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My 1980 Chrysler LeBaron (V8 318ci) just failed an emissions test. I'm coming from a state without emissions testing to a state that requires it.

Specifically, the only part that failed was the hydrocarbon count. The legal limit was 300 PPM and the reading was 454 PPM. I passed the carbon monoxide test with a reading of 2.47% and a limit of 3%. Carbon dioxide was at 11.14%.

Some cursory reading seems to indicate that hydrocarbons are basically unburned fuel. So if there is too much of it coming through the tailpipe (where they measured), could something be wrong with the return line? Other reading suggested the catalytic converter might be at fault. When we replaced the exhaust pipe ourselves a year back, a visual inspection seemed to indicate that the grate in the catalytic converter was intact, not broken or rattling.

Given that I exceeded the hydrocarbon limit by 150%, it seems the fixes needed would be substantial. What could be causing such a high PPM of hydrocarbons given the information above, and what sorts of fixes would correspondingly be needed? Thanks in advance.

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The carburetors probably just need their idle mixture adjusting to make the engine run a little leaner. If adjustment doesn't fix it, the carbs may need refurbishing.

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Most carburetor fuel systems do not have a return fuel line. If they do it is usually at the fuel pump and would not impact your air/fuel mixture in any way. However your thinking is correct when applied to fuel injected systems, where a blockage in the return line would raise fuel pressure, thus enriching the mixture.

As HandyHowie suggested, some adjustment can be made to the carburetor to lean out the mixture. Generally carburetors have idle and part throttle mixture adjustments. Past this, mixtures are usually changed with jets, needles, or a combination of both. You should not need to do this unless your new home is at a significantly different altitude. I would start a new question with your specific carburetor make/model mentioned if you require more information on this.

Catalytic converters can also lose their effectiveness. I have worked on a few vehicles where everything was in working order (as far as mixtures, ignition, etc), but they just couldn't pass until a new converter was installed. http://www.ecooptimized.com/index.php/gasoline/80-three-way-catalysts-aging-causes-of-failure-and-deactivation-.html

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