need help with an engine vibration at 1500-1800rpm.Not ignition miss,motor mounts or exhaust to body or frame contact.Installed new harmonic balancer,same thing.The engine is original with 115 k miles,1974 plymouth duster 318.Would appreciate any help.Paul  

Is this vibration occuring when you are driving the vehicle at 15-1800 rpm? Or when you rev it in neutral?
If it is happening when you are driving then the problem is not likely in the engine but the driveline.
Wheel balance, bent rim etc.
If it is in the engine, what has changed other than a new harmonic balancer, and double check to make sure this is the right balancer for your engine?
The balancer needs to be for the correct yr. of engine and the correct motor. e.g. you cannot install a 340/360 balancer since both in 74 had an external balance crank, and therefore a different balancer. The same goes for the torque converter it needs to be a 74 318 unit in case you have changed this.
If you determine that all is well with your new balancer & the converter then make sure the engine cyl. compression is o.k. A weak cylinder can cause an engine vibration/missfire.  

Thanks for the response.The vibrationis while sitting in park or neutral.elim. driveline.There is no ignition misfireand the engine and trans have not been touched internally.The balancer ordered was for 1974 318 from Jegs and looked identical to the one I took off,which also appeared ok,no rubber cracks or spinning slippage.I am stumped.  

Inspect the dist. cap, a tracer crack is hard to see but it will cause the problem you are describing by firing 2 cylinders at once. Also check the tower of the coil for a hair line crack or oil coming from the coil tower. I would also check the plugs, compression and wires individually looking for something out of the usual. Make sure your compression readings are within 10% of each other.
Make sure your plugs appear clean and are not oil soked, If you find a wet plug tip then zero in on that cylinder until you solve the problem.  

thanks for the input.the cap and rotor,are new.It is not an ignition miss.Its the same sensation if you had a harmonic balancer falling apart but it is correct,for 318 eng. and is new.appreciate any help.Paul  

Last one of these issues like yours I found a broken valve spring.  We checked everything including a compression check, then suspected a valve spring, they are hard to find because the spring will break at the bottom and it looks ok until you look real close with a bright light as it breaks at the weakest point where is goes from flat to round, and drops down on top of the broke off piece.  I have found this on several cars over the years with symptoms like yours, hope this helps, Mike  

My second instinct is you have a new balancer that is bad. I would try another balancer possibly from mancini racing rather than Jegs, or Summit. If it is a bad balancer you can certainly return it once you have determined it to be the problem.
A long shot could be a cracked flex plate that bolts to the engine to the converter, like I said this is a long shot, and happens more in drag racing type situations.  

If you order another balancer, make sure you get the engine # off the side of the block so you are sure you are ordering the right part.  

thanks guys ,I will keep you posted  

If the valve spring broke it should feel like a misfire.the sessation is like refacing a crank or flywheel and not rebalancing.thanks for the input.Paul  

I have seen a few cracked flywheels or flexplates but in every case there has been a noise problem.How about using 1/4 oz. stick on wheel weights at a given location on the balancer then move around locations ,add or subtract weight to see if any change.this would tell me location of vibration such as balancer.I appreciate the help.I read another post with a guy with a 73 318 dart with the identical problem ,even down to the same rpm range.havent seen any additional followup posting,thanks again.