Engine Vibration caused by bad distributor?

Dave in MN

Dave in MN

Well-known member
I could use some help on this one!
I was contacted by a fellow (Paul) I rebuilt an engine for with a concern of a vibration at all rpm's. The vibration is felt at all rpm and at idle it noticeably shakes the fenders. Before I looked at the engine, I asked him to check his distributor for correct timing, rotor gap and spark strength. He did so and other than equalizing the rotor gap said all was fine. He thought the vibration was from the lightened flywheel we installed. I went to his home yesterday and sure enough he had a vibration as he described. Strange thing was when I test ran this engine on my stand, it was one of the smoothest I have assembled. It has a Burlington counterweighted crank, 24# off the flywheel, V-8 pressure plate. The entire rotating assembly was balanced before assembly. The rods were weight matched but I did not weigh the pistons. (Probably should have!)
We split the bell housing from the flywheel housing removing the emergency brake cross shaft and then by dropping the rear axle and rolling it back. We checked the installation of the flywheel and it was within .002" run out and .002" total for wobble throughout a full 360 rotation. I thought it should be running pretty good. As long as we had it apart, we changed out the flywheel and pressure plate to another balanced unit and dial indicated it to have .003" run out and .002" wobble. We test fired the engine before we put the bell housing back on and it had the same vibration. I more closely listened to the firing of the cylinders and sequence of the vibration pulse and thought the vibration was more closely related to the firing of the cylinders than a single rotation of the engine. The vibration pulse seemed to be timed at about 1/2 of the engine rpm. In other words one pulse for every two rotations of the crankshaft. I removed the distributor and rotated the shaft while watching the point gap and I could see the gap was noticeably less on two successive lobes. I placed the distributor in a vice and used a dial indicator to check the difference and found the following. .022" - .022"- .016"- .012"
The question: Do you think that difference in point gap could cause the vibration we are feeling? Has anyone had a similar issue?
How much could (.010") of point gap difference change the timing of the spark delivered to the cylinder in degrees? The distributor cam in this case is supposed to be one of the very accurate (expensive) ones.
Further study:
I brought his distributor home and rebuilt it last night. There was a one piece distributor shaft (the long one) and worn bushings in the assembly. I re-checked the point gap movement and confirmed what I had gotten earlier as to the difference of point gap. After I installed new bushings (reamed) and new short (standard) distributor shaft, the measured point gap using a dial indicator was .021" - .021" - .019" - .019". This same difference was repeated with removing and resetting the rotor three times. It is obvious the .002" difference is in the rotor or (more likely) the machining of the distributor shaft. I'll send the distributor back to him and ask him to run it...with my fingers crossed for luck. Two days shipping time is a long time to keep your fingers crossed!
Thanks for input guys.
Dave in MN
 
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Well it doesn't help being that far off...I run all the distributors for the engines I
do, on my Sun machine. If you have access to a Dist. machine go run some too
see were they trigger! If it's not close, on an A, change the cam. I think it's all
part of a "blue printed engine".

Dudley
 
Wear of upper bushing, loose upper plate, worn housing, thin plate in a wide plate housing(I have found there are two plate thicknesses). Bob
 
Short comment on un-equal point-gap:, re: .021-.021 -.016 -.012"

Point gap affects ignition timing: as point gap is increased, it tends to advance the timing (points open sooner in the rotation of the cam); as point gap is decreased, it tends to retard the spark.

What you have measured with the dial indicator is resulting in uneven firing, due to uneven point-gap between the various lobes of the cam.


Let's say the lobe for cylinder # 1 is reading .021", #2 = 0.21", but #4 = .016", and #3 = .012", then you're back to #1 = .021" . The spark is correctly timed (for the sake of this discussion) for cyls 1 & 2, retarded a bit for #4, retarded even more for #3, then takes a big jump towards "advance" when # 1 fires again... my bet is the difference in cam/spark timing between #3 and #1 is the major source of this vibration. This would result in uneven loading of the crankshaft, resulting torsional vibration, which would be hard on the crank as well as the thrust bearing...

That's my theory given your comments and evidence...

Let us know what you find...

SC Frank


Worn parts can cause this, but there has also been talk of poorly-made distributor cams out there....
 
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When I bought my first Model A 37 years ago , I put a new ropop distributor shaft in it

It must have been made out of old beer cans as the shaft was worn out after 500

miles and the points gap was all over the place just like your customers .It ran like a

bucket off bolts !!! Same problem vibration and over heating !!! I'm with Frank on

this poor Quality distributor cam or shaft is whats throwing it off the last couple

thou.

PS I also bought a new repop Water Pump shaft from the same people ! You can guess how long that lasted !! Must have been my first Model A Lesson !!!
John Cochran
 
Like Dudley and Frank say:

Another possiblity: I think it could be the fact that the cam lobes are not 90 degrees from each other.

I have seen some repro cams running on a Sun Dist machine where two lobes varied 5 to 6 degrees from being 90 degrees after each other.

This cam was one of the "Model B" Repros (new one) ... replaced with an old original from parts bin and it fired much closer to the way it should.

I do not know who made the repro cam.
 
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I have several distributor shafts that are repro ---when checked for runout of the cam part of the shaft compared to the rest of the shaft --.003-.006

When checking NOS Ford shafts --out of 10, 9 are 0.0000, 1 was 0.0001 ---this is because both surfaces are ground with the same center, the repro look to be not machined at all for the bushing section, and turned in a lathe for the cam part---and it looks like the flange is pressed on.

If you want to know how far out it is in degrees a timing light could be used --1,and 4 should be the same mark, 2,3 180 out from that mark

To use a shaft that has runout you also need a cam that has runout ---then you index them to offset the runout ---then the oil pump gear has to be installed in the proper place so the dist can be dropped in without moving the cam much on the shaft----or you can find quality parts that meet ford specs
 
I feel it could be the one piece distributor shaft causing problems. The 2 piece as original, acts as a universal between the 2 shaft halves, where the cylinder head (and distributor housing port)
are not in line. Using a 1 piece shaft would have considerable wear on bushes and hence point gap.
Old Henry used the 2 piece shaft for a reason.
Just a thought.
 
My 29 stock tudor had some irritating vibrations at different speeds. The best sweet spot I had was about 43 mph which was ok because it was a good crusing speed but was a little rough getting & holding there. For reasons that had nothing to do with the vibration I put in a Zipper distributor. First drive with the new distributor "WOW" it was smooth at all speeds, vibrations gone. Had no idea but sure made me a happier driver.
 
d.j. moordigian said:
Dave,

Did some more thinking about your problem(along with mine), it could be a tight
valve...have you run the valves again?
Click to expand...

A valve hanging up was my first thought so I completed a compression test just prior to pulling the distributor: 80 - 80 - 81 - 79. With those results, I dismissed the notion that it was a valve.

Regarding the distributor cam...I don't know where it was purchased so we do not know the manufacturer. The distributor shaft i installed during the rebuild was a drilled shaft for oil to the upper bushing, purchased through Snyder's.

I did not think to indicate the cam mating area of the distributor shaft when I had it set up in the vise...it would have been easy to do...darn! I have three new drilled shafts in my parts drawer which I will indicate to see if there is a variance between centers.

Thanks,

Dave
 
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Update: The rebuilt distributor did not solve the vibration. The owner is still feeling a vibration particularly on deceleration. He says the fenders are still shaking at idle. I am having a distributor rebuilt and tested to confirm the results...I'll report the results.
 
Hi Dave, You said you ran this on the stand and it was smooth. Then it goes into the car and shakes like a paint mixer. I'm worried that there is an alignment problem somewhere in the flywheel housing - bell housing - transmission connection which is causing this problem. Have you checked these out? Is the flywheel housing properly shimmed to the engine block? Also, are the motor mounts new (new rubbers, bolts and sleeves) and the engine in proper alignment when affixed to the motor mounts?

If the engine is smooth on the stand, and vibrates in the car, then the answer must be in the car... Let us know what you find out.
 
Chris and Dudley,

If you check the original post it appears that he ran the engine in the car with the transmission and rear end removed and still had the vibration.

It must be something that is different in the test stand vrs in car. How about the fan/generator. Did you remove the fan belt and test for vibration in the car? How about tightening and/or loosening the engine mounts?


Bob
 
Thanks Guys for the continued followup.
When I test run these engines in my shop, I use my distributor, intake and exhaust manifolds, carb and other electrical components. All of these items are different when this engine is installed in his car.

We ran the engine in the car with the bellhousing not connected. So the engine only had the flywheel housing, flywheel and V-8 pressure plate installed. The clutch disc was also left out to eliminate it from the equation.
Before we split the bellhousing off to change the flywheel, I did remove the belt to take the fan and generator out of the combination. It vibrated without these items spinning.

I am having a trusted distributor rebuilder (Preacher) set up a replacement distributor for me. I asked him to use standard components in the build. He has the equipment to test the distributor before it goes in the car. We will install this replacement distributor and then take the one I recently rebuilt (and presently in the car) to his shop and test it. I will take my test stand distributor along also to test it in his car.
We did not tighten or loosen the motor mounts. That could be the next step if the distributor is not the cure.
Hey Chris...the vibration is not really to the level of a paint mixer...:) but enough that the owner is not satisfied.
Good Day!
 
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Seems like most has been checked , However I note above we changed the flywheel ?? Now if this is a balanced unit , Is it possible flywheel on 1/2 turn out ??When balancing it on the crank it should be marked so one dors not put it on 1/2 a turn out, Have seen it done & by Turning all came Good.
 
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