Info- 78 Ford F100 Custom, 2wd. 302, c4, 52k miles

So here is the story.. a few months ago the ring gear broke off my flex plate randomly. So I pull out the tranny and replace the flex plate (28 oz imbalance to match the original) and rebuilt the trans, installed a shift kit, and put a higher stall converter in while I was at it.. lemonade out of lemons right?

well everything works great except I have a vibration while in neutral and while driving at around 1500 rpms.. maybe 2000 (no tach in the truck). Mostly felt sitting in truck. Seems to go away in higher rpms.

I am pretty sure it's the original motors and thus needs a 28 oz imbalance for the flex plate. I have checked for vacuum leaks and found none and the motor runs strong.. I checked the motor mounts as well and none of them seem broke or move much when I pry on them.. I also checked the belt driven accessories and ran the motor without the fan belts.. (vibration still there)

so I started thinking about the balancer.. it doesn't look out of place or have any movement that I can tell by hand..

​I put it on top dead center the best I can and it seems to match the balancer mark.. however the balancer does seek to have a little rubber coming out.. (in photo)

how do I tell if the balancer is bad? Does this look bad? Or can you think of anything else I am missing?? ​​​​​​
Thanks in advance!!

 

Has it done this ever since you put the new convertor and flex plate in ? if so then the most likely explanation is one or the other is out of balance.

if not then there are a lot of things it could be. the vibration damper is probably the least like cause most of the time. but it does almost look like you have fresh rubber movement going on. hard to tell.

 

Regardless that damper is due for replacement. So it should be on the short list.

 

I doubt it's the flex plate either. You likely would have noticed the different size counterweight. Only thing left is the converter.

 

 

Thanks for the input guys!

So here's the thing. I was dealing with some vacuum leaks before the flexplate broke. I solved that by putting an intake and an edelbrock 1406 (600cfm) carb on there to delete all the old emissions mess.. (turned out to be the EGR and a PVS valve on the back of the intake. As well as a bad carb) It ran great but only for about two days and then the ring gear broke off the flex plate.. So I am not sure if the vibration was there before or not... I don't remember it being there though.. There is a lot of stuff going on with this truck because it sat in my dad's yard for about 4 -5 years before he passed away and was only ran maybe twice each year.. So it's hard to do the traditional "fix one thing at a time" deal..

I am thinking that the ring gear is symptomatic of a problem that existed before hand.. Ring gears usually don't just pop off of a flex plate when they are welded.. (maybe a flywheel).

I have 20 inch/lbs of vacuum, 150 psi across all cylinders, 10 degrees timing, and the motor feels stronger than it ever has (aside from the vibration) So I am feeling sure it is not a ignition or timing issue. But you would think an imbalance issue would not go away and would be present throughout the the whole RPM range..

 

Well guys, I put a new harmonic damper on the truck and it did not fix it... Bad news freaking bears...

From the looks of it everything should be right... Could there be air in the torque converter and that is what is making it off balance? I put two quarts in it before installing it.. and have cycled through the gears like 15 times....

Help...

 

I've never heard of the air bubble in the converter thing. Unfortunately, it sure sounds like the transmission needs to come back out, as big of a pain as that is.

From your description of what was done, I'd say the strongest suspects are the new flex-plate and converter. What brand are they?

Maybe the flex-plate was mis-boxed, but from my personal experience, a 50 ounce flywheel/flex-plate will start to really become noticeable at close to 3000 RPM. Maybe the converter is poorly built?

If the vibration is very slight, perhaps it's a burned plug wire which arcs at certain RPMs? Seems like I've had a bad plug wire cause a miss which would seem to go away at higher RPMs.

 

Just at a quick glance that looks like a later model (50 oz) balancer. Lots of things get changed on old trucks, nothing saying that motor is original. I would check the part numbers on the old balancer.

 

It's so easy it's hard. there are 28 oz engines and 50oz engines. Your engine is one, the flexplate is the other.
You got the wrong flexplate.
The harmonic damper was shot and should have been replaced anyway.
The reason it vibrated in that rpm range is it was near a resonant frequency. This phenomenon is quite common in rebuilt engines when the owner doesn't know and the wrong part is ordered..
R.

 

Dually noted.... however the motor is from a 78, the trans from a 78, balancer is a 78, and the trucks a 78 (photos attached). 78 takes a 28 imbalance flexplate which is what I ordered.. so unless they shipped the wrong part in the right box.... I have done ample research on the imbalance of ford motors...

truck is all original and not rebuilt according to the original first owner (who my dad bought it from). It was an old guy who took immaculate care of it and I honestly dont think would lie about it.. (though it is possible)

Just looking for some help...

I'll double check the functionality of the motor (valves, compression, and such) and if I find nothing then I guess I'll pull the transmission this weekend and test motor without trans/converter..

 

 

I have attached a photo of the 50 vs 28... mine looks to be the narrow depth one which would be less weight removed and thus less imbalanced right?

threw the two different flex plates photo in just for resources for future researchers..

 

the flexplate is B&M and the converter is a TCI "Saturday night special". Around 1600 stall... seems to perform well so far..

I will check the plug wires and probably go ahead and replace them.. I put an ohm meter on them about 4 months ago and they all came back in spec..

I may take the flexplates to a tire balancing shop and see if they can spin them up.. they should be able to mount it with a cone... I am assuming the end result would be 28 or 50 oz recommended on the opposite side of where the weight is.. haha that should tell me exactly what I have..

 

Hey guys, wanted to update.. it ended up being the flexplate.. ordered the right plate but must have been sent a defective one.. Pulled the transmission back out and went and got one at orileys and it worked. Still a little shimmy to it but it is much better.. thinking the shimmy is due to the 1700 stall torque converter..

thanks guys!