Downhill, Deceleration Vibration

[ErnieF]

Hi all,


My 31 slant window sedan has a strong vibration when holding back downhill. It does not do this at any rpm speed when on level nor when pulling uphill. It only does it when holding back going downhill, starting at about 1800 rpm and becoming quite bad by 2000 rpm. Transmission gear does not matter - it’s all about rpm speed. If I shift into neutral and coast or shift into overdrive to drop the rpm speed down to about 1500, the vibration goes away.

I expected (hoped maybe) that the problem was in the transmission. But I just installed a new Mitchell transmission and overdrive. I replaced the universal with a Bob Drake universal. I took my first test drive yesterday and the problem remains. I have talked with Steve Mitchell, and he is out of ideas.

Combined with this transmission is a new Burtz engine with a Burtz head and a Burtz cam shaft. Motor mounts are new. Fan is new. Clutch is upgraded to a new V8. New Burtz flywheel. I’ve had the engine up to 3000 rpm while it was unhooked from the transmission, and it just purrs. I have talked with Terry Burtz, and he is also out of ideas.

I have had this car completely apart and rebuilt it from bumper to bumper. There is nothing that I haven’t had fully apart, examined, and replaced as needed. In the end, almost all parts are new.

The only thing I can say, is that this is all still new. I’m up to about 130 miles on the engine, and 20 or so on the transmission. But I don’t think break-in alone would account for the vibration.

I do not feel that videos pick up the vibration very well, but I did take some. My low post count won't let me include the links in this post. I can message them to anyone interested, or I was able to post them in my similar thread over on the VFF.

I have talked to Terry Burtz, I have talked to Steve Mitchell, I have talked to Sam at Arizona Model A. I posted on the Model A Restorers Club on Facebook, as well as the Burtz Engine Facebook group. I've gotten lots of ideas, but so far nothing that has turned out to be a solution.

I have checked the universal joint (no change with the original, nor the new bob drake), rechecked the bearing preload on the pinion bearings, checked the flywheel housing for cracks, the kingpins are new and tight, brakes are all new with cast iron drums. I tried a different carburetor with no change.

I am 18 months into the restoration of this car, and have countless hours invested. I am at my wits’ end over this. Everything has been gone over. Everything is new. What could I possibly be missing that is causing this vibration??​

[Marshall V. Daut]

Check the pinion gear nuts. The repo "keeper" washer's ears are too short to bend over the two nuts' straight sides. Consequently, the nut closer to the transmission loosens up and begins to back off the threads of the pinion gear. Then the nut next to the rear end center housing will loosen up because the locking nut ahead of it has moved away. When that happens, the drive shaft starts to whip in the rear end, which causes vibrations UPON DECELERATION (because the torque is off the drive shaft) and scraping or knocking noise. If allowed to continue, there could be serious rear end problems and/or a broken pinion gear or bent drive shaft.
To check, jack the car up and place jack stands under the rear axle housings. Lie on your back right beneath where the torque tube mates with the center carrier housing and while someone hand spins the rear wheels, listen for scraping or knocking where the pinion gear is located. Have the assistant rock the tires forward and backward. If you hear scraping or any metallic knocking noise other than the bearings turning inside the rear end housing, the front nut on the pinion gear may have worked loose, followed by the rear lock nut.
This has happened to two Model A's I have fixed. I at first thought of a bad universal joint because the symptoms during deceleration are the same. But since you have a new U-joint, we can probably rule that out. My money is on loose pinion gear nuts from the way you describe the symptoms. This is especially suspicious because you have recently installed a Mitchell overdrive. If the lock washer isn't doing its job and keeping the two pinion nuts in place, this could be the source of the vibration while decelerating.
Marshall

[Marshall V. Daut]

I see that you checked the pinion bearing pre-load, but that could only have been done while assembling the Mitchell to the rear end housing. Those pinion nuts may have loosened since then due to a faulty lock washer. Or have you removed the rear end to check this by hand and are sure the nuts are still tight on the pinion gear threads?
Marshall

[rotorwrench]

When you are going down hill, the car transfers from engine driving to engine braking. A number of things happen during engine braking. The rear axle starts to pull on the torque tube instead of push as in normal drive. All of the engaged gears start to ride against the opposite sides of the gear teeth both in the rear axle and the transmission. The U-joint and even the clutch disk will have a different torque load than it does during normal driving. The engine uses compression to brake with no accelerator applied so this changes the load on the exhaust valves when they start to open so there is a lot of stuff happening in a different way than it does under normal drive.

Vibration also tends to have some noise associated with it but it may be difficult to tell where that may be coming from. Rear axle & drive shaft can get some vibration during deceleration. The transmission is generally OK unless there is frontal alignment deviation or a bad bearing somewhere. The current problematic new parts in the transmission are the countershaft rollers and the tip roller between mainshaft and input shaft. Some are crap. Old original Hyatt rollers with normal wear are preferable to the new stuff. I can't say what parts are used in the Mitchel transmission but it doesn't hurt to know that all the early Ford transmissions from the model A up through the 1950 Ford F1 use the same caged roller bearings. Mercury cars from 1949 through early 1951 used them as well.

If the vibration is up front then I don't know what to say about that. The Burtz engine is not something I have any experience with. If it's sheet metal type vibration then I'd look at the engine pans or something of that nature. Sheet metal vibrations can get loud and are usually sympathetic type vibrations that are caused by other sources.

[Gene F]

Could it be the universal joint?

[updraught]

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Sometimes it is easier to put the whole car on a dyno to find vibrations.

[Big hammer]

FAM’s or original rear motor mounts? Exhaust rear hangers can transmit vibrations, replace with a modern flexible hanger. 1800 to 2000 rpm’s is a normal range for vibrations does it go away about 2300 rpm’s and above while your coasting

[ErnieF]

Originally Posted by Marshall V. Daut

My money is on loose pinion gear nuts from the way you describe the symptoms.

I appreciate your detailed response, but the pinion gear nuts are indeed tight. The vibration was there before I installed the new mitchell as well. The nuts were tight when I took everything apart to install the mitchell. My original washer was still usable, so I put it back on when I installed the mitchell.

Originally Posted by Gene F

Could it be the universal joint?

I find it hard to believe that it is. The vibration was there with the universal and transmission that came with the car. I installed a new mitchel transmission as well as a new bob drake universal. The vibration is still the same. Surely I'm not so unlucky as to have two different universals failing in exactly the same way.

Originally Posted by updraught

Sometimes it is easier to put the whole car on a dyno to find vibrations.

I would love to do so, but do not know of any around me.

Originally Posted by Big hammer

FAM’s or original rear motor mounts? Exhaust rear hangers can transmit vibrations, replace with a modern flexible hanger. 1800 to 2000 rpm’s is a normal range for vibrations does it go away about 2300 rpm’s and above while your coasting

Original motor mounts with new pads. Muffler is a new Ares which comes with their own special hanger that has a fiberglass lining so it doesn't bind. Interesting question about the higher rpm's. I don't know - I'd have to seek out a hill long and straight enough to get up to that rpm speed. Would you expect the vibrations to quit once you get above 2300 rpm?

[Bob Johnson]

This might not be what is happening with your car. I had a similar problem. The cause was the engine mounts. When I installed the engine I tightened the rear engine mounts with the front of the engine not seated down all the way. Thus the front of the engine was not "floating" on the front engine mount. I loosened the rear engine mounts and the front of the engine dropped down. I re-tightened the rear mounts and the vibration calmed down.



If you push down on the front of the engine the front mount springs should compress a little.



Bob

[Marshall V. Daut]

Could you have a bad tire, perhaps split open on the inside of the wheel where it can't be seen? That could cause a shudder throughout the entire car, especially the rear tires when they are essentially coasting downhill and not pulling the car forward where torque might overcome any tendency to shake.
Marshall

[Will N]

Every Model A I've ever ridden in has vibrated while engine-braking.

[Big hammer]

Yep ! Isolation of the normal vibrations is the key, the exhaust rear clamp is a major problem fixable with a modern hanger, cracked engine splash pans is another

[nkaminar]

Bob Johnson had a good response. I had a similar problem with vibration when using the engine for braking, about the same RPM. I have the Float-A-Motor rear engine mounts. My solution was to loosen up the bolts that go through the rubber cushions on the mounts. That helped a lot. I still get some of the vibration but not nearly as bad.

The lightened flywheel does not help. I have that too. The crankshaft torsional vibrations will be felt in the flywheel when a lightened flywheel is used. With the original 65 pound flywheel, the node point for the torsional vibration is right at the flywheel. With the lightened flywheel it moves forward so that now the flywheel is vibrating.

[Gene F]

Originally Posted by Bob Johnson

This might not be what is happening with your car. I had a similar problem. The cause was the engine mounts. When I installed the engine I tightened the rear engine mounts with the front of the engine not seated down all the way. Thus the front of the engine was not "floating" on the front engine mount. I loosened the rear engine mounts and the front of the engine dropped down. I re-tightened the rear mounts and the vibration calmed down.



If you push down on the front of the engine the front mount springs should compress a little.



Bob

I leave my rear bolts about 3/4 turn from fully tight, just for that reason. When ever I change oil, and lube I check them. They never seem to move.

[Benson]

There is a known problem with those UJoints:

I do not know if this causes your problem but U Joint is wrong shape and size to work in a Model A clamshell.

Here is what BAAP said on the internet in 2021:

There was an issue with some of the bob drake u-joints, and that is why Bratton's stopped selling them for Model A's. Yes the splines of the u-joint are the same up to the lincoln models, but the issue is the diameter of the u-joint on the later model cars were wider. If I remember correctly the Drake u-joint was based off this wider design lincoln u-joint. So yes it will fit the splines on both the transmission and drive shaft of the Model A, but you might find the diameter is too wide and the joint will hit the inside of the u-joint cover. I'm not sure if this was corrected on later runs or not.

[katy]

Originally Posted by Gene F

I leave my rear bolts about 3/4 turn from fully tight, just for that reason. When ever I change oil, and lube I check them. They never seem to move.

What type of rear motor mounts do you have? Stock? Or FAM?

[Gene F]

I have those stock ones. The ones that are the big heavy steel housings that attach to the frame of the car, and have the two long bolts that go into the flywheel housing.

[Ruth]

I reading (on Ford barn? that some of the new u-joints were off center, causing vibrations.. Not sure.

I do know about the bigger Drake u-joints hitting the inside of the clamshell. A friend bought a new u-joint when he installed his Mitchell it hit. We ended up rebuilding his old one.

[Lawrie]

I had the same vibe in my A, over years, it had a few different engines in, but still had the rib,it was worse with and empty fuel tank.
I too tried all things but found it was cracked rear engine mounts,I made up my own set using new V8 eng mounts, finally the vibe is gone.
Lawrie

[ErnieF]

Sorry to be slow to respond, my mother had to be admitted to the hospital and life is full at the moment.

Originally Posted by Bob Johnson

If you push down on the front of the engine the front mount springs should compress a little.

Everything in the front seems seated properly. On the suggestion of others, I loosened the bolts of the rear motor mounts. I think that helped some, but wasn't a complete solution.

Originally Posted by Marshall V. Daut

Could you have a bad tire, perhaps split open on the inside of the wheel where it can't be seen?

Rear tires are new, front tires are same as new. New can still be defective, but it really seems to be rpms and not speed that causes the vibration. I can push in the clutch to drop my rpms but not change my speed, and the vibration goes away. It seems to me if it was tires, clutch or no wouldn't matter.

Originally Posted by Big hammer

Yep ! Isolation of the normal vibrations is the key, the exhaust rear clamp is a major problem fixable with a modern hanger, cracked engine splash pans is another

A lot of people are saying some holdback vibration is normal, but this seems excessive. I just put on an Ares muffler, and they come with their own rear clamp that is fiberglass lined to allow the muffler to slide. I don't have engine splash pans on my car.

Originally Posted by nkaminar

I have the Float-A-Motor rear engine mounts. My solution was to loosen up the bolts that go through the rubber cushions on the mounts...

The lightened flywheel does not help...

A number of people have suggested I try the FAM. I had originally resisted the idea, but I did try loosening the bolts of the rear motor mounts on my originals. As I said earlier in this post, I think it helped some, though it wasn't a complete solution.

Interesting thoughts on the flywheel. I'll explain below, but I'll soon have access to a car with a Burtz engine identical to mine, but it will have the original flywheel and clutch/pressure plate. It will be quite interesting to drive it and see how it compares to mine.

Originally Posted by Benson

There is a known problem with those UJoints:
I do not know if this causes your problem but U Joint is wrong shape and size to work in a Model A clamshell.

I had also heard that was a problem for a while, but it is my understanding that this was corrected in the later runs. Mine has a 2023 manufacture date. Regardless, I have checked that and mine fit tight and is not hitting anywhere.

Originally Posted by Lawrie

I too tried all things but found it was cracked rear engine mounts

Glad you found your problem! I've checked mine, and found no cracks.

Some additional information to add to all this. I did a compression test earlier this evening, and all four cylinders showed 80 pounds. I got the brilliant idea to show visually the vibration of the engine by setting a jar of water on the bell housing in front of the transmission. Overall, I think I'm happy with the engine when it's just sitting there with me adjusting the throttle. That said, I've never heard another Burtz engine in person to have any comparison. Of note, there is a slight ticking noise (perhaps more of a pinging sound) you can hear in the video if you listen closely. I haven't yet figured out what that is or where it's coming from. And unfortunately, I cannot remember for sure if I heard it before I installed the Mitchell or not. It sounds like it's coming from the bell housing area, but I can't fathom anything in there that could be doing that. I just had it apart to install the transmission and nothing seemed amiss. I still can't post the link here, but it's in one of my replies on the VFF thread.

As it happens, a friend's daughter decided she wanted a Model A as her first car, so I have a 29 phaeton sitting in my shop. It is getting a Burtz engine with an almost identical configuration to mine save it will have the original flywheel, pressure plate, and clutch instead of the lightened flywheel and the v8 clutch/pressure plate. The original flywheel and pressure plate for that car do have match marks showing where they have been previously balanced. My plan at the moment is to put my car on the back burner. I feel like I can drive it and enjoy it for now, and just try to avoid the scenario where it vibrates the most. I'll go ahead and finish the Burtz for this other car and finish rebuilding whatever it needs. Once it's together, I can test drive it and have some comparison to mine, and then decide from there what my next steps should be on my car. Sometimes I think, this vibration can't be normal and I need to keep digging to find it. Other times, I think I'm tried of digging and just want to get out and drive. That makes it much easier to lean toward the voices saying this is just how these cars are. I'm still open to new thoughts if anyone comes up something. But otherwise, I will report back once I have the phaeton going. It may take me a little while, but I'll definitely post what I find.​