Three-nuts
Well-Known Member
Greetings, I acquired a '74 Dart Custom about a month ago. Odometer reads 116K, slant 6 225. When shifted into any gear the car body would vibrate at idle, and shudder violently through the front end & steering wheel during hard acceleration. The shuddering would abate the when the load was diminished, and at highway speeds, it was smooth. This is not a harmonic vibration issue.
Replaced the 225 last week due to failing rod bearings, first on #5 and then on others. Got the replacement motor, a 1971 with a reported 44K on it, installed and sorted out. This motor runs and idles smooth as glass, got the timing, mixture and idle speed dialled in.
It still has the vibration in gear, but not quite as badly. The shudder on acceleration, however, is the same - very bad. Out of first, it improves; unloaded or at light acceleration, it's smooth. At various highway speeds, smooth as glass.
Again, I've had harmonic balance problems in prior MoPars; one was a mismatched torque converter, and the other (on three separate cars over a 20-year period: a '65 Fury, a '74 Fury, and an '83 Ram conversion van) was due to failing U-joints, This doesn't feel anything like that.
On another forum it was opined that the transmission may have a faulty lockup converter. The slant 6 forum described the difference between a lockup & a non-lockup by describing the different spline setups: the lockup had a sheath-type spline shaft with a 3"-plus smooth shaft in the center; the non-lockup was a solid shaft, splined all the way to the end. Being a '74, it shouldn't have a lockup converter, but I did take the following photo during the motor swap:
Is it possible that this is a 1978 or newer trans?
Another issue: the kickdown link was intentionally disconnected & appears to have been that way for years. When I hooked it up, I found out why: at any speed under about 50, any pressure on the accelerator would force a downshift into 2nd. I am assuming this is an adjustment issue only...
Along with the '71 motor I got the matching '71 transmission with TC & driveshaft. I have not checked the driveshaft that is currently on the car. My plan is to swap out the driveshafts & see if there is any change. If none, and if the shafts appear to be in good shape, I plan on swapping in the 1971 904, which has a solid spline.
The rear springs are original, and likely have some sag, but the car sits level. No other obvious problems were noted. The car appears to have been sitting a while, and was neglected, but not altered or abused.
Replaced the 225 last week due to failing rod bearings, first on #5 and then on others. Got the replacement motor, a 1971 with a reported 44K on it, installed and sorted out. This motor runs and idles smooth as glass, got the timing, mixture and idle speed dialled in.
It still has the vibration in gear, but not quite as badly. The shudder on acceleration, however, is the same - very bad. Out of first, it improves; unloaded or at light acceleration, it's smooth. At various highway speeds, smooth as glass.
Again, I've had harmonic balance problems in prior MoPars; one was a mismatched torque converter, and the other (on three separate cars over a 20-year period: a '65 Fury, a '74 Fury, and an '83 Ram conversion van) was due to failing U-joints, This doesn't feel anything like that.
On another forum it was opined that the transmission may have a faulty lockup converter. The slant 6 forum described the difference between a lockup & a non-lockup by describing the different spline setups: the lockup had a sheath-type spline shaft with a 3"-plus smooth shaft in the center; the non-lockup was a solid shaft, splined all the way to the end. Being a '74, it shouldn't have a lockup converter, but I did take the following photo during the motor swap:
Is it possible that this is a 1978 or newer trans?
Another issue: the kickdown link was intentionally disconnected & appears to have been that way for years. When I hooked it up, I found out why: at any speed under about 50, any pressure on the accelerator would force a downshift into 2nd. I am assuming this is an adjustment issue only...
Along with the '71 motor I got the matching '71 transmission with TC & driveshaft. I have not checked the driveshaft that is currently on the car. My plan is to swap out the driveshafts & see if there is any change. If none, and if the shafts appear to be in good shape, I plan on swapping in the 1971 904, which has a solid spline.
The rear springs are original, and likely have some sag, but the car sits level. No other obvious problems were noted. The car appears to have been sitting a while, and was neglected, but not altered or abused.
But this above makes sense. It feels like manual clutch chatter to me. I'm sure I can figure out the linkage adjustment if it's needed on the other trans. 
