Test Procedure Blazer 90 Carburetor with Thermostatic Carburetor Plunger
This is the basic Mikuni Carburetor with the addition of the Thermostatic Carburetor Plunger that provides automatic enriching of the fuel mixture at start up without the use of a choke cable.
All Starting circuits in this carburetor are basically the same except for operation of the Thermostatic Carburetor Plunger.
As this a thermostatic unit and not a magnetic device you will not see any immediate results when testing with a 12-volt battery. This device works on heat generated by the internal thermal resistor or better known as a heater element. The heater element warms up an internal thermocouple that changes length at a determined rate as it is heated up.
At start up with the engine cold the plunger and needle assembly is retracted into the thermostatic unit by the internal thermocouple (because it is cold from the outside air temperature) opening up the passageways in the bypass starter jet circuit of the carburetor. This allows for a rich fuel mixture at cold start-ups.
The length of the plunger measured from the plastic housing by the sealing o-ring to the end of the plunger of a cold unit should be approximately 8mm to 8.5mm. This allows proper clearance between the seat of the starter by pass needle jet and porting of the bypass starter jet system.
The Thermostatic Carburetor Plunger is supplied current from the magneto lighting coil and only receives current when the engine is running. It does not get its source of power from the battery. As the engine runs and supplies current to the Thermostatic Carburetor Plunger from the lighting coil of the magneto, the Thermo resistor or heater element warms up. This causes the thermocouple to expand causing the plunger and needle assembly to gradually move out of the Thermostatic Carburetor assembly. This causes the plunger and needle assembly to move down into the bypass needle jet assembly shutting down the by pass starter jet system.
To test the Thermostatic Carburetor assembly you must hook it up to a 12-volt battery and patiently observe the plunger and needle assembly. You will feel the Carburetor Plunger assembly start to warm up as the thermo resistor or heater element starts to heat up. As the thermocouple starts to activate from the heat you will notice the plunger and needle assembly gradually extends or move out of the Thermostatic Carburetor Assembly.
Total length of the plunger and needle assembly measured from the plastic surface by to Ð¾ ring to the end of the plunger should be 14mm to 14.5mm when plugged in to the 12 volt battery for approximately 8 to 9 minutes.
A resistance test is not reliable in this case because it changes so much due to temperature changes and the thermocouple itself is a mechanical device.
During start up as theengine is cold and the Thermostatic Carburetor Plunger is not receiving any current from the lighting coil in the magneto. The bypass starter circuit in the carburetor is automatically activated. After startup current from the lighting coil generated by the flywheel in the magneto warms up the heater element in the Thermostatic Carburetor Assembly causing the thermocouple to close the plunger of the bypass starter jet leaning out the mixture for proper fuel mixture at operating temperature.