How-To: Diagnose Motorcycle Vacuum Leaks

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What is a vacuum leak?

A vacuum leak is caused by extra air introduced into the fuel/air mix produced by the carburetors, which will lean out the mixture and cause poor running conditions. Vacuum leaks occur between the cylinder head and carburetor slide/butterfly, where the low pressure will draw in air through the leak area. Vacuum leaks are very common in older motorcycles.


What are sources of vacuum leaks?

The most common area of vacuum leaks are the carb holder (rubber boots). The rubber degrades over time, dries out, and can begin to crack or become brittle. Sometimes the boots look fine until you bend or stretch them, which can reveal cracking. The images below show a carb holder from a Polaris that looks fine until you bend or stretch the boot. While this particular boot was not cracked all the way through, it was replaced as preventative maintenance. You don’t want to lean out a 2 stroke due to vacuum leaks! Kiss that engine goodbye!

Carb holder vacuum leak boot


Carb holder vacuum leak rubber boot stretched cracked


Throttle shaft seals can also cause vacuum leaks and are the most difficult to replace. The leak can be caused by either worn throttle shafts, bad seals, or both. Some carbs rely on the tight clearance between the throttle shaft and carb body to minimize air leaks. Below, a Keihin carb from a Nighthawk which has felt installed on the throttle shafts, is not to be mistaken for a seal. I’ve replaced the felt and only reduced the vacuum leaks by about 50%. If you run into a situation like this, I would recommend replacing the felt with an O-ring.

Throttle shaft vacuum leak felt seal keihin CB


Fuel pump diaphragm and/or tubing leading to intake can also be a source for a leak. The fuel pump diaphragm can also leak fuel through vacuum tubing causing the motor to stall on deceleration. The video below will show you how to check a fuel pump diaphragm.


There are several other areas where vacuum leaks can occur, I mentioned the most common above. Please refer to diagnosing methods below to thoroughly check your motorcycle.


What are symptoms of vacuum leaks?

• Loss of power.
• Runs better with choke on, and in some cases the only way it will run.
• Erratic idle. You’ll never be able to set the idle. Sometimes it will idle higher or lower.
• Runs better at higher RPM.
• Sounds “boggy”.
• Do not mistake vacuum leaks for out of synch carbs. Never attempt to synch carbs without verifying that there are no vacuum leaks.

The video below takes you for a test ride on an 81’ Honda CB750 with severe vacuum leaks in the carb holders and throttle shafts. This will give you an idea of what symptoms to look for.


Diagnosing vacuum leaks

Luckily, diagnosing vacuum leaks are pretty easy, at least when the leak is fairly large and greatly affects performance. Listed below is what you’ll need to find your leak (listed in order of preference)

• Carb clean or starting fluid (these will remove paint, wipe it up immediately & blow dry with compressed air. Also keep fire extinguisher nearby)
• Propane (take the tip off and attach a long rubber hose)
• WD-40
• Water

To find your vaccum leak, choose one of the above and spray or point in the suspect areas while engine is idling. Any change in idle RPM, whether up or down indicates a vacuum leak. Below is a video on how to find vacuum leaks.


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