted sound. An active muffler consists of sensors (such as microphones), a controller, and actuators (such as loudspeakers). The controller unit processes the signals from the sensor, and computes an appropriate signal for the actuator. Numerous control systems and strategies exist, and are under continuous development. Active mufflers are best suited for low frequencies where the sound field is relatively simple. The effectiveness of active mufflers has been demonstrated for a number of situations, but several challenges are the topic of ongoing research. There is a need for ruggedsensors and actuators that can withstand high temperatures and harshenvironments. Also, high-intensity disturbances at low frequencies requirelarge-displacement, high-power actuators.

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

Is a muffler nothing more than a steel can with some baffles and tubes inside? Thats how most mufflers look, but theres a lot more to muffler science than meets the eye.

Noise Control
A mufflers job is to muffle sound. Thats why we call them mufflers. In England theyre called silencers. Regardless of the name, they all do something to change, dampen or absorb the noise-producing pressure pulses in the exhaust. Some mufflers do it by creating resistance with baffles and louvered tubes. Resistance to flow causes the pressure pulses to stack up and dissipate, lowering their energy and the level of sound they produce (which is measured in decibels).

Some mufflers turn the pressure pulses against themselves by rerouting and redirecting the exhaust through tuning chambers. By carefully adjusting the volume and length of the chambers, the pressure pulses can be attenuated within certain frequencies.

Another trick thats used to control noise is to pack areas around the louvered tubes inside the muffler with strands of fiberglass (called roving). The sound-absorbing properties of the fibrous material acts like insulation to further absorb and dampen energy in the exhaust pulses.

More recently, the science of acoustics and gas flow dynamics has yielded some new muffler designs that use reflective sound cancellation (RSC) technology. Such mufflers eliminate unwanted sound frequencies without the use of restrictive baffles or packing material. An air gap in the tube allows sounds waves to bounce back in forth in such a way that they cancel one another. The idea here is get pressure pulses that are 180 degrees out of phase with each other to collide and cancel each other out.

Making BackPressure
The pressure pulses in the exhaust also create backpressure in addition to noise. As the volume of gas passing through the exhaust system increases with rpm and throttle opening, pressure backs up in the system because of resistance to flow created by the pipes, catalytic converter, muffler and/or resonator.

As a general rule of thumb, there should be less than 1.5 psi of backpressure in most exhaust systems at idle. This can be measured by attaching a pressure gauge to the exhaust system anywhere ahead of the converter (an air pipe connection on the exhaust manifold, a diverter valve connection, oxygen sensor connection, even the EGR valve connection). If theres a restriction in the exhaust, it will usually cause a higher than normal back pressure reading and a corresponding drop in intake vacuum.

One of the easiest ways to diagnose a clogged converter or muffler is to connect a vacuum gauge to the intake manifold and watch the vacuum readings at idle. Most engines will have 16 or more inches of vacuum at idle. If the reading is lower than normal, and/or continues to drop the longer the engine runs, it means pressure is backing up in the exhaust and is limiting the engines ability to exhale. Eventually, the back up will suffocate the engine and cause it to stall if the exhaust system is not passing any gas.

Measuring a Mufflers Performance
If you use a dyno you can make a series of test runs and measure the engines power output with different mufflers behind it. Or, you can take the vehicle to the drag strip and see which muffler gives the fastest E.T.

But major muffler manufacturers use a more scientific approach. They use test equipment that measures airflow under controlled conditions. The equipment they use is similar to that which a performance engine builder would use to measure airflow through a cylinder head, intake manifold or carburetors. The only difference is that they are measuring airflow through a muffler on a test stand. The purpose of such testing is to evaluate the flow characteristics of a muffler so engineers can develop a replacement muffler that flows the same or better than the original equipment muffler or a competitive muffler.

In one laboratory test setup we saw at a leading aftermarket exhaust manufacturers facility, mufflers were fed a simulated exhaust stream of 200 standard cubic feet per minute. This is roughly equivalent to the exhaust output of a 200 cubic inch (3.3 liter) engine running at 3,400 rpm. Backpressure (in inches Hg) was measured at the muffler inlet, and a microphone recorded the sound level at the mufflers outlet in decibels. Using this setup, the manufacturer can benchmark and compare the performance of their own mufflers against OEM mufflers and competitive mufflers.

Sound Check
Many performance mufflers do a great job of reducing backpressure, but arent very good at muffling noise. A straight-through glass pack style muffler is about as simple as it gets and offers little restriction to the flow of gas. But the louvered tube and the packing inside may not provide the same degree of noise absorption as an S flow muffler or one that has multiple tubes, baffles and chambers.

Another difference that becomes obvious when various brands and types of mufflers are compared on a flow bench is that some flow much better than others. Higher flow numbers mean less backpressure, but flow numbers alone do not tell the whole story because sound control is also important. Even so, two performance mufflers that look identical on the outside and have the same size inlet and outlet pipes can have very different flow numbers. In one test, three competitive performance mufflers for a Camaro were flow tested at the same pressure drop across the muffler (20 inches H2O). Brand A flowed 700 cfm, Brand B flowed 940 cfm and Brand C flowed 570 cfm. Thats quite a difference.

Though some customers want a louder, deeper exhaust sound (and will gladly pay extra for it!), most motorists prefer a quiet exhaust, but would be happy to have a muffler that provides the best of both worlds: good noise control with less backpressure to improve flow and horsepower. Some aftermarket muffler designs can do just that, reduce backpressure from 10% to as much as 65% over a stock OEM muffler while still providing almost the same noise control as before.

A straight pipe, by comparison, produces almost no change in backpressure and does nothing to reduce noise.

The Muffler Marketplace
Polished stainless steel mufflers and free flowing cat-back exhaust systems are a hot bolt-on upgrade opportunity for shops, and can be easily installed on most cars, SUVs and light trucks. The most popular applications for these kinds of upgrades are the sporty cars that teens, twentysomethings and even some over thirties are driving, such as Honda Civic, Accord and Prelude, Acura Integra and RSX, Mitsubishi Eclipse and Lancer, Ford Focus, Chevy Cavalier, Mazda Miata, RX-7 and RX-8, Nissan 300ZX and 350Z, Toyota Celica, Matrix and Scion, Subaru WRX, and VW GTI, Golf, Passat and Beetle.

Of course, the market for performance exhaust products certainly isnt limited to these makes and models only. Almost any vehicle might be a prospect for an exhaust upgrade if the owner wants more power or wants to customize the appearance of their vehicle.

Unlike a typical exhaust customer who doesnt want to spend a nickel more than is absolutely necessary to replace a muffler, your typical performance customer will actually enjoy spending money to accessorize and customize his or her vehicle. Consequently, you can sell up and dont have to haggle price. You can sell this type of customer a performance muffler that may cost two or three times as much as a stock replacement muffler.

Better yet, dont wait for pipes and mufflers to fail to make a sale. Some people will have the stock mufflers replaced with performance mufflers as soon as they buy a new car or truck. Most premium quality performance mufflers and pipes today are made of stainless steel (usually aircraft-quality T-304), but there are lesser quality grades of stainless that sell for less. Aluminized steel is another option, but it doesnt have the durability of stainless. Even so, aluminized steel is much better than plain uncoated steel or painted steel.

Most premium mufflers and systems are plasma or TIG welded to maintain the strength and corrosion resistance of the stainless steel. Some manufacturers also use a process called bulge forming or hydroforming to form steel sheets into molded shapes using extremely high fluid pressure.

Replacement Tips
Stock and performance mufflers are available in both direct-fit or universal configurations. Youll find the greatest selection in universal products, which can be adapted and made to fit almost any vehicle. But the main drawback of one-size-fits-all mufflers is that they sometimes require cutting and welding pipes, and fabricating hangars and mounts.

When the stock muffler is removed, the existing pipe may have to be cut, shortened or lengthened. If the pipes are getting weak, they may also have to be replaced to complete the installation. New hangers are often necessary to support the muffler, or existing hangars may have to be repositioned to provide proper support and keep the system from flexing or rattling. Clearance may be another issue, especially with large oversized muffler cans that are not the same size, shape or length as the original muffler.

Most of these modifications are not difficult or expensive to make, but they do take time so be sure to take that into account when pricing the job.

Direct fit mufflers usually dont require any modifications or changes because they are designed to replace the stock muffler perfectly. Most are a simple bolt-in installation that connect to existing pipes and have mounts that line up with the stock hangars. The major drawback with direct-fit custom mufflers is limited availability for less popular models. Unless a vehicle is really popular, you may have a hard time finding a direct-fit performance muffler for it.

Sound quality is something thats harder to control with universal-fit mufflers than direct fit mufflers which have been engineered for a specific vehicle application. Different engines sound differently and produce different frequencies and harmonics in their exhaust systems. So dont expect a universal muffler to sound exactly the same regardless of what kind of vehicle is it installed on. It wont.

And finally, some performance mufflers are tunable in that they have removable baffles. The driver can change the sound by adjusting or removing the baffles.

[earlwb]

I also wanted to state that there is a old rule about mufflers, in that the muffler internal volume has to be a multiple of the engine's displacement to work correctly. They had determined this way back in the late 1930's or early 1940's. The first motorcycle company that took advantage of it was Vellocette with their big fishtail mufflers of the time.
I do not remember what the rule is at this time, and I haven't found it yet online.

Exhaust pipe cannot be too large nor too small. a 1" pipe on a 150cc engine (including the bored out versions as well) works out about right for diameter size. What you want is a good exhaust speed leaving the engine, until the exhaust reaches the muffler where it expands out and slows down. The higher exhaust speeds actually tend to suck gases out of the engine and the faster you can get the exhaust into the muffler before it cools and starts to slow down is a big plus. As the exhaust gases leave the engine the gases immediately start to cool down and also slow down as cooler gases are more dense that hotter gases. So you get a inherent amount of resistance in the exhuast system to the hot gases leaving the engine.
Thus if you make the exhaust pipe too large in diameter you actually start to have a loss of high RPM performance as the gases are fighting the engine trying to push the spent gases out.

The general consensus is you want about 1.5 psi of back pressure in the exhaust system to allow the engine to idle properly. With no back pressure the engine typically idles poorly, loads up and stalls (running too rich), and tends to bog down as you give it the throttle.
On bigger displacement engines like Harley's for example, you can typically run a straight pipe with no muffler, but you need something in the pipe to provide that back pressure effect to get it to run right at idle. Usually a 1/4" bolt run through the pip on the end does the trick, but most riders ope for the slip in mini-baffle inserts. The same method would work on a 150cc engine, but a straight pipe on a engine that turns high RPM would really piss everyone off. Harleys get away with it as they are low RPM engines, so they don't develop the high frequency sounds that drive everyone crazy.

But a straight pipe is more for racing as the pipe's length is tuned to the engine's RPM where it develops maximum power. When you aren't in the proper RPM range power will suffer a lot. On a Harley with tons of power to start with, it isn't a big deal, but on a little GY6 engine, forget it, You'll lose your low end and mid range power and torque very noticeably.
So we need a muffler or some kind on the end of the exhaust pipe.

[earlwb]

JETTING GUIDELINES



So when do we know we need to rejet and how many steps do we have to take? Fortunately for many of us, not nearly as often nor as much we might first think. If our motorcycle’s systems are all stock like they came from the factory, we may not have to do anything at all. In some cases, a slight adjustment of the mixture screws and possibly some thin shims under the main jet needle clips may help get rid of some low to mid rpm range surging.



We’re also going to stick to guidelines, as every individual configuration can be different. I’ve included some of what I refer to as "rule of thumb" information, but nowhere is it cast in concrete. What works well on your buddy’s GSCBXRZZ-2000 may not even run on your Super Tour 900 Twin. Unless you’re a fairly good wrench and are willing to chase through various configurations of parts, you should stick to known, proven combinations. In other words, ones that are known to work. You will also find that working with a carburetion tuner can really eliminate a lot of unnecessary time and frustration. Like the guy says on the commercial, "you’se can pay me now of pay me later…"



And we would be most remiss if we didn’t have a word or two advocating good old stock. Contrary to popular belief, most modern motorcycle engines do not have overly lean main jets as they come from the factory. At least those built since the early 1990s. There is no need for one thing, as the majority of EPA type testing is not conducted at wide open throttle or WOT. Plus, lean WOT operation runs the danger of damaging the engine, something else the factories try to avoid. Many Supersport racers who are required to run pure stock airboxes and exhaust systems will actually decrease main jet sizes slightly in some cases to obtain a little more power. This isn’t advisable for street use by the way, as conditions vary too much during day to day operation.



POPULAR MODIFICATIONS NECESSITATING REJETTING



Far and away, the single most popular mechanical modification to a motorcycle’s engine is a different exhaust system. There are a lot of stated and unstated reasons including less weight, better ground clearance, more power, better looks, etc.. But we know that at least 90% of you are also are looking for a little more sound. Here again, federal and local regulations may come into play as to what is legal (hardly anything) and what is not. That is because the exhaust system can have a direct effect on both emissions and noise standards. Both of these areas get harder to comply with every year, and therefore, the factories spend a good deal of time here. Factory motorcycle exhaust systems are no longer the "throwaway" item they once were. In fact, many modern systems are quite efficient as long as the rest of the engine remains stock. They also tend to be quite heavy. The tradeoff here is they are long lasting and often double-walled constructed to maintain their looks for many years.



Exhaust system changes range from simple modifications to the stock system all the way to complete trick racing systems made out of aluminum, stainless steel, titanium and carbon fiber. One of the more popular modifications on sport and cruiser style motorcycles is a new muffler(s). Commonly referred to as a "slip-on" system, the stock headpipes are often retained. This type of system can usually get by with adjusting the mixture screws and possibly shimming the main jet needles. That is as long as some form of muffler is still present.



Individual straight pipes are popular with many cruiser riders, also commonly known as drag pipes. Here, the major tradeoffs are less weight and some ear-splitting noise. It’s also another popular myth that has developed over the years that individual straight pipes give an engine more power. The only case where this is true is if the factory system is highly restrictive to begin with or the exhaust design doesn’t matter that much like on a positive displacement supercharged one.



Without going into a lot of "exhaustive" theory (terrible pun intended!), a single straight pipe provides little to no cylinder scavenging effect except at a specific rpm. Conversely, exhaust systems that pair cylinders together, when done properly, have the outgoing exhaust (and accompanying pressure waves) provide a scavenging effect between cylinders. This is why the best inline 4-cylinder systems use a 4into2into1 system. From there they may use a single muffler, or they may split back out into two mufflers for more capacity. You’ll notice that most big-bore sport bikes either have two mufflers or one very large one. This is to gain enough exhaust volume capacity while still providing adequate sound control.



So, straight pipes don’t require jetting changes, right? Actually, they usually do. See it’s a trick question. By not providing good cylinder scavenging, at least as good as the stock system, a couple of things happen. First, more exhaust mixture may remain in the cylinder, diluting the fresh charge for the next intake cycle. This may also mean that less fresh charge will enter the engine because we’ve effectively cut down on its ability to flow air. So now we open the throttle further in an effort to get some of our power back. But we’re not providing as much signal to the main jet. What this all means is we often end up having to rejet the carburetor(s), usually richer, to get some of our power and driveability back. Gas mileage usually suffers as well. The motorcycle may sound like it’s fast, but put it on a dynamometer and there may be little to no gain in power over a good stock system. This is in spite of the stock system’s more restrictive mufflers.



Another common misunderstanding is the effect of exhaust system backpressure. A little can actually be a good thing, especially if it’s timed right. Two-stroke expansion chambers use sound waves to help scavenge the cylinder at certain times, and help seal it with exhaust reversion sound pulses at others. An exhaust system can also over-scavenge a cylinder to the point where it is actually pulling fresh mixture out the exhaust port before the power cycle commences.



Now we’ll go into some of those exhaust system rules of thumb. Generally, a slip-on muffler system will work acceptably with simple mixture screw adjustments and possibly some slight shimming of the main jet needles. A couple more horsepower may be obtained by changing the main jets and needles as well. These are typically referred to as Stage I jet kits. Jet sizes are typically fairly close to the stock factory ones. Needles are usually included and tend to be more tapered in an effort to fatten up (richen) the midrange mixture.



Straight pipes, as outlined above, normally always require some rejetting, but with no other changes, it usually won’t be radically different from stock. Most aftermarket pipe manufacturers either offer or recommend a jetting kit with their pipes. It will typically be a Stage 1 or Stage 2 system with a main jet(s) and needle(s) and instructions on how to install and adjust it for your carburetor.



A full exhaust system, especially one that is significantly different than stock will generally require jetting changes to be of any advantage. They usually work best in conjunction with air cleaner/airbox modifications and possibly, changes to the ignition advance curve.

[natefromogden]

On a related note about mufflers, HP Coatings is buying me a new one for my Morphous! I took the muffler and heat shield down there for their polished aluminum Hypercoat. Its been a couple weeks and it took a week to get a call back about the part's status.

Turns out that they polish the coating by putting parts in a ceramic bead filled, vibratory tank. One of the inlet/outlet plugs fell out and the muffler got filled up with ceramic beads that would not shake out. So now they are being me a new muffler and HPC is promising expedited coating when it arrives.

I'm just glad I didn't wait until riding season to get this done!

[geetee]

Earlwb, what did or do you do for a living? ( I've got to see a therapist, my simple approach to scooten just got messed up)

[loudest143]

Excellent, informative and educational. I've got a 155cc scooter running a straight through unrestricted exhaust, essentially a straight pipe. I'm running rich, and need to see what my jet choices are. What website would anyone recommend that will show the engine size, carb size, and exhaust choice to properly align and tune the grouping of parts. I know this isn't a guessing game, and I also know that just because I read a cut n paste doesn't make it true. Not being a jerk here, I'm just saying that the information shouldn't be this hard to find. It seems downright mystical.

For the record, all the points and information submitted above and fantastic additions to this thread. My intent here is to help locate actual facts and parameters. Keep 'em coming!


loudest143
*I like my stupid loud pipe

[tp555]

The thread for my custom muuffler is here:
scootdawg.proboards59.com/index.cgi?board=tipsandtricks&action=display&thread=1191601945
Don't know if the link works.Look up exhaust mid$ style,oct 18,07 in tips and tricks.Yes I made it.
In regards to jetting simply exhaust can probably handle one size up or 2.5 more.You got to improve the intake to get the extra power. I am running the stock jet with my custom exhaust. I will go to a 110 jet next chance,(108 stock).As far as picking the 1" pipe intuition. I was a foreign car mechanic 30 years ago.Worked my way thru collage.Built lots of motors,vw air cooled,nsu,v-8's and the small lawn mower types. So I have done this before.The difference in performance with the custom exhaust and is jetting is this.I have a long 1/2 mile plus uphill road
to my house from town. Stock does 45 mph wot. Custom exhaust and 117 jet does 48mph.This was way too rich.Exhaust and stock jet does 52mph.All are wot.Intake is still stock.This worked for me.For you don't jump too far ahead with jet sizes. In regards to web sites that have all this figured this is it.It hasn't been figured out.We are doing the figuring out.I always wanted to be a pioneer. Where's my davy crockett hat?

[lewtwo]

>> Where's my davy crockett hat? <<
You got to take it from the squirrel.

[loudest143]

Tp555, your 30 years as a mechanic trumps any website. Practical experience led you to your mods, and I respect that. Your information regarding the upjetting is incredibly helpful also, as that's my next step, after getting the muffler situation under control. I am in contact with an auction site seller who is looking around for a 3 bolt hayabusa exhaust, used, that may fit my bike. So I will keep everyone informed of any details. Hopefully it will match up nicely. I was hoping to go with an aftermarket exhaust, but the stock ones are dirt cheap. Since this is experimental, and I will have to justify my expense to my accountant, (the wifey) I think it is wise to go low dough at this point. It may prevent a slaying in my household. (me!)

I appreciate all the great information in this thread! This is the great part of Scootdawg right here. Sharing information. Thank you to all!

loudest143
*you bought what?!? Why?!?

[earlwb]

Well there are a 107 or 108 OEM sized main jets. Then there is a 110, 112,114,115,120 and a 130 main jet size too. There are probably a few other sizes in there, but I haven't seen them yet. if you know how to do it, you can drill out your own jets as well. The older 100cc, 110cc, 125cc engines have a little bit smaller OEM main jet as well.
If you keep the 24mm carb and go with a K&N air filter and run a free flowing performance exhaust system, a 115 to 120 is about right. But if the hi-perf exhaust system isn't that good, a 115 might be the one that works better.
if you use a 28mm or 30mm carb you might have to go with a 130 main jet over a 120.
Now there are a couple of cheapie carbs types that have the emulsifier tube and the main jet machined as a single unit, but I haven't had one, to compare to a regular separate jet and tube to see if they are compatible or not.

Now the low speed pilot jets can be a issue, if you have installed a big bore kit, you might have to go up a size. usually you don't but one never knows. There are several different kinds of these jets out there. So you might have problems if the .038 pilot jet like www.oregonvintage.com sells doesn't fit or look like the one in your carb.
I have seen three different kinds of these pilot jets so far in carbs, all of them are different and aren't interchangeable.

Changing out your air intake does a lot more to lean out the engine that a free flowing exhaust will. On my scooter with all OEM stuff, the rubber air intake came loose once, and it caused a severe power loss as the engine was running way too lean then. Getting more air into the engine means you need more fuel too.
On my scooter I only had to remove the thin foam prefilter strip to open up the intake and make it free flowing. I got a easy 5mph improvement just doing that and of course putting in a required larger main jet.

[tp555]

My carb has a new style pilot jet that is different from the old ones.So you have to check that out too.My pilot jet has threads at the tip.The older ones have the threads in the mid section.For simple exhaust you can remove the rear cap and cut off the pig tail pipe.Then use a drill that fits inside the exhaust port.Hold the drill straight and slowly drill thru the side wall of the pipe.The pipe is bent.You are bypassing the routing inside.Sound will be louder,but should have some tone to it.
Put the rear end cap back on.I will do this to my stock muffler after I get some backlogged work done.I am jammed up with work and cannot play with my scoot.This is good and bad,but mostly
good.Just a little delay thats all.