351C Engine Vibration > 2500 RPM

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The General

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351C Q code Vert - Auto, 85K miles. Don't believe it has been rebuilt. The car seems to be pretty original including the points. (Really, don't want to debate this at this time.)

Looking for advice on what to do next. I have a engine vibration > 2500 RPM. I have put the car in neutral and park and confirmed that vibration is in just the engine. I have checked the motor and transmission mounts and these seem to be ok. I looked at the harmonic balancer and did not find anything wrong, however I do not know what indicates a bad one. I also removed all of the belts - vibration still exists.

I have purchased and older Alltest Engine analyzer a few months ago and have been able to get it working and bring it to good use. I will include some of the data from the tests. I want to know what should the points voltage be at higher RPMs? And if I can believe the analyzer saying the cam is worn.

Last year I messed with the idle, carb and points thus making the car run poorly. It stemmed from bad timing. The distributor was stuck so after three weeks of patience and work I got it to break loose. I readjusted everything and ran the Alltest analyzer with the following results:

Power balance between cylinders looks relatively even.

Vacuum at 830 rpm 15.6, @ 2700 RPM 20.7 in Hg

Points voltage 830 rpm: .28V @ 2700 2.04V (analyzer says "high")

Coil voltage 830 rpm: 7.25 V @ 2700 9.38 V

Battery: 14.05 V and 8-11 amps

Dwell @ 830 RPM: 29.7, @2040: 24.5 @ 2561 23.8 degrees

Altest Anaylyzer flashes " WORN CAM" at higher RPMS

The firing test and snap Kv test would not work > 2500 RPM - the analyzer "looses signal". Not sure why...

So to sum it up. I am trying to solve the engine vibration issue > 2500 rpm. Not sure if it is the points or the cam as indicated by the analyzer. Is there another test I can do? Any suggestions would be helpful. THANKS - Eric

mustang  may 15006.jpg

 
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The General

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What should I be looking to indicate the harmonic balancer is bad?

 
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Have you checked the timing at various RPM's and VACUUM to make sure the centrifugal and vacuum advance are working?

Sorry, don't know the advance specs off of the top of my head, but there are specs regarding timing based on RPM and vacuum. If you need them, give me a holler and I will go dig them up.

Not sure of your expertise, but remove/plug vacuum at distributor and record timing at idle and every 250+ RPM up to your 2500.

Then, you can hook up a vacuum pump to your distributor and see how the timing advances based on the vacuum you pull. Vacuum advance is notorious for going bad. Usually the diaphragm leaks and you lose all vacuum advance.

Next up....vacuum leak test. Method 1 - Take off your fan belt and then light up a big fat stogey...or light up a stogey and take off your fan belt. Start your engine and blow smoke around your intake to see if you have any air being sucked in. Helps to choke the carb so you get higher vacuum pull. Method 2, get starter fluid and spray it around your intake.

Next up...compression test. Might as well put in new plugs if you haven't done so already. If you don't have a kit, make sure you buy one with a small plug fitting and 2 or 3 foot vacuum extension.

Is your perception the vibration is due to a MISS or due to perhaps an IMBALANCE?

Is it a fairly consistent vibration?

Best of luck to you sir. Hate a bucking Mustang...and I did say bucking...

..as in Buckeye.

 
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The vibration can also be coming from the other end of the engine. It sounds like you have an automatic, so it could be a bad or wrong flexplate, missing bolts, or transmission not lined up with the engine.

For the harmonic balancer, see if you can move it any at all by hand, if so it has probably slipped. The best way to check it is to turn the engine to top dead center on #1 and see where the timing pointer points to. This isn't hard to do, but does take a special tool.

A worn lobe on the camshaft can also cause a vibration. Your analyzer is indicating that the cam in the distributor is worn, probably because of the variance in the dwell at higher RPM. Did you disconnect your vacuum advance? The vacuum advance plate will change the dwell when it moves.

 
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Since you have not just rebuilt the engine or had it apart I would pull the inspection plate and check to see if the torque converter is loose or maybe missing a bolt. You can also view it while running to see if something look odd. An engine tends to stop in the same place and the starter will hit the same teeth on the flex plate and wear them down but I doubt that difference in balance would make it shake.

If you think the shaking is coming from not running correct you can have the engine at the rpm that is causing the shake and pull the plug wires out of the distributor cap one at a time and put back and if one does not make a difference then something is array with that wire, connection in cap, rotor, plug, wire or even valve.

If you do find one cylinder that is making the shake and eliminate everything in the firing, check the compression pull the valve cover and check the rockers and push rods for that cylinder.

Good luck just keep eliminating things. You can check the cam lob in distributor with dial indicator or measure with calipers.

When I bought my 73 Mach 1 new I brought it home and took the vacuum advance off. Ford sold a kit that converted to centrifugal advance that came with different weights and springs to tune it with. You also had to change the cam in the distributor because the points sat at a different level. If I remember correct the cam is part of the shaft going down to the gear drive and not a separate part. I doubt that they still have them but you might check.

David

 

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Thanks Gents, I will work on these tests this weekend. Based on the engine analyzer - the cylinder drop test recorded even percentage drops between each cylinder indicating the cap, wires, plugs and cylinders are of even wear. So I think the compression test should be good but will check.

My gut feel is that it is an ignition problem because I cannot get the analyzer to get good data above 2000 rpm on all of the tests. Just not sure about the points high voltage is indicating something? I know the ease of electronic ignition but really want to stay original... Vibration is pretty consistent and worse as at higher Rpms.

It is an auto, So I will need to check fasteners and torque converter on he tranny.

I have pulled the covers and intake. I do have tick when starting the car cold, but I could not find any bent rods or loose rockers.

I can measure the distributor lobes with mics/calipers and indicators. I assume I am just looking for difference between the lobe heights?

The vacuum advance diaphragm does work. I believe I disconnected it during the test. (But it has been a few weeks....son graduated HS and is going to be a Buckeye... Sorry. He did not look at MI or my Flyers.

I will keep you updated on what I find out. THANKS

 

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worn distributor cam for points, points bouncing weak spring.. all the points are made in china now and are awful.

 

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Here are test results.. comments & suggestions appreciated. Thanks - Eric

Distributor/ points cam: Measured across flats (.980") and points (1.026"): Max .001"-.002" variation. Runout of high points .0015"

Power Balance: Cylinder drop @ 2500 RPM was 2.8-4.9% as each cylinder dropped out. Dwell was high @32 degrees but consistent across 700-2500 rpms with Vacuum advanced removed.

Timing: 14 BTD Stayed consistent from 700-1600 RPM with vacuum advance removed. At 2000 > the timing was bouncing/jumping 2-4 degrees). Attached vacuum pump to vacuum advance. With 5 in Hg I was able to advance timing 14 degrees to 28-30 BTD (High)?

Transmission: Pulled inspection plate and looked at Torque converter - all the bolts were tight, no teeth missing. Balance weight was intact on flywheel. Check a few of the Transmission mounting bolts and inspected the others. All were tight. I did not watch the Torque converter while running.

Have not done the vacuum leak test correctly, compression test, or tried to rotate the harmonic balancer yet.

 

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A worn lobe on the camshaft can also cause a vibration.

Vibration is pretty consistent and worse as at higher Rpms.

I have pulled the covers and intake. I do have tick when starting the car cold, but I could not find any bent rods or loose rockers.

I can measure the distributor lobes with mics/calipers and indicators. I assume I am just looking for difference between the lobe heights?
I'd bark up this tree first.

It's too common nowadays.

Especially if one/multiple lobe(s)/lifters are worn only a little, the lifter preload may still keep pressure on the rocker(s) on a visual check.

The cold tick is an indicator to me that things just ain't quite peachy in that department.

I had a 258 powered Jeep CJ with a bazillion hard miles that was so worn it acutally jumped and bent a pushrod in the driveway at idle. Thing sounded like a typewriter when it ran, was actually kinda smooth, and didn't burn any oil. It was like a science experiment! The autopsy revealed that ALL of the rockers barely moved when it ran... the lobes and lifters were equally toasted.

We just sat there and laughed... :cool:

I'll bet my lunch money here.

And I like to eat.

 
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My comment in regards to timing and dwell:

1. Refer here:

http://diyford.com/ford-351-cleveland-engine-ignition-guide-timing/

2 things of concern

1. The bouncing would be of concern to me.

2. I will try to find the mechanical advance curve for your car, but it seems to me the mechanical advance should advance the timing 10 degrees by 1800 RPM. No advance to me is an issue.

And, you should confirm(fairly easy) that when your timing mark is at TDC that your piston is at TDC as well. I think that would remove all doubt about a harmonic balancer issue.

 
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:thankyouyellow: to all who have offered info!

The General - please keep us posted on what you find and what works for you.

Ray

 
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General,

We just went through a similar exercise. I was fortunate enough to find a local resource that was able to find our issues.

Our symptoms:

"Missing", rough idle, and dieseling following a drive of more than 15 minutes. 2 spark plugs would foul routinely. Vibration noticeable at all RPM ranges, worst at idle and at 2200-2500

The findings:

Vacuum was my main culprit.

The PCV vacuum was being pulled from a runner port instead of the plenum beneath the carburetor.

The vacuum feed to the distributor vacuum advance was on the vacuum retard, and someone had put a vacuum cap on the vacuum advance.

Since my vacuum and advance were outta wack, the fella that had set up the new Holley Street Avenger had taken it way out of Holley's prescribed settings, so we ended up replacing the power valve and setting up a custom curve fuel mixture.

Last thing we found was a low secondary coil voltage that we attributed to the electric choke power being pulled off the coil. Moved to a switched power source.

Hope this helps...

Shawn

 
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+1 on Wolverine's comment. The mechanical advance should start at about 800-1000 RPM. It sounds like the advance weights are binding or the breaker plate is binding. I've seen it several times, especially on cars that have not been run in several years. The factory lube turns to "putty" at some point.

Just an idea. Chuck

 

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Thanks Gents, I will look at the mechanical advance. At the time I ran the test I thought we only had vacuum advance... until I broke open the book. So your recommendation is timely. The timing did start to " wander" at the higher RPM so maybe it was just starting to break loose. Should I spray contact cleaner or WD in there?

I stopped by Green Sales (obsolete Ford products) to pick up NOS motorcraft points, condenser and ARF 42 plugs. They did not have the cap. Although I don't think this is the problem, it nice to have Motorcraft in there... And it should not hurt.

I have ordered a TDC tool and will have it in next week. I will check the harmonic balancer and will do the compression check all at the same time.

Eric

image.jpg

 
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You might try a penetrating fluid like PB Blaster. WD-40 must be good for something I just have not determined what it is. If you have the Ford manuals you will see the breaker plate slides on REALLY fragile plastic "buttons". A little in the contact area may help. You will also see a procedure for adjusting the tension of the advance springs, access is through a small square hole in the breaker plate. Some penetrating oil,sprayed through the supplied straw, may loosen the mechanical advance flyweights. Keep the fluid off of the contact of the points, the rotor, and the cap. Let us know what you find out. Good Luck, Chuck

 

The General

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image.jpg Thanks Gents, back at it... Still more to look at.

I measured for TDC using the new tool and found it to be right on at TC (0 degrees).

All the spark plugs an found them to be in good condition and clean. The dry compression test indicated equal pressure among the cylinders - 149-156 psi.

I have taken apart the distributor to clean the counterweights that mechanically advance the timing. However have been slowed down by a stripped phillips screw.

I have noticed the mechanical advance tabs are bent all the way out which I believe reduces mechanical advancement. I will continue to work on it.

I am still looking for the mechanical advance curve for 351c 4v. to see how much I should bend the tabs...

On a side note, I found my coil is stamped with "use external resistor". I do not believe I have this resistor.

 
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