i have an 82 F-150 5.0 with a variable venturi carb. My problem is it wont pass smog, its getting way too much fuel.Does anybody know how to tune this type of carb?I've never come across this kind before.

 

The VV carb uses a stepper motor to adjust the A/F ratio under control of the EEC (II or III). It is screwed into the side of the carburetor, and you can remove it to make sure the little pintle is moving freely. It depends heavily on the EGO sensor to do the adjustment correctly, so first make sure the EGO sensor is functioning properly.

I saw this done a long time ago, so I can't remember exactly, but somewhere on top of the carb is a hex socket head that can make coarse adjustments of the A/F ratio. But I don't remember which way made it lean or rich.

 

thanks for the reply,but what is the EGO sensor and where is it located?

 

Sorry, that's the Exhaust Gas Oxygen sensor, usually screwed into a boss on the exhaust manifold. The EEC uses it to determine if the exhaust stream is rich or lean, and adjusts the stepper motor in the carburetor accordingly to compensate. It should be replaced every 50k miles or so in order to allow the EEC to work properly. It's probably around $30, so it's pretty cheap to replace. However, this does not mean there isn't something else going on in your engine causing it to run rich.

Also, just to be complete, check the spark plugs to see what condition they're in.

 

thanks i'll check those things out, though i did just recently install new cap and rotor, plugs and wires.

 

checked plugs and actually they look good replaced EGO and still wont pass smog.Its a test only smog station requirement so tech cant tell me why it wont pass other than its getting too much fuel.Any other suggestions? Carb swap maybe?

 

The other things I would try is other basic tune-up procedures. Make sure the base timing is correct, there are no vacuum leaks, and the other emission controls are working. That engine uses a couple of solenoids to control the thermactor air injection system, which helps the catalytic converters oxidize unburned fuel. Check to make sure these things are working; there is a pump driven by the engine that feeds a valve that dumps the air either at the exhaust manifold or into the converters, all controlled by vacuum, which are controlled by the two solenoids. It's hard to check these yourself without instruments. But still, look to see that there aren't anything obvious, like hoses hanging loose.

Those VV carbs can be troublesome, and a lot of people do swap them out for something simpler. If your locale does smog checks, will they let you use a different carburetor?

 

sounds like i really have my work cut out for me.I'll keep posting my findings cause i really appreciate your help,thanks.