How-To: Pressure Test Float Needle

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A very common problem I see in motorcycle carburetors is dirty or worn float needle(s). The float needle’s job is to move up and down in the needle seat to allow the float bowl to maintain the correct fuel height. In the closed or seated position, the needle must seal on the seat and not allow any fuel to flow. Over time, the needle can get worn since it is a moving part, or dirty from gummed up fuel. Let’s go over some some symptoms and a quick way to check the needles operation once it has been replaced or cleaned.

 

Symptoms

These are all symptoms of a faulty float needle:

  • Fuel pouring out of your carburetor overflow tube
  • Airbox/filters get soaked with fuel
  • Crankcase getting overfilled with fuel. Oil smells like fuel. (make sure to change oil once problem is fixed)

 

Tools

Make sure you use a blow gun with a rubber tip so it seals on the fuel inlet. Here is one similiar to mine available from Amazon.

 

Astro Pneumatic 1718 Deluxe Orange 4″ Air Blow Gun

 

Testing Procedure

Testing the float needle is a good idea while the carb is apart. I don’t know how many times I put the carb back on the bike to find out that the float needle will not seal after a good cleaning. It only takes an extra minute, and will save you time in the long run.

Here is the procedure:

  1. Set your air pressure very low, 3-5psi is plenty.
  2. Using a blow gun with a rubber tip, insert into fuel inlet and apply air pressure.
  3. Manually move the float up and down. You should hear air only when the float needle opens. If you hear air when the needle is closed, clean or replace the float needle.

 

how to pressure test float needle

 

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