Technical Chasing a vibration-Solved

I’m with Claymart. Get a short bolt and put in in a hole of the flywheel if there is one. It seems like they usually are. Make sure it doesn’t hit any thing all the way around and see if it changes anything. Cheap easy way to check balance. May have to bolt a small pice of steel on it.
I had a 300 Ford six with C4, that had a vibration for years, when I pulled it out I could see that the torque converter was machined crooked! Replaced it….smooth as silk!








Bones

 

Everything has a natural vibration frequency where it will shake the most if subjected to that frequency. A guitar string is tuned to vibrate at a certain frequency (the note) and if not tuned right will vibrate at the wrong frequency. It will not vibrate until plucked however. Suspensions have a natural vibration frequency where they will vibrate (shake) so an out of balance tire at the right speed will "pluck the string" so to speak and the suspension will transmit the vibration as it shakes. So if the natural vibration frequency is 12 cycles per second (Hz) a tire turning at 60 mph is producing about 12 Hz and that's where you will feel the shake the most. At 40 mph the tire is still out of balance but it is not felt because the suspension frequency will not oscillate as easily at this speed. Engine systems also are subject to this natural frequency of vibration, this includes mounts, accessory's turning on the engine, exhaust, etc. If anything "plucks the string " at the right frequency you will feel the vibration. If nothing is out of balance there will be nothing to start the vibration going so nothing will be felt. Sorry for the long explanation but there are books written on this subject and it drives engineers crazy tuning this out of a new car. A good example would be the cylinder de-activation on the 5.7 Chrysler Hemi. In 8 cylinder mode the exhaust produces a certain frequency at cruise but when the engine goes to 4 cylinder mode at cruise the frequency changes by half. The stock exhaust system is designed to cancel out bad drone sounds at both frequency's but when aftermarket exhaust is installed the exhaust sounds good in 8 cylinder mode but sounds horrible when 4 cylinder mode activates (exhaust drone) and inside the car sounds like crap and can cause vibrations in the floor and other items.

 

Rule out hard science and stick with the basics. Ever feel an engine that had the wrong flex plate installed? It's not pretty at an idle and sure as in hell gets worse with rpm. All I'm referring to is in regards to comments made that OP's engine is either balanced wrong or wrong flex plate.

 

Yeah, resonance points are a big deal. That's why they are called "harmonic" balancers. There is a natural RPM resonance point, but, ALSO multiples of that frequency (RPM) are resonance points. In the case of the 60 mph tire shake, it might show up again (worse) at 120 mph, and 180 mph etc. The engineers try to have engine resonance points occur somewhere other than where most people spend their time driving.

 

That sounds like something ground out like a motor mount. How about in reverse does it go away if you power brake it?

 

I won’t be able to continue with my experiments until later this week and will continue to update as I go. I appreciate everyone chiming in. Just for clarity, these are the front engine mounts

 

^^^^^ How close too the steering box is the left engine mount as compared to where the original inline 6 was?

Why were these used in later models?

 

Thanks!....

 

Original 6 was mounted to front crossmember with mounts like these below. I do wish these cars had a separate column from the box.

 

You initially said "anything" so .... you can't go back now and say "but springs and shocks change things"

 

51, I will say this, if you look at a stock 305 cast crank , early or late ,you see about 5 holes drilled in the front counterweight. on a 350 only 3, usuually in the middle possibly in the rear also. in a mass rebuilt the pistons are always heavy. Many variables apply. i have dealt with a number of cases like yours. In a GM parts catalog 305 and 350 cranks have Different part numbers. Pete.

 

Totally depends on the amount of “ out of balance” it is. Even stock OEM engines are a little out of balance. That is why people have engines “ balanced” . Like the torque converter in my situation, any part can be defective!






Bones

 

Thanks Pete. As mentioned I am going to try to eliminate the possibilities of external issues like misfire before I look internal, although I suspect it is internal. My time is cheap these days and it is enjoyable to refresh my brain on things like timing, carbs, etc. I have built or overhauled several engines over the past 50 or so years but until I retired 2 years ago my focus was on my regular job and that old adage that “ if you don’t use it you lose it “ is true. Can’t believe how many little techniques and details I had /have forgotten! I have tried or at least seriously considered each of the suggestions posted.
Bones…I do think I have a minor internal vibration but it seems to be amplified by the front mounts. As well, I do think I have an intermittent misfire that complicates the issue so I will try to address the external issues before attacking the internals. I am searching my garage as we speak to see if I can find something to use as a temporary motor mount cushion just to see if it changes the magnitude of the vibration.
Cheers

 

Is the steering rag joint all wore out?????.... Just curious...

 

It isn't the GM part number from a catalogue that confuses things, it's the fact that both part numbers use the same cranks with identical casting numbers. Part# A is machined or 'adjusted' to work with the 305 piston/rod combo and Part# B is machined or 'adjusted' to work with the 350 piston/rod combo. Although there are two distinct GM part numbers, they both started with the same casting. The question here is, how does one determine the difference between a 305 or a 350 crank when the order tag is no longer dangling off the end of it?

The chances of having a 305 crank in a 350 may be like "I in a million" but like Jim Carrey said in Dumb and Dumber ... "So you're telling me there's a chance"

 

Dueces… this car is one where the column is mounted directly to the steering box so no rag joint. I think the fact everything is solid means any vibration transferred to the frame is amplified through the car…hence my obsession with the front mounts. I found some softer and smaller biscuits (see below) that I am going to temporarily try once I get the car back up on stands later this week.

 

Cool!.... Thanks for the info...

 

Dang, do you have a long term plan to stick in a rag joint? That must be really annoying.

 

You are on the right track with the softer mounts.

 

49…it wasn’t an issue at all with the 235…just this shaker
BobSS396… it is worth a try. Spent some time with a card against tailpipe (single exhaust) and noticed a small and somewhat intermittent miss at idle…just an occasional variation in the exhaust so I am also going to check that out further. Too damn hot outside to work on it past noon.

 

Noticed your motor mounts.Those mounts should have another large rubber washer on the bottom side and a shouldered bolt from the bottom to the top with a nut.
Seeing a bolt going down has me wondering what is on the underside.

 

Hoop…they have the large rubber mount, flat washer and locknut below. I made sure they are seated properly and right now they are snug…but I have experimented with everything from very tight to finger tight. Thanks for checking!

 

I thought I might be on to something. Hope you find it soon.

 

I am curious if anyone else has this 350/350/chassis engineering mount setup with stock steering and suspension in their fleetlines. It would be nice to hear that this mount setup can be smooth. I have held my hand around the mount and blipped the throttle…little to no movement…felt like a solid mount which is part of the reason I keep coming back to it. Hopefully be able to get back up on stands tomorrow and try the soft mount to see how different it is.

 

Just a couple observations.

Purchased a V8 conversion kit in the 90's Trans-Dapt 4196 mounts, or similar. When new the cushions were supple and pliable, now they are closer to rock hard. This conversion went unuesed and has sat in the basement shelf for ~20+years. One of the older fellas at the speed shop told me if the upper conical cup starts to touch the lower centrical cup that was how you could tell it was time to change them. No idea if this was true. I've seen plenty of this style mount used without the conical covers and flattened.

When you noted the engine is a late 70's model I thought of all the 305's I've encountered. A/G/F body or truck. Q-Jet, TBI, or TPI. They all had a 'thrum' around 2K rpm in Park/Neutral. I just call it the '305' shake. Rpms wouldn't change and power seemed consistent, but that thrum could be annoying. It was really bad in a '84 Sierra long bed, especially if both tanks were low. If the engine was below or above the rpm it was fine but if you held it at the rpm it would have a weird thrumming, and it would shake the column/steering wheel. Did compression test, full tuneup, etc etc. Even went and replaced the intake manifold because a casting defect on the original looked more like a crack and was wondering if it was causing a lean misfire. No crack, just a casting artifact. Truck was factory stock and mostly original. It has the correct damper on it as it had the goofy 12 O'Clock marked on the damper/timing cover that you you could only see by looking down behind the water pump with a timing light. A PITA if there ever was one. I never did replace the mounts, they were the 80's style bolted to the frame with a through bolt bracket on the engine type.

 

Thanks Mike…I remembered this setup as always having a vibration but it does seem worse now. Makes sense that the mounts have turned to hockey pucks over 30 years…time to try some new ones I guess.

 

As the title suggests, I have…with help from the HAMB…solved the vibration issue. As I mentioned in my last post, I swapped out the old Chassis Engineering biscuits for some temporary softer biscuits…fired it up and vibration is basically gone. Obviously these temporary mounts are not suitable for anything but a test but I now know exactly what the problem was. The old biscuits are so hard they could be used as hockey pucks.
Does anyone have any recent good experiences with some of the biscuits currently on the market?
Anyway, thanks everyone!!! I am going to enjoy Friday night.
Cheers

 

Glad to hear. Enjoy driving now.

 

Or you've found a method to hide the symptoms of another problem that still is there. Hard to know the difference.

 

Gson…very true. I will keep working on the intermittent misfire but at least now I know that the installation should work. At first it felt like it could have been any of a dozen issues. One by one they have been remedied or eliminated as a possibility. As I mentioned early on, the engine actually felt quite smooth running…but the vibration seemed amplified in the body and steering. It is smooth now, not like a new car but not annoying.