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How to Diagnose and Repair Intake Manifold Leaks

Written by  November 30, 2007

Every once in a while I like to report back on some of the malfunctions that have plagued many of us in the past, such as your intake manifold. When it comes to intake manifolds, these items are under extreme heat at all times and don't always receive the proper cooling, which the front of the engine normally gets. The temperature at the mouth of the intake will vary, but often exceeds 240 degrees. Unlike the exhaust system, the intake manifold on many bikes is not a metallic-type material like on the American V-twins. In fact, most metric motorcycles have a rubber-type intake manifold that is mounted to the cylinder head. The newer model also have an o-ring for the gasket material instead of a cork-type material, which is intended to keep the cost down and also make removal and installation easier for servicing.

Intake leaks can cause a wide variety of abnormal running conditions, such as:
The engine taking longer to settle down when the throttle is revved. This almost acts like a carburetor synchronize issue, but don’t be fooled because the carb sync only works on the idle circuit.
The engine starting hard and requiring more choke than usual. This is what may happen to Harley-Davidson owners.
The engine warming up, but when you are riding down the street and engage the choke, the engine stumbles and wants to die.

You can often diagnose an intake leak by listening for a popping sound or visually looking for a gap in the mounting point of the manifold. There is, however, another way to diagnose a potential problem with the intake system, which is to take an over-the-counter can of contact cleaner, and while the engine is running, spray around the manifold sealing area and listen for a rev in engine rpm or a stall condition that will happen instantly. Don’t be afraid to use plenty of the spray, because and at this point, you are looking to fix your motorcycle the quickest and most inexpensive way you can.

If the engine has an intake leak you have a choice to either repair it yourself or have a reputable shop do the work for you. The decision is up to you and your budget; so if you decide to tackle this repair on your own, I highly suggest getting a service manual to locate any and all parts you will need to repair your motorcycle correctly the first time.

This is a common problem on most four and six cylinder inline motors. When the carburetors are serviced, the intake manifolds are removed as part of the service and were either replaced or resealed. The carburetors were then reinstalled and the synchronizing was checked and adjusted. Many Harley-Davidson models up through 2006 came with carburetors, which are now being replaced by fuel injection. Don’t think that this won’t happen to fuel injection applications as well, because it will. The same stresses and heat is present, which can cause an air leak.

The sealing surface can be repaired with a semi-drying sealant such as Yamabond 4 or Honda Bond. These sealants are found in your local Honda and Yamaha dealerships for around $6-9. The equivalent product at your local auto parts store would be the Permatex aviation-grade semi-drying sealant, which also works extremely well for this repair.

Once you remove the manifold, clean the surface with fine sandpaper, a wire bristle brush, or equivalent. Make sure the aluminum surface is completely clean to accept the sealant and for reinstallation of the manifold. You will also want to visually inspect the manifold for cracks and tears, and if found, the manifold must be replaced. Depending on your motorcycle, they should be reasonably priced, and since you have them off, it makes sense to do the job right and get your bike back on the road.

Installation is simply the reverse of removal. You will want to tighten the mounting bolts or clamps to torque specs, reinstall the carburetors and throttle bodies, and start your bike to check the manifolds using your contact cleaner. In most cases, this will fix the problem, and poor running conditions will be eliminated. Your bike should also idle correctly after the engine has been revved and the throttle closed.

For those of you who decide to tackle this repair, you will have a better understanding of how something so simple like an intake manifold leak can cause so many issues. This should also get you pumped up for the next repair you decide to take on.

I want to wish you and all of our brothers and sisters in the military that are serving over in the Middle East this holiday season a Merry Christmas, and we look forward to when all of you come home.

Merry Christmas and B-safe out there!

Dave Miller