Case of the shakes

[dcducks1]

So, I put in a Hi-Po 289 into the Comet over the winter. It has a mild cam with just a little bit of chop, nothing radical. I replaced the tranny and engine mounts and added headers before the motor went in. Once the car was up and running, the engine has a little shimmy too it but nothing crazy. I drove it with open headers to the shop for the exhaust install and when I drove it home it seemed like it had a lot more vibes to it. At speeds between 20-40....there is like an oscillation which goes away if I let off the gas or put it in neutral and coast. If I am parked, hold the brake, and put the car in drive I can feel the vibe. I have checked the exhaust and everything has an isolator mount. It definitely feels like it is in the drivetrain. I first thought exhaust and then cancelled it out. Then I thought u-joints but it vibes when I am in gear and sitting still. The u-joints feel tight and I don't have any noise when I shift in or out of gear. The only thing I can think of that is still "moving" when I am in gear and at a stop is the torque converter. Anybody got any other ideas? I still plan on changing the u-joints because they might be adding to the oscillation.

Dave

[dcducks1]

Update : I got under the car yesterday and did some investigating. I found the passenger header is very close to the bell housing but not touching.....this shouldn't be my problem. The headers bolts were a little loose so I snugged them down, both engine mounts and tranny mounts looked snug and in good shape (they are new). I need to take off the access panel off of the bottom of the bell housing and check the torque converter bolts to make sure they are snug but I am still recovering from right shoulder surgery so I'm a little limited on what I can be doing (should be doing). I should probably check the bell housing bolts too while I'm at it. I found the bolt missing on the apron between the bumper and grill which would rattle (very rarely but at times). Easy fix, it's not the main problem, but every little bit helps. Overall, everything looks pretty solid under the car. I suppose I need to disconnect the exhaust from the collector and see what that does with the vibes. As I mentioned before, all exhaust connections have isolator mounts. Still want to change out u-joints and maybe have the driveshaft checked for balance. I still get the vibe when in gear and with the brake on, but u-joints and driveshaft might be giving me some of the harmonics around 30-40 mph. Just spit balling right now......feel free to throw something at the wall to see what sticks.

Dave

[poboyjo65]

do you have the right balancer & flexplate on it? If it is a true hipo I think they had a specific balancer. also later 5.0 engines had a different balancer & flywheel/flexplate, make sure its not from a 5.0. does it run good? a dead miss will make it shake too. you might look at the motor running at night to see if any sparks are jumping from plugwires to block/ground. I'm just spit ballin too!

[popscomet]

I bet it's in the rear gear....like a pinion bearing.....when I have to take my comet on a LONG LONG road trip,I have a 3:00 single track I install.with this gear I get a small VIB,,nothing to get alarmed over,so I live with it....then when I get home,I put my 3:50 posi back under it and all is OK !! JMO...POP

[ants875]

if you feel this vibration at a stop that rules out the driveshaft and the rear axle, I second the thought of the flexplate or the balancer. did you rebuild the original motor or get a new one? if so you need to know what balancer the motor is supposed to have, the 5.0 motors have a 50oz imbalance and that would make for a definite vibration on the wrong motor.

hope this helps

Ant

[65clevorcyclone]

Id take a look at the flex plate and converter. Get a dial idicator and check the runout and see if its within spec. Was the engine balanced before it was assembled?

[popscomet]

the vibs go away when he kicks it out of gear and coasts,thus unloading the drive line.....pop

[poboyjo65]

Pop he says it vibes in gear sitting still in his first post.

[popscomet]

Pop he says it vibes in gear sitting still in his first post.

WENT back and re-read his post ,,reckon I over looked that part ..thanks JOHN !!!....pop

[dcducks1]

The motor was rebuilt when I got it. I used my original flex plate and put a new 28 oz harmonic balancer on it. It was a 69 289 block and I don't believe they started using the 50 oz until the 80's. I'm not sure that I believe it is an imbalance between the flex plate and the harmonic balancer because 1) it runs good other than the lobe chop at idle, 2) my vibe is most prevalent between 30-40mph AND I can "drive thru the vibe" which I would think would continue to get worse if the two items were mismatched, and 3) I don't get the vibes in neutral and revving the motor. My ignition seems to be firing fine with no misses or errant sparks. My focus tends to be on 1) the torque converter, 2) the exhaust. I don't recall the vibe when it was straight headers but noticed it after I got it from the exhaust shop. I will disconnect the collectors this weekend and see what happens. Thanks everyone for the advice. It definitely helps get my "gears" moving as I work my way thru this.

Dave

[dcducks1]

Poboy - you are a smart fella. I called a motor guru that I am lucky enough to know and told him what my motor was doing and he asked me to look at the freeze plugs to see if the block was raised around the freeze plugs or flat. My original block was flat and the codes said it was a 1966. I told him they were raised and he asked me to find a block code which was E5. This is a 302 block, 1985 or newer which MUST have a 50oz flex plate and harmonic balancer. He asked how much I had driven it and I said less than 100 miles. He said it should be fine, don't drive it anymore and swap these components out. Sooooooo, next weekend I will put it on a lift and separate the tranny enough to get the flex plate out and swap the balancer off the front. Good troubleshooting Poboy and thanks for the help.

Dave

[dcducks1]

Well, after 7 hours of busting knuckles, the Comet is all put back together and running like a dream. No more shakes, rattles, or rolls! I drove her for about an hour today and it drove perfectly. Thanks for everyone's inputs, always great help and advice here at Comet Central.

Dave

[popscomet]

GREAT news ! reason I mentioned 3rd member was cause I misread your post,,but when I run a certain 3:00 gear I have for long trips ,I get just a tad VIBS in the dr shaft at certain speeds not bad ,but I can feel it,no one else in the car notices it ,,then when I put the 3:50 posi back in it's all gone ,,,again glad your back up to SNUFF !!!! pop

[poboyjo65]

Glad you got it fixed!

[ants875]

Nice to hear you got it fixed! now go out an drive the wheels of it!

Ant