Carburetor and Intake

Repair Cracked Motorcycle Carb Boots, What You Need To Know

Cracked carb intake boots are not difficult to locate or repair, here is what you need to know to do it yourself.

Example of a set of motorcycle intake manifold rubbers

Weathered, cracked or split rubber motorcycle carburetor  intake manifold boots cause erratic idle, and lean burning conditions.

The carburetor boots are also called the carb holders, or intake rubbers, along with a bunch of other names.

What the hell are carburetor boots?

Carb boots are the rubber ring that attaches your carburetor(s) to your engine. They are black and have a hose clamp that attaches them to your carburetor, and they bolt to the engine.

What If My Carb Holders Leak?

If your Intake rubbers leak your engine is sucking air in through places other than the air cleaner. This will usually make your engine idle erratically. Sometimes it will idle to low, other times to high, and once in a while it might even idle just fine.

It also can cause your motorcycle engine to run lean. This means its is getting too much air and not enough fuel. This can mean it idles poorly, but if it becomes severe enough it can burn a hole in your piston. Do not ignore the problem, it’s not going to get better.

How To Check For Leaking Motorcycle Intake Manifolds?

There are a couple of cheap easy ways to check if your carb holders are leaking or cracked.

Start up your bike and let it warm up, until it idles as good as is can. I know your having idle trouble or you wouldn’t be reading this. Warm it up until it is stable as it can be, this will make it easier to test and diagnose your vacuum leak, if you have one.

Option Number One

With your motorcycle running spray around the intake rubbers with starting fluid, carb cleaner, or WD-40.

If you hear a change in engine speed, it means your motorcycle engine has sucked some of the spray into the engine through a crack or split carb holder.

Option Number Two

Again with the engine running at idle, move an unlit propane torch around the rubber intake manifolds. If you hear a change in the idle speed you have located a cracked or failing rubber intake manifold.

Option two is my preferred method. It does not make a mess and you do not have to worry about accidentally spraying flammable liquid onto a hot exhaust pipe.

Testing for leaking carb boots using an unlit propane torch

Testing for leaking carb boots using an unlit propane torch

My Carb Boots Are Cracked, Now What Do I Do?

There are a few ways to repair leaking rubber carburetor holders. They are all temporary fixes though. Best used to get back home or to ride a little while you are waiting for your replacement carb boots to arrive.

Temporary Carb Holder Fixes.
  • You can try wrapping black electrical tape around the outside of the carb holder.
  • Brushing on liquid electrical tape over a crack carb rubber.
  • Coat the outside of the carb holder with plastic tool handle dip.
  • Smear RTV silicone over the outside of a failing intake rubber.

All of these have been used, but none of them are permanent. None of these products stand up to exposure to gas. So you can only use them on the outside surfaces of the carb boots.

One Other Place to Check

A less common place for your holders to leak is the flange where they bold to the engine block. Or even a deformed carb rubber may not be sealing tight around the carburetor body.

These area are worth checking, but cracks in the intake rubber itself are much more common.

Age, and the weight of the carbs vibrating and bouncing tend to fatigue motorcycle carburetor intakes. It’s not an uncommon problem. It’s also not that difficult to diagnose and fix.

You can handle it. Grab some tools and jump in there. Save a few buck and learn something while you repair your own motorcycle.



  1. Pingback: Motorcycle Idle Troubleshooting « Long Beard Bikes - October 6, 2012

  2. Pingback: The Right Way To Clean Your Motorcycle Carburetors « Long Beard Bikes - October 7, 2012

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