Technical Chasing a vibration-Solved

I know this is a lot if work, but you could disassemble the engine and have the crank balanced, check each rod big end weight, and the piston/ pin/rod little end weights. You could go cheap and reassemble with same rings and bearings if good condition. At same time have valve seats freshened up. Maybe you have a unmatched set of rods. Or cheap pistons with too much variation. Also take flexplate and dampener in with crank.
That would eliminate if the engine is out of balance and make it smoother than it was. May not eliminate your problem. but does ensure it isn't internal engine.

 

Did you check the flange on the back of the crank to make sure it is a 350 crank and not a 400 crank?
I'm simply wondering if the 350 you bought is actually a 400 or is a stroker.

I've seen a very similar vibration when the truck my brother thought he had bought with a 400 had had a 350 swapped into it before he got it. He had a 400 flex plate put on when he had the transmission rebuilt and it had a very similar vibration., That was pre internet and it took me and my students a couple of days of checking to finally figure it out and an air chisel to fix it.

 

Well, since you brought it up, try running it with the valve covers removed. Look at the inside of the covers and see if there's any witness marks from the rocker arms. Solid or hydraulic lifters?

If you're running a mechanical fuel pump remove it temporarily and see if anything sounds or feels different.

 

Chased a vibration on a buddy's 34 cabriolet. It was engine/rpm related and easy to duplicate but we just couldn't pinpoint it. I finally borrowed a Kent-Moore vibration analyzer. It basically told me what I already knew. However, I was able to move the pickup around and find the spot with the maximum amplitude and it was at the transmission mount. It turned out the lip of the cross member was against one side of the mount, but you couldn't see it. We relieved the lip and no more vibration.

 

The documentation I got with the motor showed it to be 30 over 350…seems to me the crank was 10 over but I will have to check. No other mention of modifications. Block number showed to be 350. Flex plate, converter and tranny were from a 1976 Canadian Pontiac with 350sbc. I am quite confident they are the right ones but anything is possible. One question, when I unbolted torque converter and slid it back I only had about 1/8” travel (to maybe 3/16”-didn’t measure unfortunately but it wasn’t a lot)…is this normal?
I think where I screwed up was painting it Chev blue…got the old Pontiac pissed off.

 

I think you're looking for a solution to a problem that isn't there.

 

Claymart…just your run of the mill 350 with hydraulics. Runs quiet and no rattles,ticks,etc.
Jaracer…I was crawling around under it looking for any interference, did have a couple close tolerances but I clearanced everything I could find including trans mount. I will look all that over again. To be honest, when I first put it together with the 350 I assumed the vibration was minor contact so didn’t stress as I was more focused on all the other stuff.
Thanks for the suggestions!

 

427sleeper…like my neighbour suggested-nothing a couple burnouts can’t fix, except being retired I can’t afford the tire (not posi haha).

 

I've no idea if it would actually prove out, but there are some different engine tests that can be done, e.g. "cylinder contribution" that aims to see if each jug is doing its fair share of the work, by grounding out individual cylinder ignition wires and noting the RPM drop. Another torture test is to run on two cylinders at a time, each pair should produce about the same RPM. Point being anyway I'd want to try stuff like that before tearing the engine apart?

Vibrations can be really tough to find sometimes. I know on the trucks, they have relatively long driveshafts, or two piece with hanger bearings. These d/s bearings are always trashed, or the d/s is missing balance weights, or the d/s isn't clocked or indexed correctly.

With any rotating assembly There's "balanced" (within spec, or barely) and then there's balanced it is amazing just how quiet and smooth a car or truck can ride when care is taken in wheels & tires and the rest of it.

 

Mr48chev. I will confirm the crank once I get it back on stands next week. Thanks for the chart!

 

Truck64…yes, I was going to head down the path of pulling wires to see if one cylinder was not contributing until I checked temps on the manifolds at each port and they were virtually the same for all. Actually surprised me that they were so close. Still might try pulling wires to look for differences.

 

I'm just spitballin' on that idea, maybe that test would only reveal bad compression or something like that.

 

I had several 305 and 350 cranks on hand and went looking in my stash for a 350 crank to use in an engine I was assembling. Unfortunately I had never tagged the cranks when I removed them (years before) as I assumed the casting numbers would tell one from the other ... this (when I went looking for a 350 crank) is when I found out that GM used the same casting number for both cranks. Apparently (according to the internet) you can lay a straight edge across the notches in the weights to determine which was which. I tried that and wasn't comfortable with the results as I wasn't able to definitively determine to my satisfaction, which was which so I did the next best thing ... I knew for a fact that several of the 350's I had taken apart were painted orange and that all of the 305's were painted blue so I located a crank that had traces of orange paint on it. Although you can run a 305 crank in a 350, it ends up being a very expensive route due to the costly work needed to rebalance the 305 crank to work with the weight of the 350 pistons/rods (assuming one would get the 305 crank balanced). It is apparently much cheaper to start with a 350 crank. I ended up using one of the orange 350 cranks with a .020 block/rod/piston I had on hand. Engine runs great and is still in use in my daily 80's Regal.

Great story right

This got me thinking. There are specialty engine rebuilders who will rebuild an engine you bring to them and there are (or were) large volume rebuilders who sell rebuilt engines they source themselves. I assume the large volume engine rebuilders don't want the added expense of balancing a reciprocating assembly as it would hurt profitability. So ... what happens if their latest hire, we'll call him Sammy Simpleton, is tasked with assembling a 350 on a Monday morning (still hung over), goes to the crank pile and ends up grabbing a 305 crank by mistake simply because the casting numbers are identical? Could this be what you are experiencing? Is it possible that you do have a 350 but it has a 305 crank in it? For all I know, an unbalanced 305 crank in a 350 would knock the fillings out of your teeth or maybe it would be just what you are experiencing ... I just don't know how bad a vibration it would cause. Did the rebuild tag happen to state "Quality Control by Sam Simpleton"

I'd be looking elsewhere myself (something barely contacting the floor/exhaust/mounts etc ) for your vibration but just wanted to put this out there.

There was a thread not too long ago about a terrible grinding noise coming from a left rear wheel bearing. Turned out it was a parking brake cable occasionally contacting the driveshaft ... sometimes it is something easily overlooked. I wouldn't be in a rush to swap back to the six

 

I think its your Muffler Bearing that's giving you the whole Problem.!
it Depend's on the Type .?
I use Tefallon type some people use different kind.
So check your Muffler Bearing's
Ha Ha

Just my 3.5 cents

Live Learn & Die a Fool

 

Borntoloze…the rebuilder was a volume rebuilder so who knows…engine runs nice with no strange noises or anything and is still running after 5000 or so miles. I originally thought it was just some interference with the body/frame when I did the swap back in 1990 but decided to chase the vibration down now since I am retired and have time (but no money!). No panic on finding it other than the desire to get it nicer to drive, plus if it was something like a fan, damper etc. it is best to fix it right. Otherwise it might be like Ken Schrader used to say…don’t know what it is that broke but it must have been important…
I am not rushing back to the 235 but decided to see if I can get it back. It was a low mileage rebuild that ran smooth and quiet.
Thanks!

 

A 50merc…maybe the new silicon based muffler bearings would help…but I don’t think they are traditional.

 

You got another carb you could stick on there?. Might be some kind of mis-fire.
I would buy me one of the small tachs just so i could tell exactly what RPM

 

You read my mind…but I think I will just do a careful overhaul of this quadrajet as I know it hasn’t been messed with.
I also figured I would try testing for vacuum leaks around the carb base with propane.
Before I do either of those I an going to spend a bit of time making sure the exhaust and mounts are not the cause.
Thanks!

 

I vote for an exhaust pipe clamped too tight against the body. If not, you have an out of balance engine.






Bones

 

I'm still stuck on the mounts any or all of them. It could be as simple as the through bolt on the motor mounts up against the frame or as said before a mount rubbing on the frame.
One more thing, are the transmission to engine bolts all tight? It happened to be me once, to much late night thrashing.

 

Well i just tracked down an annoying buzz of sorts. But mine is a 51 Chevy, 261, splits. I thought the same, pulled fan belt, etc, no luck. Put car on ramps so it was still sitting on its weight. Got the noise to go with a pribar on gas pedal. Get under and touch stuff. Exhaust gets hot tho. Lol. Burt my elbow. But surprisingly found 1 exhaust hager a half turn loose where i have it boted to cross member. Tightened and no noise since. Knock on wood.... i have about 8 hangers by the way. Dont overlook the small stuff. Get uder it and touch, feel and use a stelphascope if need be. Good luck. Keep us posted

 

Engine to trans bolts are tight. I still have suspicions about those biscuit mounts as well. Pretty confident trans mount is good and clear of interference. I removed trans access panel and checked everything for interference. Started it with access panel off and vibration still there. Put my hand on trans and other hand on steering wheel…feels like wheel has more vibration than trans. This leads me to think interference for mounts, exhaust or some other engine/trans component with the frame or body. Hope to dig deeper this weekend.
Sure appreciate all of the suggestions!

 

Thanks for the input getow….I will be going under it for sure. I am preparing a check off list starting at the back of car and going forward. I sure hope it is an exhaust issue….easiest to fix.

 

Ever drive a car with a lokar shifter and experience the lokar buzz?

 

31Vicky…never have. I might have just found something!! The transmission dipstick tube appears to touch the firewall in one spot so I will pursue that tomorrow. I have checked the tube previously but this time I ran a wire up along the tube and there is one spot where it is a no-go. Hope this is it but don’t want to get ahead of myself here. I will update once it is clearanced.
Thanks all!

 

easy check, stick rag behind it.

 

Nope..not the problem. It was just the clip holding vacuum line to trans tube and it didn’t make any difference when clearanced…damn

 

Biscuit mounts don't offer the isolation of a stock chevy mount

 

Sunbeam…that is part of what I am wondering. Am I expecting too much from these mounts? It is an old car, it is going to vibrate and make noises. I just would like to minimize that if possible.
Right now I feel like my dog does when he gets all excited chasing his tail haha.

 

Inject some humour here