Was recently having problems with my 64 Dart (225), but changed the points, rebuilt the carb, and fixed a vacuum leak. All was running relatively smoothly (apart from a slightly rough idle caused by a bad 6th cylinder. Soon after, while driving on the highway, when I hit 50 MPH, the car started making a deep bass rumble, kinda' like when I was a kid and put your face in front of a fan and started talking like Darth Vader. The faster I went past 50, the louder and deeper and shakier the vibrations got. When I took my foot off the gas, the vibration went away (so it wasn't anything in the wheels), but as soon as I put it back on the gas, it came back. I eventually slowed all the way to 30, then slowly accelerated back up, and it was gone. Happened just like that again a few days later, did the same thing, and it was gone again. Now, however, the last 3 times I've gotten it up to 50, it's done it with regularity, to the point where I'm having to drive 50 or slower on the highway, thus matching my speed with what people think of as my granny car (I suppose the Frost Blue coloring has something to do with that). Anyone ever had this problem or have ideas what could cause it?

Many thanks,
Jeremy

Could be a loose/sloppy front or rear universal joint. Could also be soft engine and/or trans mount(s), or improper rear axle pinion angle caused by sagging springs.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Dan,

Could those still be viable causes even though the vibrations instantly and completely disappear as soon as I let off the gas and reappear as soon as I reapply the accelerator?

Thanks,
Jeremy

I'm not Dan, but the answer is yes.

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"When you find a big kettle of crazy, it's best not to stir it." - Pointy-haired Boss

1964 Valiant V200, 225/Pushbutton 904
BBD, CAI, HEI, LBP, AC, AM/FM/USB, EIEIO

I am Dan, and the answer is what slantvaliant said!

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I would take a look at the front B&T joint. A quick test: remove the four nuts, that hold the joint to the trans flange. Place 2 flat washes on each stud, between the two flanges, and reassemble. Then road test, to see if there is an improvement.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

hello jeromy! i feel your pain i have a 68 d100 adventurer with the same situation i would call it a harmonic vibration around 55 mph my conclusion on the problem is in the engine, cause just about everything else has been replaced,whl bearings, hubs /drums, axles, shocks, rear gear, driveshaft, trans, torque conv/ flex plate,rims/ tires, just to name a few things and it still has the vibration. it broke 3 or 4 flex plates so i replaced the sb 727with a sl6 727 and got rid of the spacer plate. it quit breaking the flex plate but still had the vib. it eventually vibrated the torque converter bolts out enough to smack the block and broke it so if it was the in the engine i will know when i swap it out in the next week or so.i will let you know the outcome and maybe you wont have to spend 3000.00 and two years of headaches, if there is a bright side to this i will have a brand new 68 adventure when i replace the engine.

I seem to remember my Dart doing something similar to this shortly before I had the transmission rebuilt.

Your transmission mount could have went south on you. 30 years of operation tends to turn the rubber into dust.

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2