Technical Chasing a vibration-Solved

I’ve never known anything out of balance getting better with more speed.

 

You never met my old classmates….black beauties, bennies and eye poppers.

 

Exactly..... It will get worse.

Have you done a compression test? Someone suggested a cylinder balance test. I would try that also. Simply pull one plug wire at a time at the distributor to see if a cylinder isn't contributing.

You may have a valve that isn't adjusted right. It sounds more and more like you have a misfire problem, not a balance problem. You usually won't feel an out of balance engine vibrating at that low of an rpm unless it is way out of balance and that would shake you out of the car!

 

Thanks for the responses. I am going to pursue the misfire theory over the next few days. It was just an off the shelf rebuild. Just noticed it was 1991 that I did the swap, not 1990.
Block numbers are 761313379 K03 31 CMJ 8554 which I believe show it as a GM of Canada 1976 350.
I think I have eliminated most possible sources other than internal or some misfire. The steady vacuum in a reasonable range is interesting if it does have a misfire.

 

I really like the sound of this. It would be fairly easy to add a couple of washers to one converter bolt at a time, fire it up and see if the vibration changes or gets better or worse. Maybe put a little dab of paint on the converter and flex plate to make sure they don't get out of phase.

 

Claymart…flex plate did not have any weights so should be correct for 350. I do think I see how it could help me evaluate if it is a balance issue as it should either get worse or better, at least change a bit. If no real change then it would lead one to look back into engine/body/frame contact. Thanks for the idea.

 

. . . Couldn't have said it better myself. It does my heart good to see someone take a more logical approach to this kind of troubleshooting instead of just throwing their wallet at it.

 

My wallet is a little too thin to throw…it would just blow away!

 

does the lower column shifter lever need to travel further for a th350 over a powerglide

 

I adapted the original 3 on the tree shift arm to work for both the power glide and the turbo 350

 

No, it's wires run next to each other for a long distance that causes the problem. The magnetic field in one wire induces a voltage in the adjacent wire when it collapses. Crossing the wires prevents this from happening. That's why signal wires like a vehicle speed signal are always twisted.

 

If you had access to the old Stewart Warner strobe style wheel balancer, you can use it to balance an engine in the car. I worked with a guy who was quite a mechanic and he used the wheel balancer to balance drive shafts and engines. It was common for someone to replace a 318 torque converter in a car with a 360. The 360 converter has a large balance weight. Paul made number marks on the converter and used the strobe to find the heavy spot. He rotated the engine 180 degreed and spot welded a weight from a junk 360 convertor on the 318 converter. This was all done in the car.

 

Look under the hood at your wires at night to see if there is any spark jumping. My $.02.

 

Small block MOPAR V8's and the v10's had Technical service bulletins to correct a missfire caused by induction if wires were routed incorrectly so you are correct. Big block Fords in the 60's also had a problem they called crossfire, two plug wires had to be crossed to prevent this from happening under load. From what you describe your vibration as in neutral and park sounds like a first order crankshaft vibration. These usually show up the worst just off idle and get better over 1200 rpm or so only to start up again at a higher rpm. I used to teach classes for Chrysler corp. and we would put stick on weights inside the vibration damper to cause this type of problem so students could learn to use the vibration analyzer and dial in the frequency of the crankshaft vibration. Also had a 390 Ford rebuilt by a company in Portland and it shook right off idle. They finally went through the engine again and found where they had misplaced a decimal point in the balance calculations and the engine was balanced out of balance by 200 grams. You have done all the correct steps to isolate this in my opinion without having an electronic vibration analyzer or other equipment. Good luck with the back, I wasted mine 30+ years ago and surgery was successful.

 

To that end I'm not persuaded it's a misfire either (though I don't know ...)

BUT, I will say if someons wants to look for that there is nothing that will compare with an ignition oscilloscope in terms of NFA and/or saving time, if there is an ignition defect somewhere, it will show it instantly. See if you can find someone who has one, the SUN and ALLEN units get all the love but lots of other less expensive models were made. A few shops still use them that cater to old school iron.

 

GM put springs on the bolts for exhaust manifold to exhaust pipe for a reason. It does absorb vibration.

 

I read a few posts, and you kept saying the parts were new. The definition of New is Never Ever Worked. So the new part might be junk.

 

Do you have a Radio in that Car.???
Turn it up so the Only thing you Hear is the RADIO.!
I want to tell you a Little Story when I put my 327/300hp in my Merc.
My 63 Chevy Impala Sta.Wagon 9 Passenger Fully Loaded with
every Option was Hit Head on by a guy who was day dreaming at 85 miles per hour
The impact was my car was braking down to 30 mph so impact was105
I was going to Fix it, but I changed my mind, instead i put the Engine & Tranny
Powerglide in my 50 Merc.
There were No Vibrations with this Engine & Trany in my Mercury.!
I have had this Car up to 85 and No vibration, I run Radial Tires
That's all I wanted to Say.!

Just my 3.5 cent

Live Learn & Die A Fool

 

I just wanted to Say the Engine & Tran's have been in my Merc. for about 42 years.!

Just my 3.5 cents

Live Learn & Die a Fool

 

I appreciate these comments. I agree that it seems internal however since I can’t tear it down till Fall at the earliest I have decided to check out everything other than that just for fun as my hourly rate is pretty cheap haha.
Yesterday my focus was initially on two things, making sure the carb was set and working properly and looking very closely at motor / trans mounts.
I went back to basics on the carb, checking for air leaks at base with propane and double checking the idle/air mixture screws using a vacuum gauge. No leaks found and mixture screws set to achieve highest vacuum(17-18” and absolutely steady).
The mounts were easy to evaluate as I have the trans tunnel cover removed. Rear trans mount behave exactly like it should with it visually allowing some movement but not slopping around at all. While I was doing this I noticed no rotational movement of engine/trans when I blipped the throttle and there doesn’t see to be much movement at all in front mounts when I blip the throttle. My conclusion here is that these front biscuit mounts don’t really dampen vibration at all and basically transfer it to frame…right by original steering box. I should note that I adjusted the tightness of the mounts through bolt to just snug, not compressing the rubber really at all. Mount is assembled correctly with rubber above and below.
After doing this I sat down and read C9s old post here on the HAMB entitled “A Reprise on “Got Time””. This post reaffirmed so much of what I was taught by my dad and made me think about vacuum and timing. I tested the vacuum can on the HEI and it was good. I had been using ported vacuum for advance but decided to try manifold vacuum. Had to do minor readjustment of idle mixture screws to get back to max vacuum.
I am detecting a slight off idle misfire so I am going to pursue that deeper. I think if this engine was sitting in a stock 1976 Impala the soft factory mounts would mostly mask the vibrations but sitting in the 51 with this setup it tends to amplify them. As mentioned by stanlow69, new parts are not always good parts so I am going to refocus on the ignition system next. I don’t have access to any analyses or fancy tools but will do my best with my old timing light, multimeter, etc. Worst case scenario is that I have a nice running engine that vibrates. Like I said, I am enjoying the challenge at this point.
Hopefully I can check it out in the dark soon looking for arcing. It stays light up here pretty late.
Thanks for all the feedback.

 

If that engine is pulling 18" of steady vacuum at 3000' elevation it's almost impossible that manifold or gasket or vacuum advance leaks of any sort are a problem, not enough to cause the symptoms you describe here anyway.

Btw, I found an ignition oscilloscope for $40 on that auction site, they were damnably expensive 60 years ago, they aren't anymore. I didn't understand any of the finer points of using it at first, but I could tell in about the first 30 seconds after hooking it up that the ignition system on the engine in question was pretty screwed up. It quickly became one of my favorite tools.

Maybe that engine doesn't have a misfire problem, but this is a really good opportunity to buy more tools! I think you should buy one. We're all here to help you spend money.

New parts doesn't mean they are any good, somebody said, and that is exactly correct, especially these days.

 

Truck64…I will look around, can never have enough tools. Unfortunately living in Canada means stupid high shipping costs, even though I am less than an hour from Montana. I am going to carefully look at as many of the new parts as possible. I have a pretty good stock of “old” stuff that was taken off various vehicles as part of general maintenance like plugs and wires, caps and rotors so can play with that.
I agree on the vacuum, tells me the suck/squeeze/blow elements work ok so now I will focus on the bang part.

 

Isn't there an eBay set aside for Canucks? Find one local to you, beg borrow or steal it. Probably have to troll the nursing homes, somebody up there has one, eh?

 

We have a variety of options. I will look around a bit.

 

My Vacuum is 18 degrees & I run a Dual Point Tack Dist. GM which
I used to use in my Race Car L88 back in the Day!
My Plugs I set at 35 & Timing I set at about 38 Total and when you
adjust the Carb you turn the adjusting Screw in until it stunbles you bach
off until it Smooth' out
I hope this Helps, My Engine is complete Stock with the AFB Carb. on it.

Just my 3.5 cents

Live Learn & Die a Fool

 

I used a Hurst front Mount to put my Engine in & the Trans Mount is
OEM GM.
Mabey this will Help.?
Dom.

 

My initial thought was to agree with the above but then remembered that a tire imbalance is most noticeable at 60mph. Both above and below that speed, the vibration is less noticeable.

 

Yes. The cranks are identical, right down to having the same casting numbers. The only difference in the cranks is they are balanced differently to wok with the different weights of the rod/piston combo.

 

Does an engines rotating assembly use springs and shocks? This isn't a washing machine that has the capacity to a said degree of unbalance to cope with it.