How an IMPCO Model 425 Mixer Works


Propane gas from the converter flows through the main low-pressure fuel line to a valve inside the carb.  This valve is your lean adjustment.  There is a bolt on the back of the carb to adjust it.  It is simply a door that opens and closes based on how far in and out you turn the bolt.  This leads to the main valved port that goes up into the center of the air intake.  Hanging from the top inside of the carb is a diaphragm connected to a steel plunger that goes down into the propane port.  As vacuum increases, this diaphragm opens further and pulls the plunger off of the propane port letting more fuel in.  The shape of the plunger is the main determining characteristic to affect performance of the carb.  IMPCO did a lot of testing during the design phase of the carb to get max power and fuel economy out of this plunger.  Idle speed is adjusted on the side of the carb.  The adjustment screw is literally labeled with an arrow cast into the cover.  As this screw is turned in, it acts on a lever that pushes against yet another diaphragm.  As this diaphragm is opened further, more unregulated propane is dispersed into the engine at idle.  A rebuild kit for these carbs consist of the two diaphragms mentioned above and costs roughly $40.  On the off chance that something goes wrong with your carb, 99.9% of the time it is going to be one of these diaphragms since they are the only moving parts in the entire carb!