Bad Vibrations 351w

My newly rebuilt 351w in my 69 XR7 vibrates more than normal. It is barely detectable at idle, but increases linearly with rpm, up to 3000 which is a high as I went. All tests done in neutral, with rear on jack stands. Rebuilt C4 and torque converter. The engine components were not balanced, but my rebuilder said that was not necessary unless it was going to be raced. It has .020 over cast pistons, original rods, reground crank (not original) .010 under, and a Howard (mild) cam. The distributor has been rebuilt and recurved, Pertronix ignition, carb is a rebuilt Autolite 4100. Here is my troubleshooting so far:

  1. Noticed damper had slight wobble, so it was replaced, no change.
  2. Disconnected all drive accessories and removed pulleys and belts for quick test, no change.
  3. Disconnected and capped all vacuum ports, except for vacuum advance.
  4. Varied timing from 5 to 20 degrees to see if there was a sweet spot - no change. Set at 10 deg now.
  5. Vacuum at 20Hg at idle
  6. Confirmed cam break in by watching pushrods rotate while engine is cranked. They all rotated, but not all at the same speed.
  7. Reconfirmed firing order
  8. Compression was 160 on all cylinders
  9. Checked the firing of all cylinders by removing spark plug wires, one at a time, while engine running, no weak cylinders found.
  10. Carb adjusted for smoothest idle
  11. Checked for vacuum leaks around manifold and carb with carb spray, no effect at idle.
  12. Flexplate is from a 289, 157t, 28oz and is currently the prime suspect.
  13. Plugs are Autolite 45 and appeared normal after about 30 minutes of testing.

Any other ideas anyone? Any comments and suggestions would be appreciated.

You should not have a C4. You should have an FMX or a C6.

Did your builder compare the weight of the old pistons with the new? The new pistons will weight a bit more. Even if the pistons are close to each other, the total added weight needs to be within a certian percentage to calculate the bob weight. I wish I could elaborate more on that but it is something I leave to the machinist before I assemble the motor after having a block bored.

Doesn’t the 351W have a different balance factor?

No every 351 is 28 oz like every 289.

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My old boss years ago chased a dead cylinder that turned out to be a bad headlight switch

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Thanks Royce - are you suggesting that the C4 could be the cause of the vibration?

That is really an odd problem. I do not think that is the cause of my issues since I have good spark at each cylinder. Thanks for the suggestion.

Thanks for the reply - I will discuss this with the machine shop that do the work.

A C4 will work behind a 351W. I’m on the flex plate band wagon. But check everything else out first.

Thanks Badcatt - I am trying to figure out how these flexplates “go bad”. What are the possibilities?

I think you probably do not have a C4. If you do it would have nothing to do with any vibration.

A few pieces of information that would probably be helpful here would be is the 351W the original engine? If not do you know what year it is? Do you know what the C4 transmission originally came from?

Now a few things to think about. In a later build 1968 Cougar with a C4 the transmission had a vibration dampener on the end of it. A C4 was never used in a 1969 Cougar but it could be in a 1969 Mustang with either a 302 or a 351. When it was used in a 1969 Mustang the flex plate was a 164 tooth version.

Randy Goodling
CCOA #95

Flexplates do not “go bad”. They just are chosen inappropriately.

Thanks Royce, yes, it is a C4. The po removed the original 351/FMX and replaced it with a 289/C4 during the “energy crisis” days of the 70s. This was common as many people were seeking better fuel economy. I have replaced the 289 with a 351, but kept the C4. Though not the correct combination for Cougars, it was common with other Fords of the day.

Thanks Randy. The 351 that I have is not the original engine, it is however a 69 engine, from a 69 Mustang. The C4 came from a 66 Mustang, along with the 289 that was in the Cougar when I bought it.
My understanding is that tooth count matches the bell housing - 157t is for a C4, 164t is for FMX and manual transmissions, and both have the 28oz imbalance up until around 1981.

I confirmed that my flexplate is the correct 28 oz, and it does not appear to be damaged in any way.

I may be an idiot, but since the vibration increases with RPM, could it be driveshaft or rear end related? Or is the vibration strictly the engine and not while driving?

Has anyone mentioned motor mounts?

All testing has been done while in Park.