1975 Dart swinger origional 318. I just purchased the car and at @ 1800 rpm's either in park or while driving in gear it has a vibration. The vibration is about a 5 on a imaginative scale of 1-10, 10 being the worst. The converter DOES NOT have weights. The balancer appears to have holes drilled for balancing?? The engine was built prior to my buying the car and supposidly it has a mild cam, but in my 20+ yrs experience it appears stock. I have removed both drive belts and vibration was still there. Rubber does not appear to have slipped in the harmonic balancer. Any thoughts?? Much appreciation in advance.  

Look and see if exhaust is laying up against something like the body of the car leaf spring ect. Can you tell where [front, back, side to side ]where it's coming from?
If it was rebuilt maybe they didn't rebalance the motor.  

The vibration is not exhaust or engine mounts. I have made sure to check for good isolation of anything like that. The vibration shakes the entire car some but it mainly felt in the steering wheel and pedals. If you run the rpm's up at the carb the engine appears to vibrate side to side. The more torque the more the vibration. In park it is very noticable but not quite as much as while under load by either power breaking or while driving. It comes in right at 1600-1800 rpm's and when the rpm's get out of that range it will settle down, however if you continue torque the vibration stays constant (ie manual 1st I can make it vibrate at a rpm range up to 4K steady).  It just appears to be either a bolt on item like torque convertor or possibly harmonic balancer. Previous owner stated both were changed out at time of rebuild.
If I had to describe it as feeling like anything it would be just like pulling off a plug wire and make it a 7 cylinder.    

have you check and made sure all the plug wires are routed correctly? no matter how simple this seems and how many times you do it sometimes it happens as did on my brothers 67 coronet. his was a little rough started messing with the timing and happen to notice that he had two wires switched. fixed his problem and its a quick and easy check.  

did you confirm that engine is 318 not 360  

I will go run the firing order for sure. I have not checked that since buying the car. Thanks for a simple suggestion.
UPDATE:: checked firing order real quick and it is correct.    

I definetly did make sure that by the numbers on the block it is a 318, appears to be the origional engine according to how I have managed to trace down numbers from the car so far. Build date on motor is July 1974 and it is a 318, this is from the block.  

So you are not sure if the engine has been rebuilt? Like mentioned, if it was it may not have been balanced.

Also check to see if the damper is not for a 360. There should be a part number or you can check the back of the damper. It should be soild all the way around if its for a 318. the 360's had a notch cut out on the back side.  

If it had been rebuilt, which it appears to have been at the very least apart and resealed, in my understanding the crankshaft would have been balanced from the factory, and assuming it is the origional crank would it have needed balanced?? I guess my thought is and speaking from my background of knowledge if the engine has its origional crank and was simply polished, or possibly turned down I doubt that I would have had IT balanced. But not knowing what has been done it the real issue    . I am chasing a man made problem that will not be easily found I doubt. I am prepared to pull it before whatever is doing the vibrating causes internal parts to become external    
I will check the balancer closer, but it did not appear to have any flat spots, was the same thickness all the way around but with some holes which appear to be machined holes drilled into the face. Thanks for the replies.  

Damper part # is a 2951673 and is definetly NOT a replacement. It has some of the old factory blue paint on the back side of it. The vibration with all pulleys off was still present running with only the damper installed. Unless I am wrong this balancer looks to be for an older 318. I believe the one I need from researching that in fact my block is a 1974 model would be a 4095633 ??? If I am doing my homework correctly I have learned that I have a offset bolt pattern damper which would explain why only 5 bolts were installed on the crank pulley part # 37691-41 and the 6th bolt hole does not line up.??  

Hi,I am having the identical problem on a 74 Duster that I just purchased,I changed motor mounts and harmonic balancer,still same problem.At that certain rpm,about 1500-1800 the dash,doors and the entire interior shakes then goes away outside the rpm window.I was going to stick on 1/4 oz. wheel weights on the balancer one at a time at certain places to look for any improvement to eliminate poss.crankshaft endplay or balance problems.The engine is original with 115k miles,never torn into.I will let you know once I find the problem,and please do the same.thanks alot,Paul  

Is the trans mount good? I have seen them cause this type of vibration.