Tech: Fixing Overheating Carburetors
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.