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Classic/Antique Car Repair/Carb Backfires
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Question
1975 Dodge Dart Sport with 318 V-8. It backfires through the carburetor. I've heard 3 possible scenarios for the cause and would like an expert's input.
1. Mixture too lean (or was it rich?).
2. Mistimed.
3. Bad intake valve.
I have a guy locally that is willing to look at it but he's the one who mentioned the valve problem and estimated $1500 for repair. If you could provide a short dissertation on why a car backfires through the carburetor, I'd greatly appreciate it!
There are two more common cause that you have not listed, a crack in the distributor cap. A rich mixture will not cause back fire up through the carburetor but lean will. A lean mixture causes a very high temperature in the engine and the cylinder will develop hot spots. These hot spots will ignite the incoming air fuel mixture when the intake valve is open and boom. On old cars this was also the cause of radiator overheating and can on older non catalytic equipped cars be spotted by a bone white ash in the tail pipe. The timing will usually not cause popping through the carburetor but a more common cause is the spark plug wires mixed up and out of sequence. The burned intake valve can be detected with a simple engine compression test. Low compression on one cylinder and you have your problem. The second one not on your list is a bent push rod or worn cam lobe on an exhaust valve. This prevents built up compression and exhaust from getting out of a cylinder and the next time the intake valve on that cylinder opens, pow.
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| Rating(1-10) | Knowledgeability = 10 | Clarity of Response = 10 | Politeness = 10 |
| Comment | Thank you very much, Brad! I "fiddle" with cars a little bit-but if I don't know what I'm doing I take it to a mechanic-so I totally understand everything you discussed. Now I have confidence that the mechanic I take it to will not be able to "take me for a ride" by saying it's one thing without the evidence why. Thanks again and have a wonderful day! | ||
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Answers by Expert:
Brad Sears
Expertise
All automotive including antique and collectible. However if the car has been modified I can only answer in general terms and maybe get you pointed in the right direction.
Experience
Automotive tech instructor.
Syndicated auto columnist 1970's though the early 1990's.
Syndicated auto radio talk show, Ask Brad About cars, CBS Radio 70's through 90's
TV Show "Last Chance Garage" 1980's PBS-TV syndicated.
Auto instructor for the following companies:
Fram
Autolyte
Holly
Carter
AMF
Ford Motor
University Of Conn
Blue Hills Technical School
Sugar River Technical Center
Grew up in a family garage in Needham Mass and turned wrenches from the age of 14.
Publications
Manchester Union Leader, Nashua Telegraph, Motor Service Magazine, Yankee Magazine, Popular Mechanics (Saturday Mechanic early 80's), Los Angeles Times, New York Times, and lots more.
Education/Credentials
More than I care to remember.
Basically Franklin Technical Institute in Boston, Northeastern University, Fitchburg State Teachers College, Tufts University, and a lot of factory schools along the way.
Awards and Honors
Moto Award winner. And much more.
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