Still can't figure out why my 351c is vibing at 3k rpm

DarkoStoj · Apr 2, 2006

I have been driving my 66 mustang for 4 months now as a DD and the vibe is starting to bother me. When I first got it there were a lot of things I fixed that I would have thought fixed it, but I still have the vibes.

-I replaced the harmonic balancer, the original one was spun and just toast
-I checked the flywheel to make sure it was 28oz
-It had a newer clutch/pressure plate, but the throwout bearing was toast, so I changed that
-The drivers side motor mount was separated and the motor was laying on the header, fixed that and nothing
-Replaced the rear ujoints, this fixed my driveline vibration, but still have the motor vibration.

Vibes are worst at 3,000 rpms. It seems like the whole car shakes, and its worst through the shifter. When I'm driving on the E-way it feels like the vibes are pulsating also.

Gud T.B. Blown · Apr 3, 2006

it could be that your driveshaft might be too short or that your yoke is too long. first check to see if your yoke has @1" sticking out of your toploader. if it is much more than 1" that could be your problem. if not, remove your rear ubolts and slide your driveshaft into the toploader to see if it slides in all of the way without bumping/rubbing into anything inside the toploader. either of those two things will also cause driveline vibration, i know from experience because i had both problems on my car.

bswor · Apr 3, 2006

When mine started vibrating it was becaause the tranny bolts started backing out, all thanks to the exhaust shop not tightening them back correctly. Which cost me a tranny rebuild. I'm sure you have already checked this though.

DarkoStoj · Apr 3, 2006

pushrod power said:
it could be that your driveshaft might be too short or that your yoke is too long. first check to see if your yoke has @1" sticking out of your toploader. if it is much more than 1" that could be your problem. if not, remove your rear ubolts and slide your driveshaft into the toploader to see if it slides in all of the way without bumping/rubbing into anything inside the toploader. either of those two things will also cause driveline vibration, i know from experience because i had both problems on my car.

I don't have any driveline vibrations, it is coming from the motor and they are at their worst at 3,000 RPMs

LMan · Apr 3, 2006

Friend of mine just had this *exact* same problem, and it turned out that his flywheel was not balanced correctly. You might try swapping in another flywheel and see what happens. Or remove just the clutch/PP and test it.

DarkoStoj · Apr 3, 2006

LMan said:
Friend of mine just had this *exact* same problem, and it turned out that his flywheel was not balanced correctly. You might try swapping in another flywheel and see what happens. Or remove just the clutch/PP and test it.

well it is a 28oz balance flywheel. What are the odds that it would not be balanced correctly? I did pull the trans to make sure it was a 28oz when I first got it, and the flywheel looked like it had a recent resurface job, along with a new clutch, and pressure plate.

Dark Knight · Apr 3, 2006

Do you have a fan clutch? My 84 had one lock up and it made a pretty good vibration.

DarkoStoj · Apr 3, 2006

Dark Knight said:
Do you have a fan clutch? My 84 had one lock up and it made a pretty good vibration.

its got a flex fan.

Since the vibration pulsates, would that mean anything?

bsedwebt70-5.0 · Apr 3, 2006

I had a flex fan cause a vibration on my 1972 351c in a 1970 mustang years ago - cheap NEW fan - I took the belt off and ran the car a block or two and the vibration was gone. Easy way to tell if that is your problem - It also convinced me to go with underdrive pullies because it had so much more power.

DarkoStoj · Apr 3, 2006

bsedwebt70-5.0 said:
I had a flex fan cause a vibration on my 1972 351c in a 1970 mustang years ago - cheap NEW fan - I took the belt off and ran the car a block or two and the vibration was gone. Easy way to tell if that is your problem - It also convinced me to go with underdrive pullies because it had so much more power.

I'm going to try that tommorow

MitchGT · Apr 4, 2006

Does it vibrate with the tranny in neutral, or just when you are driving it? Maybe the distributer has too fast an advance curve?

DarkoStoj · Apr 4, 2006

MitchGT said:
Does it vibrate with the tranny in neutral, or just when you are driving it? Maybe the distributer has too fast an advance curve?

it vibes a little with the trans in neutral, but it gets a lot worse when you're driving it

LMan · Apr 4, 2006

DarkoStoj said:
well it is a 28oz balance flywheel. What are the odds that it would not be balanced correctly? I did pull the trans to make sure it was a 28oz when I first got it, and the flywheel looked like it had a recent resurface job, along with a new clutch, and pressure plate.

Damfino, but my buddy would have bet a few paychecks that his f/w was in balance also ...after he tore his hair out checking everything else, he finally started checking the "no effing way in hell its THIS" items, and lo and behold....

Remember, start with the easiest thing to check and work from there. Pull the fan belts and accessory belts..then move to bigger things.

DarkoStoj · Apr 4, 2006

LMan said:
Damfino, but my buddy would have bet a few paychecks that his f/w was in balance also ...after he tore his hair out checking everything else, he finally started checking the "no effing way in hell its THIS" items, and lo and behold....

Remember, start with the easiest thing to check and work from there. Pull the fan belts and accessory belts..then move to bigger things.

The thing that really kills me is that I already HAD all that stuff out, but didnt take it somewhere to check the balance.

I just pulled the belts off the motor and took it for a quick spin around the block, no change in the vibrations.

Could timing have anything to do with the vibrations?

LMan · Apr 4, 2006

ehhhh, I doubt it....but here I am presupposing. Wont hurt to reset timing, though, thats easy stuff.

Have you run the motor at 3k with the trans out? with the flywheel/pp off? with the driveshaft out?

maybe you did, and I missed it in your previous posts...

DarkoStoj · Apr 4, 2006

LMan said:
ehhhh, I doubt it....but here I am presupposing. Wont hurt to reset timing, though, thats easy stuff.

Have you run the motor at 3k with the trans out? with the flywheel/pp off? with the driveshaft out?

maybe you did, and I missed it in your previous posts...

How would I be able to run the motor without a flywheel? I was going to run it with just the flywheel, bellhousing, and starter when I had the trans out, but never got around to it.

Also, I noticed a weird way that the clutch linkage is setup. There is a horizontal bar that goes from the frame that bolts to the side of the block, and that is what the clutch linkages rotate on, there is some play & freedom of movement, so I never thought much about it. Could that cause some sort of vibration?

slackr · Apr 4, 2006

Compression test?

DarkoStoj · Apr 4, 2006

slackr said:
Compression test?

What does that have to do with anything? I've been meaning to do one lately cause my oil pressure is a little low, and I think I might be burning some oil. I did change the head gaskets cause I thought the little bit of smoke I had was from burning coolant, and when I looked at the cylinder walls they looked great, no sludge in the lifter valley, and there was no ridge on the cylinder walls. I'm not sure if the carb is tuned or not, and I didn't even look at the timing.

It makes sense that my oil pressure is low because if the flywheel was causing the vibrations that would prematurely wear the main bearings giving me low oil pressure, but that doesn't explain the burning oil part, because I do burn some oil.

View attachment 479406

krash kendall · Apr 4, 2006

Is it just the angle or are pistons #5 and 6 different from #7 and 8?

DarkoStoj · Apr 4, 2006

krash kendall said:
Is it just the angle or are pistons #5 and 6 different from #7 and 8?

they look the same to me. I probably would have noticed if they were different when I had everything apart.

btw I like that picture in your sig, is that you & your kid?

Still can't figure out why my 351c is vibing at 3k rpm

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