My carburetor was just rebuilt, so why doesn't my Mustang pass emissions testing?

Dear Car Talk

Dear Car Talk | Oct 01, 1992

Dear Tom and Ray:

I have an '83 Mustang V6, 3.8 liter with an automatic transmission. It has 106,000 miles on it. After a tune up, I was told I had a fuel leak and needed to have the carburetor rebuilt or replaced. So I had it rebuilt last January. Before this, I averaged 15 miles per gallon. Afterwards, the mileage did not improve. After a required emissions test, the carbon monoxide level at idle was 6.29, instead of the allowable 1.20. At 2,500 RPM it was 8.12. I took it back to the person who did the carburetor rebuild. He could only get it down to 1.25. And now my mileage is worse, and the car is hard to start and hesitates or stalls when leaving stop signs. Any suggestions?
P.S. Please don't use my full name. I don't want the Deparment of Motor Vehicles on my trail!

TOM: Don't worry, Sandy, your secret is safe with us. By the way, are you related to the Krasners of Springfield?

RAY: What your mechanic is doing now is doctoring up the X-rays. He's probably using every trick in the book to get your car to pass the emissions test. He's retarding the timing, lowering the idle speed, and doing the ancient carbon monoxide reduction incantation.

TOM: But not only does that hurt the car's performance, it also completely ignores what's really wrong with your car.

RAY: And what, pray tell, is that, brother dear?

TOM: I have no idea! I thought you knew.

RAY: Well, I do have a couple of ideas. The mechanic could have done a lousy job rebuilding the carburetor last January. It could be a bad catalytic converter at 106,000 miles. It could be low compression, an ignition problem, or a hung up EGR valve.

TOM: What I'd do is take it to another mechanic and have him start from scratch. After checking out the car, he may tell you that you need another carburetor rebuild. And he may be right.

RAY: But before you do it, we have two suggestions. First, buy a new one, or one that's been rebuilt at the factory. Rebuilding carburetors is a tricky business, and it's very easy to screw up. Ask my brother.

TOM: Second, make a deal with him before you agree to do the work. Tell him that if the new carburetor doesn't fix the emissions problem, you want him to take it back, and give you your money back.

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