Zenith Carburetors

[Doc Holliday]

In followup to the GPU 220 Spark Plug topic, I have determined that my plugs are "fuel fouling", and not "oil fouling". This was based on the fact that my plugs were black (but not really oily) and that there is sometimes (usually) a presence of black smoke during idle.

I am assuming that "fuel fouling" is caused by too rich a mixture, and that I could resolve this issue by "leaning out" the carburetor. Our current fuel configuration is as follows (in order of fuel flow):

25 Gal Aluminum tank with bottom fuel valve
Fuel/Water Separator filter
Electrical Fuel Pump
Standard 220 GPU Mechanical fuel pump
Clear inline fuel filter
Standard 220 GPU Zenith carburetor.

Now my question: On the Zenith carb, there is a mixture screw near the intake manifold side which I have heard described as an "idle mixture" screw. Our local airboat guru set this screw approx. 3 1/2 turns out. Is this the screw that I would use to adjust the mixture (even when I'm not idling) and hopefully resolve my fuel fouling issue? I also assume that I turn the screw "down" to lean the mixture.

I have heard many horror stories about running too lean (increases head temp thus shortening engine life). We do not have thermocouples installed on our heads to provide a real-time reading of engine temp (does anyone do this on their airboat?). Do I have to rely on the plug color to know if I'm running too lean or not?

Thanks in advance for any suggestions........

 

[Anonymous]

The best way to determine which way too adjust your fuel system is to read your spark plugs. Once your jetting is close you need to put in a new set of plugs to read your jetting accurately. If you are anywhere near the ball park here is what you read. The very outer metal ring (the part that the ground strap connects too) shows how rich the idle circuit is. If it’s black your idle mixture is too rich at idle. Remember your idle circuit only functions when you are idling it does not affect the motor at WOT (wide open throttle). The white porcelain part shows your fuel mixture at WOT if it’s gray and your motor is an aircraft it’s lean and if it’s too black it’s rich. Your ground strap (the part that reaches over the electrode) also indicates your fuel mixture at WOT. If your ground strap is black your rich if it’s green and gold your lean if it’s really green your approaching Chernobyl leanness. Aircrafts are a little tricky because their spark is so weak at idle due to running a magneto. Water-cooled motors A.K.A. car motors run leaner because they don’t use excess fuel to cool the engines. When running a water cooled motor you like a nice goldish green coloration. If you’re running nitrous it’s a completely different ball game. I hope this helps.