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They can be called “APV” by Arctic Cat, “RAVE” by Ski-Doo or “VES” by Polaris. If you own a 2-stroke snowmobile made in the last 15 years, the engine is likely equipped with “variable exhaust power valves”. If that is the case, you probably know that you have to be quite diligent about their cleanliness and operation. This is one engine component that has to be monitored regularly to maintain peak performance.
Variable exhaust power valves are moving devices located at the engine’s exhaust port. Every brand’s variation is there to provide the same essential service and that is to vary the height and width of the exhaust port according to engine RPM. This vastly broadens the power band. At low RPM, the valves are in a closed position which promotes more complete combustion. This improves low-end torque and minimizes unburned gases in the exhaust stream. At high RPM, the valves are wide open which allows for optimum exhaust flow. This enables the engine to rev freely and produce maximum top end horsepower. Older systems were actuated by a mechanical linkage which opened and closed the valves based on engine RPM. Modern systems are actuated by electronic servo motors which precisely position the valves throughout the RPM range. Before the advent of the power valve, engine builders had to determine one size and shape of the exhaust port and exhaust pipe. This predetermined whether the engine would provide its optimum power delivery at either the high, mid or low RPM range. The power valve allows the two-cycle engine to provide usable horsepower throughout its operating RPM range.
The modern power valve system has evolved from a number of different variations over the last 30 years. Yamaha was the first company to find success with this concept when they integrated a variable exhaust power valve system in their 2-stroke Grand Prix road-racing motorcycles in the late 1970’s. The technology then made its way to Yamaha’s production road and motocross bikes the early 1980’s. Other motorcycle manufacturers soon came up with their own systems. By the early 1990’s, virtually all high-performance two-cycle motorcycles, ATV’s, personal water-crafts and snowmobiles were equipped with variable exhaust power valve systems.
Keeping the power valve system clean is of paramount importance. As power valves are located directly in the stream of the exhaust flow, carbon deposits can accumulate on the valves over time. The deposit build-up can alter the shape of the valves and change the engine’s exhaust flow characteristics. In extreme cases, the exhaust port can become completely clogged and expensive damage can occur. Deposits can also can hinder the valve movement or leave them stuck in one spot. Any of these scenarios will drastically impair the power delivery and output of your engine.
The task of maintaining your snowmobile’s power valve system is not complicated. Your service or owner’s manual will give specific instructions on how to remove and clean the system. Be careful when cleaning the valves, as you do not want to scratch or roughen the surfaces. Your manual will also offer a specific service interval and it should be followed religiously.
Once your valves are clean, there are ways to prevent premature valve depositing. First and foremost is to make sure that your machine is tuned correctly. Overly rich carburetor jetting is a major contributor to this malady. Rich jetting will allow too much fuel into the combustion chamber and this promotes carbon formation. If your sled is fuel injected, make sure that the system is operating correctly. Also make sure that your oil injection system is delivering the proper amount of oil. Perform regular spark plug readings as described in your manual. If your plug readings are off and you cannot determine the cause, contact your dealer.
Another major contributor to power valve depositing is the use of poor quality 2-cycle oil. The cheaper base stocks and additives used in lower grade oils can be cooked into carbon deposits very quickly. High quality synthetic two-cycle oil is notorious for its clean burning characteristics and will dramatically lower the rate of deposit formation. Good quality synthetic oils use sophisticated base stocks and high temperature additives which are engineered to avoid power valve depositing. They also have an effective detergent/dispersant package which will allow for much cleaner operation. When it comes to two-stroke injector oil, you definitely get what you pay for. Spending a little more on an oil that is specifically engineered to provide clean power valve operation will not only improve your engine’s performance, it will save the headaches and expense that come with premature power valve depositing.
Snowmobiles operated at low speed and steady RPM over extended periods can also promote deposit build up. During this type of operation, the valves maintain a stationary position and this allows deposits to form. If the valves are constantly changing positions, there is less chance of deposits forming and increasing in size.
Variable exhaust power valves have revolutionized the capabilities of 2-cycle engines. With a little bit of maintenance and care, they will offer trouble-free performance.
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