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

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Mar 27, 2024
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Tupelo, MS
My Car
1973 Mustang Mach 1
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.
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.
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.
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.
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.
I had a very similar vibration problem with a 302-2v engine in a Mustang. Turned out to be a bad balancer.

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