1968 GTO bad vibration
 

Got my 68 GTO on the road and it needs some help. It has a fresh built 400 and a rebuilt turbo 400 both stock rebuilds. My problems is a real bad vibration. This vibration goes away as soon as I put car in neutral . If what I have searched is correct this rules out drive shaft back and sll suspension / wheels and tires. Is this correct? It seems to shift through gears but vibration is bad. So how do I figure this out ? Any thoughs to rule out other parts? It has a 13 inch stock converter. I'm replacing tranny mount to rule this out. Any help is appreciated . Doug

Well you have ruled out the drivetrain, so now you need to unbolt the torque converter from the flexplate, push it back out of the way and start the motor.

Having no vibration after doing this rules the motor out and I would take a guess that tranny wise a balancing weight has come off your converter as that's the only thing big enough in diameter to make a bad vibration like you discribe.

Before backing off on the Converter just loosen the bolts some first and see if the locating nose of the Converter was out of place.
Hopefully you will get to the point of singing The Beach Boys song with your 68!

A driveshaft could be an issue. What speed is this vibration? is it "road speed" related or "engine speed". Driveshafts will usually have a vibration at lower speeds, since it usually turns 3-4 times the wheels speeds.... depending on your rear gear ratio. An engine misfire also is more pronounced on the gas, than it is off the gas. Do you feel it slowing down in gear? Not enough info to guess at this time. ; )

when you say the vibration stops rolling down the road in neutral, is the motor still running , or no?

Putting it in neutral and the vibe goes away would indicate that the vibration is from something that is static then rotating with the neutral gear change.... Input drum section of the trans,. It's a fresh build could be some bad or missing bushings I have seen elliptical wear in pump housing bushings from out of balance input drums on 400's I've torn down for rebuild..... If a bushing is missing a drum or shaft won't be supported it will drop with gravity then when a clutch pack is engaged it will begin to rotate off center,.

Even if it goes away in neutral I wouldn't rule out a u-joint. BTW you do mean driving down the street and put in neutral right?

When I say Driveshaft, I really meant Universal Joint. I would for sure, if you dont find anything obvious, pull the driveshaft and feel the universal joints by hand, not for looseness, but for tight spots.

Thanks for help so far. Yes I am talking about putting in neutral when driving down street . It does not matter at what speed but when I go to neutral all is ok..( smooth rolling) I had someone tell me driveshaft but I thought that continued to spin even in neutral? Im going to drop out driveshaft and make sure it's not the problem. I had shop install new u bolts and thought he checked balance but I can bring back if you think that is where to start. The transmission was rebuilt but don't have much information on shop an quality( I had it done). The torque converter is a 13 inch as far as I know is stock and have no idea on condition or how to check. I installed everything after rebuilds. I also just bought new transmission mount just in case.Doug

The drive shaft is still spinning when your rolling down the road in neutral so don't waste any time on checking the driveshaft or its joints.

Your issue is with the trans if the motor itself has no vibration when run at any rpm out of gear.

Steve thank you. So I'm down to transmission. Could it also be torque converter? And how about the trany mount. Is it worth replacing it or would this not cause my problem. Doug

Spinning? Yes, load on it? No. A bad u joint CAN cause a vibration that comes and goes. Does vibration stop "OFF the GAS" or just NEUTRAL?

Was the motor balanced with the flex plate? Are you using a stock or after market flexplate?
So the vibration goes away once the motor drops to idle rpm? Does it vibrate sitting in the driveway, in nuetral, and reving the motor to cruise rpm?

I will add a few things (this is my dads car)

Engine was built at a speed shop. Using the stock flexplate and was run on an engine dyno. As far as we can tell it runs spot on and has no misfire or other symptoms.

The vibration starts at low speed and continues to get more pronounced as speed increases. It is present on and off throttle. It goes away at any speed if you shift to neutral and let the car roll.

Other symptoms that we have noticed in our very limited test driving (less than 15 miles):

The car is not accelerating the way the dyno numbers say it should. It has some kick to it, but absolutely not what it should.

Our last test drive we could smell what to me smelled exactly like burnt clutch as if someone were riding the clutch pedal in a manual.

When you put the car in gear it's like getting kicked in the ass, it really slams into gear, often times stalling out. We thought the idle may just be too high, but now I think this is related too.

Lastly, transmission came from the shop that rebuilt it with the TC already seated...supposedly.

Well now, that's a lot more info. Pull the trans.

In the transmission's defense. It actually does shift in to all the gears nicely.

Still think it's definitely the transmission?

Slamming into gear? What was the Burnt Clutch Smell?

It slams in to D and R. The shifts from 1-2 and 2-3 feel good and proper.

The burning clutch smell was present at the end of our test drive, no idea it's cause, I assume internal clutches inside transmission though.

I'd say tail shaft bushing might be suspect here, based on symptoms/speed correlation, if its externally wet with fluid look no further.

Guys just want to add flex plate was not with engine builder for rebuild . I took it off old motor and put on the new rebuilt engine following all of Wallace torque specifications . . I'm not sure if this is an issue and could cause the problems I'm having . Thanks Doug