Your motorcycle hasn’t been performing at its usual level, and you’re worried there may be a problem with its mechanics. An intake air leak is a sudden onset issue that can make your bike go from running normally to roughly almost instantly. Learning the symptoms can keep you from shrugging your shoulders and deciding your machine has just gotten old. Whether you’re not sure how to find the source of the leak or how to fix intake manifold leak, this article answers all your basic questions.

What Is an Intake Leak?

Your engine runs off a mixture of fuel and air. Messing with the mixture can increase performance (think aftermarket air filters and fuel injection modules) or decrease it (as with an intake leak.) An intake or vacuum leak occurs when you lose the vacuum seal somewhere beyond your fuel delivery system. This means that the leak could be occurring almost anywhere between the cylinder head and carburetor slide. This leak causes air to dilute your fuel and oxygen mix.

What Are the Symptoms of an Intake Leak?

Unless you like browsing around mechanical articles, you probably found this blog because you suspect you’ve already seen some carburetor air leak symptoms. Losing power is a big one, but that may not be noticeable on an old bike. The first one you’re likely to notice is an erratic idle. Usually it’s idling higher, but it might change suddenly for no apparent reason. You may also notice it seems to smooth out more at higher RPMs.

What Happens When You Don’t Fix It?

Short term you’re looking at performance problems. The aforementioned loss of power is the main issue with rougher running and poor gas mileage the consequences of the bad fuel mixture. It also raises engine temperature, especially if your motorcycle is air cooled. The major reason to take care of an intake leak is the damage it can cause in the long term. You could wind up losing your engine to a hole in the piston.

How Do You Diagnose an Intake Leak?

If you want to know how to check for intake leaks, there’s a few tricks. Try pulling the clutch in and closing the throttle. The engine revs should fall immediately, or you’ve likely got a vacuum leak. For finding the leak itself, start your motorcycle up and leave it idling. Grab a propane torch, bottle of penetrating spray oil (such as WD-40,) or even a squirt bottle with water in it. For the propane, turn the torch on but don’t actually light it. Then apply the gas or liquid in areas you might find a leak. Any changes to the RPMs indicates that you have an issue.

What Are the Most Common Spots for Leaks?

The rubber motorcycle intake boots are a good place to start. While rubber is an excellent sealing material, it does break down over time. Changes in temperature, UV rays, and the effects of fuel cause cracks to form in the rubber. If the spray test indicated problems but the boots outwardly look fine, they may still be causing issues. Try pushing or pulling on the intake boots to see if holes or cracks appear.

The throttle shaft seals also cause leaks but are difficult to diagnose. Not only are they hard to access compared to the intake boots, the problem could be with the throttle shafts, the seals, or both.

Breaches in one of the vacuum hoses can also be the source of air intake. These are usually easier to see because the hose is longer, and it’s hard for cracks to hide. If you don’t see anything, you can always try flexing the hose. Leaks in the fuel pump diaphragm can be air or fuel, although the latter tends to stall your motorcycle during deceleration. The spray test may reveal the leak, but you probably need a vacuum pump to diagnose this part.

These are the most common areas for leakage, but anywhere beyond the fuel delivery system could be the culprit. Even misaligned intakes can cause a gap in the seal with a V-twin cylinder head. The spray test is generally pretty revealing, but there’s no shame in seeing a mechanic if you just can’t find the problem.

How Do You Fix the Leak?

Diagnosing the leak is often the most difficult part of the whole process. As discussed above, an intake leak isn’t just one part failing, it’s one of quite a few parts failing. Thankfully, the most common causes are relatively simple and, quite often, cheap to fix.

If you’re seeing cracked intake boot symptoms, the good news is that this part is the easiest to replace. Just make sure you get a good seal before you start it up or you’re back in the same spot. While you could use a silicone sealant on the crack, you’re only delaying the inevitable. Replacing an intake boot is cheaper than blowing a piston top. The vacuum hoses aren’t much harder, although some are a little tough to get at.

If your carburetor needs a tight seal, the throttle shaft could cause issues just by loosening a bit. The felt spacers often used on these setups do not work as effectively as o-rings. Carburetor o-rings are cheap, so there’s no reason to cut corners by using a felt for the repair. A worn throttle shaft is a more involved fix, so you can either tackle it yourself or call your mechanic.

If your V-twin intake isn’t aligned correctly with the cylinder head, you can seal off the breach by reinstalling it. This is often as simple as loosening the head, letting the intake settle to a flush position, and then tightening it. While it may initially look off-kilter, if it’s creating a good seal then you’re better off leaving well enough alone.

Intake leaks are potentially damaging, but they’re usually easy to fix. Now that you know the common leak sources, you can find the spots in need of repair. Identify the right product then shop the best aftermarket fuel and air parts that you can use to seal off the breach.