Q: '72 Mach 1 with 351C. Runs, idles, revs in park. Sputters, chugs and coughs in Drive. Any suggestions?

asked by on May 05, 2017

Engine idles and revs in Park with no problem. When under a load (in Drive) in sputters, coughs, chugs and backfires. Have rebuilt the carb and checked for vacuum leaks. Any suggestions to trouble shoot?

My car has 36000 miles.
My car has an automatic transmission.

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Check for proper advance of ignition timing. In particular, verify that advance curve you observe matches factory spec. Measure fuel pump output/pressure against factory spec. If engine compression is good and balanced, apart from timing check and fuel pressure check, you have to diagnose the fuel (carburetor) and ignition systems and the fastest way to do that is put the engine on an automotive scope to look at secondary firing patterns in all cylinders. Even though you have rebuilt the carburetor, faults in the carburetor are still a very plausible cause. I remember as a teenager rebuilding a carburetor (66 Falcon) and the engine then ran just as you have described. I took it to a shop, they put it on a scope and told me to buy a new carburetor. Problem solved. If you desire a diagnostic, the service to request is poor operation/sputtering diagnostic. If you request that service, the responding certified mechanic will get this taken care of for you promptly. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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