Yet another problem: bad vibration between 1700 and 1900 rpm

[Pentroof]

This build is a fresh 390 in a 1970 F100 with a new TCI C-6 and street converter.
Motor mounts are not stock, but have rubber isolators (front crossmember is from a late model Crown Victoria)
Stock transmission mount.
New flexplate, zero balance. No issues with starter engagement.
New Pro Products balancer, nice and tight.

I have a vibration that starts around 1700 rpm, is worst at around 1750, and then diminishes as rpm is increased. Above 1900, it's not really noticeable. If I rev the motor quickly, it revs right through that zone freely and you would never know. While driving, I feel it on the way up and down.

Vibration exists while driving, as well as parked or coasting in neutral and revving the motor. No connection whatsoever to vehicle speed.

[Pentroof]

The only accessory I question is the power steering because of the age of the pump, even though I went through it and had no concern.

I just pulled that belt and had no change in vibration.

The fan is an original steely and had no issues before this build. Alternator is a brand new 3G.

Next move is to pull the inspection cover and, well...inspect. Maybe a flexplate/converter issue?

Any other suggestions?

[machoneman]

Make sure # 7 and #8 plug wires are widely separated (not parallel) to avoid cross-fire.

[BruceS]

I had a stock aftermarket converter installed that was out of balance on my Fairlane SBF with C4.  Vibration was most noticeable in the 1500 rpm range and higher.  Had the tranny shop change to a different converter and problem went away.  From then on, I've only used "name brand" converters.  Btw I have had an ATI Street Fighter in there now for a number of years with no issues. 

[Pentroof]

Make sure # 7 and #8 plug wires are widely separated (not parallel) to avoid cross-fire.

Yep, well aware. That's why the wires look like this...
.

.
7 and 8 divert immediately at the distributor and are furthest apart in the wire separator on that bank.
Coming off the plugs, 8 goes to the rear and 7 is routed forward, under 5 and 6.

[WConley]

I agree with Bruce.  At that kind of rpm, it's something big and heavy that's out of whack.  I would suspect the converter...

You mentioned that idle quality is OK.  If anything was wrong at idle, especially switching in and out of gear, I'd look hard at the engine mounts.  Seems like you've got that covered though!

[Pentroof]

I agree with Bruce.  At that kind of rpm, it's something big and heavy that's out of whack.  I would suspect the converter...

Yeah, that's what I'm suspecting. Just looking for some other potential low hanging fruit. I guess I'll crank up the A/C in the garage. If it is the converter, I'm screwed. I bought this TCI tranny package almost a year ago and it sat in a crate in my garage. I've only had the truck running and driving for a month or so, but have been focused on ironing out the little details and don't have any real miles on it.

[My427stang]

Make sure all the bellhousing bolts and converter nuts are tight first, amazing the odd feeling you can get if things loosen up.

[HvyFt4spd]

 The inertia ring or crank hole itself can be off center on the balancer.

[Pentroof]

I removed the inspection cover and checked the TC bolts. All good.

Then I started the motor and took a few seconds of video. It looks like there is a wobble, but you can't tell until you look at it frame by frame. Some guys suggest using a timing light, but I found reviewing the video on my phone in edit mode was quite revealing. Simply dragging the slider at the bottom of my iPhone from one frame to the next shows the flexplate is moving position.

Here's the good news! It's not possible to lower a C6 out of a 67-72 F100 because of a crossmember that's riveted in. So, I either have to move the motor as far forward as possible, remove the motor altogether, or cut the crossmember and fab some brackets to bolt it back in.

Since the tranny came from TCI, I should probably plan for it coming out on a regular basis, so I'm leaning toward modifying the crossmember.

[Barry_R]

Can you unbolt and push the transmission back an inch or so to disengage it from the flexplate.  Just enough to start it up without the converter spinning?

[blykins]

Any chance it’s a fuel related issue causing an imbalance or stumble?  I’ve had some jacked up carbs that have created imbalance issues that would shake the engine.

[Pentroof]

Can you unbolt and push the transmission back an inch or so to disengage it from the flexplate.  Just enough to start it up without the converter spinning?

Barry, I think that's my next move.

[Pentroof]

Any chance it’s a fuel related issue causing an imbalance or stumble?  I’ve had some jacked up carbs that have created imbalance issues that would shake the engine.

This motor has a Holley Sniper TBI. I suppose it could be a bad transition area or gap in the mapping, but I don't see a discontinuity in the AFR.

I can't rule out fuel at this point. I do have the timing issue (different thread) and need to dive into the tables.

Have you experienced a TBI unit causing a seat of the paints vibration?

[C6AE]

I have a '67 F250 with a C-6 (yes, I did that!)
Just grind off the rivet heads, punch the rivets through and you can move the cross member diagonally enough to remove it. I use nuts and bolts to reassemble...