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Learn How To Track Down Motorcycle Vacuum Leaks Like A Pro

Some of the biggest carbureted motorcycle repair headaches can come from the smallest of problems. A small vacuum leak can cause big problems with your carbureted motorcycle's drivability, performance and even fuel economy. In order to put this problem to bed, you'll first need to learn how to recognize the signs of a vacuum leak as well as how to track it back to its source.

Understanding the Symptoms

Vacuum leaks are often caused by small cracks and holes in the various rubber components on or near the carburetor. The most common culprits include the carburetor manifold and intake boots, throttle shaft seals, fuel pump diaphragm tubing, vacuum hoses and caps. All of these rubber parts can degrade and become brittle with time and use, allowing cracks to form on the surface. These cracks can tear apart, leaving holes that create the vacuum leaks you're trying to track down.

A vacuum leak may cause one or more of the following symptoms to occur:

  • Your motorcycle constantly loses power, and it often runs better at higher RPMs and/or with the choke in the open position.
  • The engine has an erratic idle that's hard to properly correct through conventional efforts. Sometimes it may idle high due to the leak.
  • The engine constantly hesitates and sounds like it's bogging down at times.
  • The motorcycle becomes hard to start in cold weather.

Keep in mind that some of the symptoms of a vacuum leak are also similar to those of an out-of-sync carburetor. It's a good idea to check for vacuum leaks first before you start messing with your carbs. This way, you can be certain that the issues you're experiencing with your motorcycle are being caused by your carbs and not because of a leaky vacuum line somewhere.

What You'll Need for the Job

Now that you understand the potential sources and causes of a motorcycle vacuum leak, it's time to pinpoint the source of the leak. For this, you'll need something that can help you detect the leak as the engine is running. Your options for this include a can of carburetor cleaner, WD-40, or a propane torch with its tip removed and replaced with a 1-inch length of rubber hose. These tools make it easier to track down hard-to-spot vacuum leaks.

Depending on how your motorcycle is designed, you might need to remove the fairings and fuel tank so you can have better access to the engine. If you have to remove your fuel tank, you can use a standalone auxiliary tank to keep fuel flowing to the engine during your diagnosis.

Tracking Down the Culprit

Once you have complete access to all of the vacuum hoses, caps and various fittings, you can start up your bike and let it reach its normal operating temperature. Spray the carb cleaner, propane or WD-40 around the areas where you suspect the vacuum leak is located. Immediately after you do this, you'll want to carefully listen for any change in the engine's idle.

What you'll be looking for is the leaky component to draw the chemical of your choice into the engine, thus changing the engine's idle RPM momentarily. If the idle increases or decreases after spraying the area, then there's a good chance you've found the source of the vacuum leak.

Afterwards, shut the engine down and take a close look at the pinpointed area. You may need a flashlight to spot the cracked, degraded rubber components that are the source of your troubles. In most cases, you'll need to completely replace these components to solve your vacuum leak problems. If you don't have the parts on hand, however, you can apply a small amount of RTV silicone into the crack as a temporary stopgap.

If your motorcycle requires more extensive repairs, or if you don't feel comfortable performing this task on your own, contact a company like Monarch Honda for assistance.