Exhaust System

By: Ron Milam

 

      There are four main systems on a trials bike that affect its performance. Engine characteristics, brakes, suspension, frame geometry and rigidity. The first three of these are fairly easy for an owner to change and hopefully improve. These are also areas where performance slowly degrades during the first few years of ownership. This happens so gradually that it often goes unnoticed until the opportunity arises to ride a fresh mount. By the time this is discovered many points may have been lost in competition. Many bones may have been broken in practice. Multimillion dollar sponsorship opportunities may have been lost, etc. Maybe that is going a little bit too far, but if you keep your bike performing like it did when it was new, you won’t be so shocked and feel so bad when you ride your buddy’s new one.

        All current trials motorcycles and most older ones have wonderful engine characteristics. They pull strongly from sub-idle speeds to chug you around that impossibly tight turn and rev freely to get you up that step or over the top of that hill. That is, when they are running properly. But with time you may find that the crispness that impressed you enough to buy your bike in the first place is just not there anymore.

What happened? I am going to make a few assumptions here, like you have cleaned your air filter and oiled it properly,and  you have good fuel and no clogged fuel filters, bad sparkplug ,etc. If your bike is only few years old a good place to do start is your  exhaust system. Many people do not understand the effect that a properly designed exhaust system has on trials bike performance. Proper cylinder scavenging is just as important to us at 1000 rpm as it is to a motocrosser at 8000 rpm. Your exhaust system has  certain internal volumes located at certain distances from the exhaust port. These features are important because they cause the formation of both negative and positive  pressure waves in the pipe. When the exhaust leaves the cylinder and expands into a larger volume section of the pipe (the first cone in an expansion chamber ), it causes the pressure at the exhaust port to be reduced or even go negative. This helps suck those nasty exhaust gases out of your nice cylinder and make more room for the fresh new charge to get in. The pressure is so low that some of that nice new charge can  escape as well. Then a split second later the escaping exhaust hits the area of the pipe that has a decreasing area (second cone of an expansion chamber ). This causes a positive pressure wave that travels backwards to the exhaust port and pushes the escaping fresh charge back inside the cylinder just before the piston closes the port. Trials bikes do not usually have expansion chambers, but in many bikes you can see a tapered header pipe followed  by a section with a reduced crosssectional area. Some bikes have the pipe designed differently, but the main point is that the entire exhaust system is critical to getting the proper fuel air charge in and out of the engine. Unlike a motocrosser, the exhaust system of a modern trials bike has two mufflers. One is that bulbous thing that gets bent in and rubs your rear tire. The other is that critical second section of the pipe. If you cut your exhaust pipe open you would find that the second section is double wall. The inner wall is most likely made of perforated metal. These holes can become plugged with carbon deposits and therefore the volume between the walls is lost. This change in volume changes the scavenging characteristics slightly. The same thing happens when the secondary muffler becomes plugged or has the packing blown out . Its volume is changed  and the pipe characteristics change. Your carburetor is jetted for a certain set of conditions. If your exhaust system, slowly over time, changes those conditions, the result will be reduced engine performance. I have usually seen this  cause a rich condition at lower rpms. What to do? Be of good cheer trials bretheren. The remedy to this problem is technically easy for anyone to perform. It is however, one of the messiest jobs in all of  motorcycledom.

       First you should procure some replacement muffler packing material from your friendly dealer. Next remove the entire exhaust system. Pour out any oil that may have collected inside. ( Don’t laugh. There is probably some in your pipe at this very minute.) Be sure to spill some on your pants . It will permantly give them that professional mechanic look. I like to do this job in stages. Start with the most coarse work and then move on to the finer parts. Scrape out what you can reach with your favorite scrapping implement. Gasket scrapers, putty knives, screwdrivers, brushes, etc all have their place as you reach further and further into the pipe. I have had good success using a length of steel banding material bent double so that you must spring it closed to fit it into the pipe. Twist it  as you insert it and you will be rewarded with a small pile of black yukky stuff that can be saved and used later for practical jokes.

          The next part is not so easy. There are several ways to remove the remaining residue. Some people plug one end of the pipe and fill it with a strong detergent. Some people like to burn it out. I am one of the later group. I usually do not have a lot of time to spend waiting for the detergent to do its job. I also like to play with fire. I have heard of people putting their pipe in a barbecue grill and heating it from the outside. That may work well, but I just use a propane torch to heat the inside. Oily carbon is an excellent fuel and once you get it hot enough it will burn readily. If you choose to do the job this way, don’t sue me if you burn yourself, your yard ( did I mention this is definitely an outdoor project? ), or any other person, place, or thing. I’m not recommending anything. I’m just telling you how I have done it and achieved excellent results. If any of your parts such as the silencer core are aluminum, be careful. That carbon-oil mixture burns hot and you can melt the parts if you get carried away. After cooling, any residue is easily removed. You can use this  opportunity to clean up the pipe and give the non plated portions a nice new coat of paint.

         On assembly, be sure any o-rings, gaskets, etc are in good condition and installed properly. A little black silicone sealant will assure pressure tightness on those questionable areas. The system should be pressure tight when properly assembled. After all of this hard work, reward yourself with a nice long test ride. At the least your bike will be much quieter and at best it will pull much more cleanly from idle to midrange.