Vibration when running engine at 1200-1700 rpm’s and crusing about the same.

Joeaus08 · Apr 23, 2024

I have a 1973 Mach 1 Q code and I noticed running the engine at 1200-1700 rpm in park you can feel a recurring vibration. Any ideas? I had a similar one on my son's 2006 and I replaced the counterbalance and it fixed the problem.

I'm going to act like my wife telling me what is wrong with it. It goes, "waaaa, waaaa, waaaa, waaaa", about a second to second and a half apart - continuiously. When driving you can't feel it while its pulling (gaining speed) until you get to a cruising speed of about 45 then it is like it is above. It has a c-6 auto and a 4:10 rear end.

This afternoon I am going to start at the harmonic balancer and go through all accessory drives and cooling fan looking for the problem.

TommyK · Apr 23, 2024

Sounds like a bad U joint

Stanglover · Apr 23, 2024

TommyK said:
Sounds like a bad U joint

He said it does it in park, so likely not a U joint.
Basically, I have no idea.

rcadd1ct · Apr 23, 2024

Check timing then carb

That rpm is where carb is transitioning from idle to main circuit but also where the advance on timing is active.

Easier to check timing so I would do that first.

72mokk1 · Apr 23, 2024

During my first real engine tuning number 1 cylinder was dead, I thought my timing light was broken but it turned out some crud was on the node in the cap.

Joeaus08 · Apr 23, 2024

I took off the fan and all belts to all accesories, vibration still there. Pulling the counter balance this afternoon, can't feel anything or see any movement while engine is running.

c9zx · Apr 23, 2024

Search on "finding true TDC with a piston stop". That will tell you if the outer ring on the damper has moved, changing engine balance. Chuck

Don C · Apr 23, 2024

Has the vibration always been there, since you had the car, or did it start all of a sudden?

jakosaur · Apr 24, 2024

Slow down! You’re charging ahead after a major mechanical problem without checking the low-hanging fruit first. This sounds like most likely an ignition or carburetor problem. Off the top of my head, a bad diaphragm in the distributor might cause it, or really anything broken in the distributor (they can fail in weird ways), or a bad ignition coil, or something sticking or gummed up in the carburetor. These are all much easier fixes than tearing apart the front of your engine, and more common failure points. That’s why all these parts are on the top of the engine.

70cobrascj · Apr 24, 2024

Joeaus08 said:
I took off the fan and all belts to all accesories, vibration still there. Pulling the counter balance this afternoon, can't feel anything or see any movement while engine is running.

I found my original was moving when trying to time it. Timing kept moving around and I couldn’t get it right. I made a mark on it and the mark would change every time I fired it up.

jakosaur · Apr 24, 2024

Or a vacuum leak, which can be very hard to find and can truly drive someone mad.

cjstaci · Apr 24, 2024

Vacuum leak, carburetor, timing, any of which can be the problem. Check the spark plugs. Look for any that look lean, take note of the ones that share the same intake runners.
As far as the harmonic balancer goes, visually inspect the rubber ring and bring #1 to tdc at end of compression stroke to make sure the balancer is at tdc at the pointer.

Joeaus08 · Apr 24, 2024

Don C said:
Has the vibration always been there, since you had the car, or did it start all of a sudden?

Just purchased in March. Since i bought it.

Don C · Apr 24, 2024

In that case, it's also possible it has the incorrect balance on the flywheel/flexplate, or a problem with the clutch disk or torque converter. It could also have loose or missing bolts on them.

Idaho Chris · Apr 24, 2024

Don C said:
In that case, it's also possible it has the incorrect balance on the flywheel/flexplate, or a problem with the clutch disk or torque converter. It could also have loose or missing bolts on them.

I had the flexplate problem that Don C alluded to. I don't know if my old thread will help you, but here is a link:

https://7173mustangs.com/threads/please-help-me-diagnose-vibration-1966-289-4v-solved.41589/

72conv · Apr 24, 2024

I had a very similar vibration problem with a 302-2v engine in a Mustang. Turned out to be a bad balancer.

1973 Mach-1 · May 23, 2024

I have that SAME vibration, but noticeable when driving over 2000 RPM. I have changed/updated everything that's been mentioned. Still- Thrummmm- Thrummmmm- Thrummmmm- Thruuuuummm.
The only thing that I haven't done it to replace the rear tranny seal. I also have a leak there.

rcadd1ct · May 24, 2024

1973 Mach-1 said:
I have that SAME vibration, but noticeable when driving over 2000 RPM. I have changed/updated everything that's been mentioned. Still- Thrummmm- Thrummmmm- Thrummmmm- Thruuuuummm.
The only thing that I haven't done it to replace the rear tranny seal. I also have a leak there.

Try turning the driveshaft u-joint at the yoke 180 and see if it helps

Spar_ky · May 24, 2024

I have had some weird problems with my 73. that sounds like vibration. The plate under the carb with the EGR valve, had deteriorated and let the exhaust gases enter continuously. The distributor plate that the points mount on, the advance plate, moved in an elliptical pattern rather that curricular.

steves73 · May 24, 2024

If it's the natural V8 harmonic, mine was the transmission mount rubber insulation. It was so old it was no longer separating the vibration from the chassis/floor.

Vibration when running engine at 1200-1700 rpm’s and crusing about the same.

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