motorcycle-engineMotorcycle Vacuum Leaks are a seemingly small problem that can cause big headaches when you’d least expect it. Learning How to Diagnose Motorcycle Vacuum Leaks quickly and easily will save you a lot of trouble in your riding career. Vacuum leaks are a common problem, especially on older motorcycles and bikes stored outside. Heat, changing weather conditions, and sunlight exposure all cause rubber parts to dry out and crack, and if that rubber part is subject to your carb’s vacuum, you end up with a vacuum leak! This article will teach you the easiest way to Diagnose Motorcycle Vacuum Leaks so you can spend more time riding, and less time wrenching!

Tools Required:

  • Can of Carb Cleaner, Brake Cleaner, WD-40, a Propane torch, or a spray bottle of water. – If you use the propane torch, you will need to remove the torch tip and replace it with a 1-2′ length of rubber hose.

As always, take proper precautions when working with flammable chemicals, protect your eyes, work in a well ventilated area, and keep a fire extinguisher handy just in case.

Depending on your bike, you may or may not need to remove fairings and raise or remove the fuel tank. You will need full access to all vacuum hoses, vacuum caps, intake manifold boots, and air intake fittings in order to thoroughly search for any vacuum leaks that may be present. Should you need to remove the fuel tank completely, I suggest using an auxiliary tank like the one pictured on the left to run the bike while you hunt for those pesky vacuum leaks.

Once you have full access to all vacuum hoses, vacuum caps, intake manifold boots, and air intake fittings, start your bike and get it warmed up to operating temp. Next, pick your chemical of choice and spray it around each of the areas mentioned above, one at a time, waiting several seconds in between sprays. As you work, listen for any change in your motorcycle’s idle. If you hear a change, you’ve found a leak! If the leaking part can be tightened, such as the clamps on your intake manifold boots and air intake boots (if present), try that and test it again. If it cannot be tightened or tightening does not seal the leak, it will need to be replaced. With the intake manifold boots (the rubber boots between the carburetors and engine), there is usually an o-ring between the boot and the engine that is a common source of leaks. The vacuum caps, also known as ‘sync caps’ on your carburetors are another common source of vacuum leaks as they are often overlooked.

In the event that you have sprayed all of the parts listed above and your motorcycle’s idle does not change, your problems are most likely something other than a vacuum leak. The carburetors themselves are a likely suspect, though it could also be something as simple as a clogged fuel filter, petcock, or possibly an air filter depending on the symptoms you are experiencing.

Katana-carbs-1What a fun week. OK, fun may not be the right word, but I digress. It’s been a long process getting my ’03 Suzuki Katana 600 running again after it inexplicably refused to start one morning a few weeks back. I ended up having to replace the fuel petcock, and inline fuel filter, flush the entire fuel system and tank, and rebuild the carburetors. This was all due to some water-contaminated gas, I assume, since I filled up the night before at a Circle K and the gas I drained out of my fuel system was a mess.

While it was down I also decided to get some other work out of the way in the form of a valve adjustment, new vacuum hoses, new fuel lines, and a general cleaning of pretty much everything on the bike. If nothing else, she will be pretty! haha

I’ll have to go get new gas in the morning and (hopefully) the bike will fire up so I can start tuning. With any luck, ill be able to put this expensive, and frustrating issue behind me. I usually commute daily on the Katana, rain or shine, so not being able to ride has been a major disappointment.

Thanks to RideNow Powersports for a couple of the parts, and to Arizona Motorcycle for the carb cleaning.


Should anyone be interested, here is what you need to refresh your fuel system:

Fuel Filter:

I got a glass-bodied one with a paper element, but this one is the same brand.

Carb Rebuild Kits 

Petcock Rebuild Kit

Step by Step Suzuki Katana Carb Rebuild Instructions:



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