If you've installed a harmonic balancer and/or a flexplate/flywheel designed for a later model engine such as the 1982 and up 5.0/302 HO, there is a chance that this is your problem.
The balance factor, (Referring to the crankshaft counterweight), for all Ford V8 engines is 28 ounces with the exception of the engine listed above which has a balance factor of 50 oz. The parts will interchange, but the counterweights are different. The difference can be checked by comparison, as obviously, the counterweight is larger on the 50 oz. example.
All factory 50 oz. balancers will have 4 bolt holes for the crankshaft pulley, the factory 289 balancer will have 3 bolt holes. (There are aftermarket 50 oz., 3 bolt hole balancers available, FYI, but never factory issued). If you have a C4 automatic transmission, the only way to get a 157 tooth flexplate with the 50 oz. balance factor is through an aftermarket source, again the factory only issued the 28 oz type. The 157 tooth is the only size flexplate that will work on the Mustang with the C4. There are no factory C4 flexplates that are 50 oz. they are all 28 oz. There are factory 4 bolt balancers with the correct 28 oz. factor, but they were installed on 302's starting in 1970. The 289 ceased production in 1968.
The balance factor, (Referring to the crankshaft counterweight), for all Ford V8 engines is 28 ounces with the exception of the engine listed above which has a balance factor of 50 oz. The parts will interchange, but the counterweights are different. The difference can be checked by comparison, as obviously, the counterweight is larger on the 50 oz. example.
All factory 50 oz. balancers will have 4 bolt holes for the crankshaft pulley, the factory 289 balancer will have 3 bolt holes. (There are aftermarket 50 oz., 3 bolt hole balancers available, FYI, but never factory issued). If you have a C4 automatic transmission, the only way to get a 157 tooth flexplate with the 50 oz. balance factor is through an aftermarket source, again the factory only issued the 28 oz type. The 157 tooth is the only size flexplate that will work on the Mustang with the C4. There are no factory C4 flexplates that are 50 oz. they are all 28 oz. There are factory 4 bolt balancers with the correct 28 oz. factor, but they were installed on 302's starting in 1970. The 289 ceased production in 1968.
