Harmonic vibration issue

[471_Magnum]

The curse of the yellow Plymouths continues.

Just a little background to get everybody up to speed:

Bought a 340 powered 72 Roadrunner back in July. Got it home. Had a fuel tank leak immediately. Replaced the tank.

Generally speaking, the car ran like shit. Completely stock build on the motor. Couldn't get out of it's own way. Unstable idle speed and bogged with any aggressive throttle input. Very hard starting. Basically, it was way too lean and idling past the transfer slots and consequently pulling vacuum on the advance.

After a couple weeks of tuning, I got the thermoquad to behave reasonably well and the timing settled down. Recurving the distributor perked up the performance.

Fast forward a few weeks to Mopar Nationals, the transmission goes out in route. You can read about that mess in another thread. 

Got the transmission repaired and reinstalled a couple of weeks ago. During the course of reassembly, noticed a corner of the block was broken off that held the bellhousing dowel pin. The dowel pin was held in place by JB Weld. WTF!?!? Contacted the PO who claimed other Mopar guys told him it would be okay like that.  

Lacking the funds to replace the short block, we just bolted everything back together pretty much as we found it. One change we made was to swap torque converters. Friend had a 2800 stall laying around that I figured would help perk things up some more.

So upon test driving the car, everything functioned okay, but I didn't register any performance improvement on the butt dyno. Apparently the stall on the new converter was the same or close to that of the old one.

Last week I got around to installing the TTI headers that I purchased in Columbus. The headers really help with the performance on the low end. The car can actually due a respectable burn out now. Only have taken a few short test drives though.

So Saturday I finally got the car out to enjoy it... or so I thought.

While driving to a local car show, I had the opportunity to wind it out a bit. Was doing about 60 or so and punch it. Did pretty good for a second or two, and then fell on it's face... like it ran out of gas. I let off and coasted down a bit and it eventually recovered. Along the rest of the drive, it exhibited the same behavior some more, even without getting on it hard. I have a clear fuel filter and there was no sign of fuel boiling, vapor lock or blockage. Only thing I could figure the problem to be was an internal clog on the carb, a stuck float, or a fuel pump going bad. Seeing as I have run a couple of tanks of gas through the car, I felt the internal blockage was unlikely. Long story short, while nursing the car home, I grabbed a new fuel pump. Swapped it out when I got home and the problem seems to have gone away. Didn't get much of an opportunity for a test drive though.

So tonight I got another chance to take the car for a test drive. I took it out on the highway to attempt to duplicate the previous conditions. I couldn't get the car to fall on it's face as before, so it seems the fuel pump may have been the issue. I still feel an ever so slight bog. It's a weird sensation, almost like it's trying to seize. I may just be in my head. It's very subtle if it exists at all. I'm going to recruit a friend to test drive it to see if they notice it.

While winding it out a bit though, I notice a harmonic vibration around 60 or 70 mph. I also notice the same harmonic vibration while fast idling in park. It's serious enough that I'm scared to put the car on the highway for an extended period. I can't pinpoint the RPM since the car does not have a tach.

No temperature or oil pressure issues indicated on the gauges.

Going to check a couple of the obvious things. Make sure all the converter bolts are tight (I torqued and loc-tited them). Already visually checked the balancer.

Next step might be to swap back that converter, but I can't say for certain the vibration wasn't there before the transmission repair and converter swap. I just haven't had much opportunity to wind this thing out before.

There are no weights on either converter. The one in the car came off another 340.

Any thought?

Was hoping to drive this car to MMW, but now that's in jeopardy. While I'm not afraid to drive the damn thing until it blows, I'd sure like to avoid it.

Any ideas?

Needless to say this car has me...  

[471_Magnum]

 

[firefighter3931]

It sounds like an engine balance issue because the vibration is there in park as well as in gear and at speed. I would reinstall the old converter and check the flexplate closely for cracks. Have a look at the flexplate/crank bolts while you've got it apart....i've seen those come loose before.


Ron