I have a 65 Dart 2 door hardtop, 225 slant 6, there is a bad shaking and shuddering every time I take off. It starts around 20 mph and lasts untill I hit about 26 or 27 mph. I took it to a shop and they checked the drive shaft (OK), U joints (OK), tires are new and were installed and balanced 2 months ago, front end was checked (OK), wheel bearing are OK. They couldn't spot any problems and suggested that the torque converter may be off balance. I think maybe the engine mounts are worn. They look like they are original. Maybe there is a flat spot in the carb or distributor on aceleration. I've owned the car about a year and this problem started about a month ago. Any suggestions what I can check?

My first guess would've been U-joint/driveshaft related, but I'm onside with your engine mounts idea. Replace all three of them. Get these left and right ones, and this trans mount (last mount on the page, "2-stud" item).

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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 have a similar problem at about 65 to 70 on the freeway. I get a loud vibration I think is from my engine mounts being shot? So I'm hoping after I replace that it should fix the problem. I can't really see the mount on the passenger side but the driver side is shot. any thing else you think I might look for? I just replaced all the steering linkage, upper ball joints and bushing, front and rear socks, brakes and did a tune up. So I'm thinking its got to be the mounts. Thanks for your time guys! -R-

Wait a sec, wait a sec…yours is a '65. Tell more about how that shop checked the U-joints. The front U-joint on a '65 is a ball-and-trunnion item which won't necessarily show any easily-spotted wear on a visual inspection (or a "grab the driveshaft and shake it" check) but which produces exactly this symptom you describe when it gets worn, which at this late date it almost certainly is.

Strongly recommend cross-and-roller front U-joint conversion. See here and take note that it has been reported the bolt-up flange part number has changed (or is incorrect in the linked article); it is now 2-2-349.

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

It is a long shot but I had new tire that after a few months go bad-the tread twisted in one spot and made the tire go in that direction every turn. Felt like a bad vibration. It was odd-just a small section of the tread went bad. Check you all four tires 100%. Jack it up in the air or have someone drive it slowly while you look at the treads

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50 chrysler,54 Plymouth, 64 Valiant conv 4 speed, 66 Valiant V8 wagon, 70 Challenger R/T 440+6 conv 4 speed,80 Colt, 98 Neon ACR,84 Honda V45 Magna
Taking care of 57 300C conv,48 T&C conv. Missing my 67 GTX and 36 Ply coupe

My 1968 Australian VE Valiant with slanted six was shuddering on take off. It was a sagging engine mount which put the driveline off centre. Replaced mount and now smooth as silk.