Tech: Fixing Overheating Carburetors

Tech: Fixing Overheating Carburetors

Watch Kyle remove his Amal carburetors on his 1969 BSA Lightning that had some overheating carburetor issues. Installing new phenolic spacers fixed the over heating problem.
Overheating carburetors are a common occurrence on some British motorcycles. An overheating carburetor can cause serious performance issues. Finding and diagnosing the problem can be tricky, but once found, it can be fixed in a quick manner. Below we will explain to you how to diagnosis and fix an over heating carburetor. 

What Causes Overheating Carburetors?


Most (if not all) motorcycle carburetors have an insulator block or gasket that sits between the manifold and carb flange to prevent heat from the cylinder head spreading to the carburetor body. When a carburetor is not properly insulated, the heat from the cylinder head can transfer over and cause the carburetor to over heat and ultimately "boil" the fuel. When fuel is exposed to extreme heat, it can be very difficult to ignite.

The nature of a motorcycles design also plays a role. For example, the BSA A50 & A65 where known for having over heating carburetor issues. The fault was not using the correct insulator block but the fault was also in the design. Unlike Triumph, many BSA models mounted there Amal carburetors directly onto the intake manifold allowing heat to dissipate directly to the carburetor.  


What Are The Symptoms?


Symptoms of a carburetor overheating can be tricky to diagnose. Below you will find the most common symptoms.


Hard to start after the engine is warm

Engine starts to surge at operating temperature

Amal carburetors are too hot to touch

Bike has a hard time keeping a consistent idle

Throttle response suffers at operating temperature


How To Fix An Overheating Carb


We have found that most overheating carburetor problems on a vintage Triumph motorcycle or BSA motorcycle is usually fixed by fitting an insulator block. There are many sizes, thicknesses, and materials of an insulator block. Materials range from phenolic composite, rubbers, paper, and wood. Each material has its pros and cons. For British bikes, I prefer to us a phenolic spacer due to the material. Using a paper insulator block or wood is more prone to deteriorate from fuel. Phenolic composite insulator blocks are a sturdy material that will not warp your Amal carburetor flange.    


When choosing an insulator block, you may have to experiment with thicker material blocks. Sometimes heat can still be transferred from your cylinder head if the heat is excessive.This is where having a variety of spacers will come in handy.



  • Andy Ryan

    Is there a spray that can cool the carburettor

  • Classic British Spares

    @Tim – Yes, so do we. Cheers

  • Darren

    Burlen sell the phenolic spaces for Amal carbs. Cheers.

  • Classic British Spares

    @Tim You can use it on either carb. See the link below. Thanks for reading today!

  • tim

    Where do you get the block that you used.
    I have 2 A65 Ts one 67and on 69 road bike.
    Does it differ in the heat depending on which carb you use. Cosentric or Monoblock ?
    Thanks for the Info.
    Tim G.

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