So I bought a 1975 Ford e250 camper van with a blown 351W (thrust berring gone). Got a rebuilt motor from ATK cause it has a 3 year warranty. While the motor and trans were out I rebuilt the c6 with new torque converter. I installed all of the stock components on the new motor except for I put a mild edelbrock performer cam in it. At idle there is a slight vibration you can feel throughout the van. Once you get up to freeway speed it feels as if the wheels are going to fall off. Since then trying to fix the problem of have replaced: all 4 tires, had the driveshaft balanced front and rear brakes, wheel berrings.....then once I figured out it still did it in park...intake (edelbrock performer) carburetor ( summit 625? CFM), fuel pump, motor mounts(this helped a little), flex plate, crank pulley, fan, installed msd box, new plugs, cap and rotor, plug wires, harmonic balancer (twice the second one a fluidamper) trans mount, and the vibration is still there. It's not as bad as before but still uncomfortable to drive on the freeway. To explain the vibration it feels like 4 pulses that are dependent on engine speed the higher the RPM the closer harder the pulses are. It's a 28oz imbalance and I odered the flexplate and fluid damper from summit racing. The motor has around 400 miles on it driving to the lake and back but I fear taking it on any longer trips. My next step was going to be pulling the Valve covers and inspecting the valve train. Anybody got some ideas???

 

No one single idea?

 

The symptoms sound like a balance problem. Do you know if the engine was balanced by the rebuilder? I've heard horror stories of mass production type rebuilders, so I would tend to avoid them.

 

I called and asked if they balanced the motors or at the least just weighed the pistons and rods and got it close. The answer I received was "we should". My guess is they've got a stack of pistons/rods and just slap it together. I'm almost to tha the point of pulling the motor having changed just about everything else. I did pull the valve covers off tonight and didn't see anything out of the ordinary as well.

 

Run the engine with the valve covers off and examine the actions of all the rockers. You could have wiped a cam lobe, this can cause what you're describing. Look for a rocker that's not depressing a valve as much as the rest. As for the internals, you have to get them really out of whack before you'd notice an imbalance. They do though assemble them from parts out of many motors, they're all reconditioned though. I'd be more concerned with the torque values, than the mix of parts. I did a build a few years ago around a roller 5.0 that was a reman like yours. The rod bolts were overtorqued but everthing else was fine. I replaced those bolts with ARP's then ran the snot out of it for 3 years until it cracked a cylinder wall. Those rods ? I sold em to another guy to use in his build.

 

I did pull one of the valve covers and ran it mostly looking for a broken spring. I ran it for about a min until oil started hitting the exhaust manifold. At least on that side everthing looks to be in order. On a compression test everthing was good and within 5 lbs of each other. The motor only has about 400 miles on it. And it has had the vibration since mile one. I followed the break in procedure for the cam exactly as it was written I did mineral oil and put the bottle of zinc in the edelbrock recommended.

 

Well the breakin problems weren't all caused by the lack of zinc in the oil. Most were caused by poorly machined lifters. You can follow all the steps everyone tells you too, that will not prevent a failure due to one or more bad lifters. Run it again and watch the pushrods. If any one is not rotating, you've got a bad lifter that's also sitting on a ruined cam lobe. (from the bad lifter) . I had two cam failures in the last flat tappet motors I built, in both, there was a single lifter failure, both failed to rotate at startup, both killed the cam lobe inside the 20 minute breakin period. As for a broken spring, that's not going to be your problem, that would result in a dropped valve after running it that long, as there'd be no spring tension to prevent the valve locks from letting go.

 

I'll pull the other VC tomorrow and check that side. The side I had the VC off of all pushrods were spinning. The edelbrock cam I put in before installing the motor came with new lifters as well. I still have the cam and lifters that came with the motor in a box in my garage(all numbered whee they came from and everything) I'll pull the other VC In the morning and report back.

 

Maybe this is a good time to check the plugs again to see if any cylinder is weak or dead.

 

Since you mentioned the new torque converter I thought I'd mention and old 454 I had that I couldn't track down a vibration that was directly related to engine rpm. It was an $18 flywheel missing it's weight.

 

Yeah, I replaced the flex plate and balancer already. I also pulled the other valve cover and checked all the pushrods are spinning and rockers are rocking evenly. I pulled and made sure all the push rods were the same length and not bent. Once I get another day of sunshine I'll hit the manifolds with a temp gun and see it they're running the same temp. I did a compression test shortly after putting in the motor and dry they were all 150-152 lbs. I've swapped the plugs a few times up and down in heat range hoping that would help. Think it could be a distributor problem? As that and the exhaust manifolds are the only original things left on the motor.

 

I thought all of ATK's motors were 50 oz. imbalance motors. If you do really have a 50 oz. motor and 28 oz. flexplate and damper, that could easily be your problem.

 

As a follow-up I went to ATK's website. All of their 302 based engines are 50 oz except the 331 and 347 strokers. The 331 & 347 strokers, and their 351 are all 28 oz. motors, so you probably do have a 28 oz. motor as you thought if you have a 351.

 

So I finally had the time and pulled the motor. Tore it down, took the pistons, crank, balancer and flex plate to the machine shop for a proper balance. While that’s in the shop and the engine out would it be a bad idea to send the torque converter out for a balance since it is “new” as well?

 

You don't need to balance the converter, the oil filling it does that.