Background

I like the big, LONG 2-Into-1 exhaust system which Honda designed for the Aero. I also like the sound of some after-market pipes. I haven't found a manufacturer who is making anything that I like yet. I would prefer a 2-into-1 exhaust system that was as LONG as the Aero's, or 2-into-2 where BOTH pipes were VERY long. No one has created one that I like so I thought I'd try to create my own.

I purchased an extra exhaust system for my Aero from someone who had put on an after-market set. I thought I'd experiment with the exhaust trying to create something unique.

Note: I'm looking for another Aero exhaust system to allow me to do some more development work. If you have one contact me. I want to keep my costs down, so please consider that my ideas sometimes do not work, so I don't want to spend a lot of money on it.  I really only need the 'muffler' section, I don't need the header pipes. I'd also prefer that the chrome is in good to perfect condition, but this is not an absolute requirement. Contact me if you have replaced your Aero's exhaust system and want to donate  your old factory muffler section (or sell it inexpensively).

Originally I thought I would simply gut out the rear muffler cylinder of the factory system. I was planning on using an after-market automotive 'cherry bomb' resonator to add some sound quieting (If you'd like to see an exhaust system that was done this way, see Pop's thunder pipe project page). I had hoped that this would increase the low frequency sound of the exhaust. After thinking about it, I knew that I'd have to create some sort of combiner that would bring the two cylinder's exhaust stream together without creating too much resistance to the flow of the exhaust gases.

I finally decided to install two internal pipes and keep them separate. I could then install a standard motorcycle baffle into each pipe. I could then choose between different baffles, or even slide the baffles up and down the exhaust pipes to tune the system for best performance.

I started to probe around the inside of the factory system. Externally, it appears to be a 2-into-1, but actually, it's a 2-into-2 system with an external shell to make it appear to be a 2-into-1! There is a cross-over pipe however, so the two exhaust 'sections' are connected to each other.

I haven't been able to find anyone who can tell me what function the cross-over serves. Some tell me that because factory exhaust systems have to be very quiet to meet federal noise standards and quiet also tends to be very restrictive. They say that the cross-over allows both exhaust 'pipes' to be used by both cylinders increasing the flow capacity, thereby decreasing the resistance to flow (and therefore is less restrictive), increasing the top-end (high rpm) power that the engine can deliver. Others have told me that the cross-over somehow increases the scavenging (which helps to remove spent exhaust gases from the cylinder), thereby increasing the low-end (low rpm) power which the engine can develop. Which is it? I don't really know!

WARNING!

Do NOT attempt this project if you're not good with your hands, and able to think things through. The Aero exhaust system is not inexpensive to replace if you're not happy with the results, or are unable to get it to work properly. After these modifications your motorcycle may not meet local or federal standards and therefore may not be legal to run on the road. I do not know how your Aero will perform after these modifications have been made. You may also have to rejet your carburetors to get the engine to run properly. I'm not responsible for your work, nor available to help you solve problems. You are on your own for any or all modifications which you attempt. There certainly could be errors or omissions here which could keep you from completing your project.

Note: I've had some requests as to how much I'd charge to do this modification for you.  I'd have to charge $200 for my labor. Shipping and parts would be separate, as will any welding costs. If you are local, and want me to help you perform this modification yourself, contact me and we can reach some agreement.

You would have to consider several factors:

1. You may not be happy with the results. You may find the system too loud, or it may not sound as you imagined it. If you are not happy with the results, there is no going back. A replacement exhaust system from Honda is not inexpensive.
2. There is the possibility that the chrome on the muffler may get scratched or damaged during the process. I'm NOT responsible for any scratches or other damage.
3. You may not be happy with the performance of your engine after the modifications.
4. This may void your warranty, or may affect other aspects of your motorcycle.
5. Modifications to your exhaust system may prevent you from being able to pass your local motor vehicle inspection or may cause you to get tickets from the local or state police.
6. You must agree to all that I have said in my 'warning' paragraph above.

On to the project!

You will need two 24 inch long by 1.75 inch OD (outside diameter) exhaust pipes and two 1.5 inch OD to 1.75 inch OD adapters. Any auto parts store should sell these. I paid about $14 for mine. I also purchased some high temperature (1500F) flat black paint to repaint the welds and external parts which were cut to help them from rusting. You'll also need a hacksaw, a few blades, a large cold chisel, a good 5 LB hammer and some safety glasses.

Find someone who will weld it all back together for you. I went to a local Meineke muffler shop. They are familiar with welding exhaust pipes and have the proper TIG welding equipment for this type of work. They charged me about $35 to do the welding. So my total parts and labor cost was about $49 (not including the cost of the Aero Exhaust system).

I also installed a K&N HA-1187 air filter, as these are supposed to decrease the resistance to airflow to the carburetors. This is NOT required. Since I was planning on rejetting my carbs anyway, I decided to attempt to decrease the intake resistance also, and the K&N filter was the simplest and easiest way to attempt this.

I purchased a Factory Pro CRB-H34-1.0 carburetor rejet kit for the Aero. The Factory Pro kit has the widest selection of jets to allow you a lot of choices to get your pipes setup just right. Since this is not an 'off the shelf' exhaust system, no one can tell you exactly which jets to use. Therefore the Factory Pro kit will give you the ability to use any pipes on your bike. This costs about $80.

View before you start

Here is the external 'muffler' after being removed from the Aero. You can see the two exhaust clamps where the exhaust header from each cylinder is connected to the main muffler section.

What do you cut?

Here is a view of the front of the exhaust. If you look closely, you can see that the cuts have already been made. I placed it back together so you can see where to make the cuts. Don't make one mistake that I made! You DO want to cut at each weld. The weld seam is just in the correct location. The mistake that I made was to cut straight through the whole assembly in one cut! You want to cut the circumference (around the outside of the shell) and not straight down through the whole thing. This is because the two internal exhaust pipes are friction fit inside the muffler. If you cut only the perimeter of shell, you'll leave these exhaust pipes sticking out a couple of inches, this protruding pipe will allow easy access to the pipe to allow you to weld on the 1.5 inch to 1.75 inch couplers. If you cut these pipes off flush as I did, it makes it difficult to weld on the couplers.

After the Cutting

Here's how it looks after I finished the two cuts. If you look into the main cylinder on the left, you can see one of the internal exhaust pipes. That part is only about two inches long. It's friction fit into an internal baffle. If I had simply made a circumference cut around the shell, that pipe end would still be connected to front part making it easier to perform welding on the couplers and pipes later.

Inside the front

This is the 'inside shot' of the front part. You can see the two exhaust pipes here. There is also a front part of a baffle here. If you cut straight through as I did, you'll have to remove this front baffle. Note that you can also see the front part of the crossover pipe on the top left. That crossover doesn't connect to one of the pipes, but actually is connected to the area created by that front baffle. It's hard to see, but that front baffle has some sizable holes in it.

Inside the rear

Here's the inside of the rear of the shell. You can see the second baffle and one of the internal pipes which leads to another section of the muffler. What you can't see clearly is that the pipe which you can see here is actually just lightly pressed into place. I was able to simply wiggle it and remove it. After I found this I realized I should not have cut straight through the hole thing! The welder would simply have to deal with it later. This baffle and all other internal pipes and baffles now have to be removed.

Here's some of the leftover rubble

Here's all the junk you have to remove. I placed them so you can see the relative position as they were installed inside the original shell. You can see that there were four baffle plates here. There was another one to be removed at the cut you made to open the shell for a total of five baffle plates (some were perforated, some were solid). I used a big cold chisel and a 5 LB hammer. Wear safety glasses, and be careful NOT to scratch your chrome!!!

Inside the end product

Here's a shot inside the tip after the pipes are all done. You can see the baffles which I installed as well as the bolts which hold the baffles installed. The pipes were tack welded together at this end and also to the external shell at this end. It was a tight fit to get them installed, so I may not have needed to weld them, but I didn't want anything vibrating! I also had to file down the tops of the acorn nuts (which are used to hold on the tip) a bit to make room for the pipes. Be sure to leave enough of the threads on the acorn nuts to allow you to reinstall the tip. This would be one ugly pipe if the tip couldn't be put back on!

I also cut the original exhaust tail extensions and bolted them to baffles (photo not shown). I purchased two 1.75 inch to 1.375 (1 3/8) inch couplers. I had to do some work on the couplers with a nibbling tool and a file to get them to fit over the end of the ends of the 1.75 inch exhaust pipes. I fit everything together, and then welded the exhaust extensions into the couplers. I then used some high temperature RTV silicone to create a bit of a gasket inside the coupler. These were slid on over the 1.75 inch exhaust pipes and bolted on with the same bolt which holds the baffle. I then painted this with the high temperature flat black paint. .From the external view, it really looks like the stock factory exhaust system. The only clue to a change is that the cross-over pipe is missing. If you want it to look just like a factory system, don't cut off the cross-over pipe completely and re-weld the front cut that you make when starting.

Sound please!

So how does the result sound? Here are some recordings to help you satisfy your curiosity. I tried to make each recording in the same manner to be sure the results would be relatively comparable. This worked, but I made the mistake of leaving my garage door open and the bike was sitting right in front of the garage. I can hear the sound echoing from inside the garage, it really detracts from the recordings, I can hear it, you may not. It certainly did distort the drive by sequence. Note: These sound clips were taken before final tuning of the carburetors and before I bolted on the tail pipe sections.

Here's the sound of the stock Aero without any changes. I noticed after listening to this recording that the bike wasn't fully warmed up yet.

Here's the new modified exhaust system sound after the initial re-jet. There's still more work to be done.

Here's the sound of a drive away, drive by, drive back sequence. There is a lot of echoing going on from the garage here. I'll have to redo this sequence soon!

This system is working well for me. I spend some time trying different configurations. I ended up with the baffles mounted so the 'cups' in the baffle are facing down stream of the exhaust flow. That is, if you look into the rear of the exhaust pipes you can 'see into' the open end of the cups in the baffles.  I also used the fiberglass packing with the baffles. With the exhaust tips installed, it looks like a factory system, but sounds and good as any above average V-Twin on the road.  Another plus of this system is that I passed my state vehicle inspection without the inspector even making a comment about my exhaust system. Of course, while driving in to the inspection station , I kept the RPMs as low as I could.

If you don't install the rear exhaust extensions as I did, it will sound a bit louder. The exhaust note tends to have a bit of a hollow sound to it. This is caused by the chrome tip. You can hear the difference that the tip makes by  letting the bike idle, and simply install and remove the tip when the exhaust extensions are not installed.   This is what prompted me to take the extra time to install the exhaust extensions.

If you want that BIG DEEP sound similar to the V&H pipes, then simply do not install the baffles. It's that easy! You will be amazed at how deep (and loud) this will sound.

I recently rode with another Aero which had V&H Straight Shots installed. His bike had a noticeably louder and deeper idle. However, when following behind us, I was told that it was impossible to tell the two bikes apart from one another. I consider this a compliment, as I spent about $49 on my pipes and maintained the factory look (which I like). The V&H pipes cost about $350 and are a two-into-two system. I know that I can make my pipes sound very loud and deep at idle, but I thought it was a bit too loud, so you can have this sound if you like it by making your baffles to suit your personal taste.

Another idea which I have not tried yet is to get a 1.5 long coupler. These are about 12 inches long and are 1.5 inch pipe with both ends flared to allow a 1.5 inch pipe to be added to each end.  This will 'telescope' nicely inside of the 1.75 inch exhaust pipe. Simple take two of these, and drill a series of holes through the 1.5 inch center section. Then mount these inside of your 1.75 inch exhaust pipes as a baffle. You can also wrap fiberglass around it when installing it to change the exhaust note a bit . This will be significantly louder than the baffles which I purchased, but should be quieter than running without any baffle.

I'd also like to find some alternate chrome tips to try. I'm keeping my eye out for something which might fit and will be a bit different. I did see one system which someone created by using a chrome 'turn out' exhaust stack designed to be used on a truck.

Rejetting the carburetors

There seems to be a lot of myths about rejetting the carburetors. If your bike is running properly, there is no need to 'rejet' your carburetors.

What does it mean to 'rejet' or 'jet' your bike? This means you are going to change the air to fuel ratio (or mixture) being delivered to your combustion chamber. There are several ways to do this.

Do you need to 'rejet' your bike? This depends on how your bike is performing. If you are happy with the performance of your bike and it is not running poorly, then you probably do not have to rejet. You should also determine if your engine is running within the design limits that it was designed to handle. You don't want it to run too lean or too rich (see below for definitions of lean and rich).

These days, most bikes are designed to run on the lean side, especially during the idle part of the throttle curve. This allows the bike to pass federal emission standards. Unfortunately it also causes many bikes to demonstrate a bit of hesitation (sometimes called a 'flat spot') in the throttle response. I can feel this in most bikes as they are setup from the factory. I complained to my dealer several times about this. They said the bike was running normally. They are not allowed (by federal laws) to change the carburation in the idle circuits. This is precisely where the problem was! The hesitation (or 'flat spot') went away when I richend up the idle circuit on my bike by going to a larger pilot jet (see below for a description of the pilot circuit). One might think that simply adjusting the pilot screw (sometimes called the idle mixture) would solve this, but on my bike I had to both change the pilot jet and adjust the pilot screw.

Your carburetors are setup to provide the proper fuel to air mixture for your particular setup. Any changes made to the air intake, engine, or exhaust system may require you to change the fuel to air ratio which your engine requires to run correctly. When you 'rejet' a carburetor, you are changing the jets which affect the supply of fuel for the mixture.

The idea is to not allow your engine to run too lean (not enough fuel) or too rich (too much fuel). Lean running can cause the engine to run too hot, and can damage internal components. You could experience melted spark plugs, burned rings, burned valves or even burn a hole in the top of your pistons! Running too rich can cause your plugs to be fouled by carbon build up. Running too rich will also cause poor gas economy.

A properly tuned carburetor will allow the engine to develop the optimum power over the entire throttle range. To properly rejet the carbs you can make changes from idle, through mid range to wide open throttle.

There are four changes which you can make to the mixture with the Factory Pro rejet kit. You can change the main jet which affects all throttle ranges, but mostly adjusts the wide open throttle mixture. The mid-range (partial throttle) is affected by the tapered needle. The idle is controlled by the pilot jet. The fourth adjustment is made to the floats by adjusting their height.  The Factory Pro kit give you control of all four of these areas. They supply a wide variety of main jets, an alternate pilot jet in case you need to richen your idle circuit and an adjustable set of needles to adjust the mid-range mixtures.

My carburetor settings

I'm currently using the following jets and settings. This is being refined over time. If I had access to some proper test equipment such as a dynamometer and a exhaust gas analyzer, I could do this quickly! Since I don't have access to this, I'm taking my time to make changes and evaluate the results.

I started using a larger set of jets and am working my way down. I didn't want to start too lean. At first I was getting some plug fouling (carbon buildup on my spark plugs) so I knew that the system wasn't setup correctly yet.

With the baffles and exhaust tip extensions which I have installed I'm currently using the #45 pilots supplied by the Factory Pro kit. The pilot screws are set to about 2.5 turns. My needles are now set to the #3 (from the top) position. I'm using a #172 main jet in the front carburetor and a #178 main jet in the rear carburetor. I'm still testing this configuration and think it may be a bit lean at wide open throttle, so I may go to the next larger main jet size.

I have had a slight flat spot which occurs just as I crack open the throttle a bit when maintaining a steady speed on the highway. This condition has been there before any modifications have been made to my Aero. I complained to the dealer about this several times. When I changed the pilot jet from the Honda installed #42 to the #45 supplied with the FactoryPro kit this flat spot disappeared. All I can say is: WOW, jetting can really make a difference in how the bike handles. It almost seems like a different machine. I seem to have much more low end 'grunt' (power available when going at low speed).

The Aero handles great with this system on it! I recently took a trip of over 350 miles riding two-up. With the two of us on the bike, we were at the factory load limit (About 350 pounds) of the Aero. I had NO problems cruising with traffic which I noticed a few times that we were going about 30 MPH OVER the speed limit (don't ask, I think YOU can do the math yourself). I also had plenty of power to pass when I needed it. No, this didn't turn the Aero into a crotch-rocket, but I feel the bike handles just fine on an interstate riding two-up, thank you!

Rejet tips

Here are some things I've learned. I definitely recommend that you have the Honda service manual for your Aero. It definitely is an asset and well worth the money. The front and rear carburetors are different. The service manual shows that there are different main jets in the front and rear carbs. There are other differences in the carburetors also! When you work on your carburetors, only work on ONE AT A TIME so you do not mix up any internal parts.

When you install the needles, the Factory Pro needles are a bit longer than the Honda needles. To allow the new ones to be installed, you will need to cut off a small amount of the plastic 'stud' which the needle hold down spring is pressed onto. Just cut this stud about in half to make room for the longer needle. Don't cut off too much! If you're not sure how much to cut, simply cut a SMALL amount off, and then re-install the spring, and attempt to install the Factory Pro needle. If the needle holder will install, and there is some spring action to the needle (if you press in on the needle, you can feel the spring resisting its movement) then you have cut off enough. If the needle holder will not install, you will need to snip off a bit more. Go slow and take your time! You need to leave enough of the stud to hold the needle retaining spring.

Even though the service manual does not point it out, the slides inside the carburetors are different for the front and rear carburetors.  The needles mount in the slides in the middle of the three holes at the bottom of the slide. The front and rear slides differ by the size and position of the remaining two holes.  The FRONT carburetor slide has two 3MM holes. The REAR slide has two different diameter holes which are smaller than the holes in the front slide. If you mix up the two slides, you can tell the difference by comparing the holes in the bottom of the slides. Again, the FRONT slide has two holes which are the same size and are larger than the holes in the rear slide.

The screw on the clamp which holds the rear carb to the intake manifold is difficult to reach because of all the tubing on the left side of the engine. I rotated my clamp about 45 degrees to the 'right' as viewed from the top. This makes the screw on this clamp accessible from the top of the bike and MUCH easier to get at. If you only change your jets once, this is not a big deal, but since I'm doing mine a small step at a time, this has made the job much easier.

Dog gone squeak!

This is the infamous 'dog bone' that creates all the high pitched squeaking noises from the exhaust system after a while. It's located under the bike. Look under your bike from the left side, looking toward the right, just in front of the rear tire. You can see the 'dog bone' there. Remove the two bolts, remove the 'dog-bone' (actually the muffler system support bracket) and apply some waterproof grease to the fittings.  Be sure to remove the all-metal bushing from the top end of the 'dog bone' and place some grease in the groove before replacing it. It appears to me that Honda forgot to lubricate this during the factory assembly procedure!

You can use any type of grease for this, however waterproof grease will last a lot longer. You don't really need any special type of grease. WD40 or other spray lubricants will work for a short time, but if you use them you will be giving it a squirt every now and then.

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