Fuel Injection Mixture - or Why are My Plugs Fouling?

In the article Vacuum Leaks, Smog Test, and Blazer`s Automotive I had a shop find my vacuum leaks and get the Guacamole Bus ready to pass the Colorado emissions test.

They found leaks in my S-boot (I knew there were some there) and my intake manifold gaskets (that was a surprise). They slopped more goop on the S-boot and cranked down the nuts on the mainfolds. They also made some adjustments, but never really specified what those adjustments were. But the bus passed the smog check.

Then I went through and replaced the intake manifold gaskets and the S-boot, as described in Replacing the Intake Manifold Gaskets. Everything is tight as a drum.

Too bad my mixture was WAAAAY off.

My plugs kept fouling, and I was getting like 12mpg. Ugh. Thinking the shop had only messed with timing and the idle mixture screw on the Air Flow Meter, I started going batty trying to figure out why the bus was running so rich.

I pulled the cold start valve out and pressurized the fuel line. No seeping from the valve. It couldn`t be a leaky injector since that would foul only one plug.

I pulled the cover off the Air Flow Meter (AFM) again. The glue on the fine adjustment screw for the wiper is still present, so they must have adjusted the big black wheel. That wheel controls spring tension on the wiper. Essentially, it controls the amount of fuel delivered for a given volume of air. A tighter spring gives less fuel, looser spring gives more.

I initially moved the wheel 5 notches, but still had fouling plugs and poor running. It took me a few days to realize they must have gone more than 5 notches!

I ended up with a total of 12 notches! In the pic below, the white dot is where the arm was pointing when I started. And you can see where the arm (silver bit of wire) is pointing now.

Without a gas analyzer I can`t get it perfect. But I got it close. Here`s how:

  • Remove the cover from the AFM

  • Mark which notch the arm is in

  • Start the bus

  • Rev the engine - Watch for smoke from the tailpipe

  • Move the black wheel one notch at a time, revving the engine after each change

  • When you find a notch that has no smoke and no fuel smell, you`re there!

As I said, you can`t get perfect with this method, but it`ll get you in the ballpark until you can get it to a shop with a sniffer.

While I`m at it, I`ll describe how I set the idle mixture. The screw is at the top of the AFM, often covered with a silver cap. Remove the cap if it`s there. Using either a screwdriver (early busses) or an allen wrench (late busses) adjust the mixture until when a small vacuum line is pulled off the idle rises about 50 rpms. This gives a "rich best idle" which will help with light throttle response and lower head temps in traffic.

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