The Stovebolt Site Forums The Shop Area The HiPo Shop Chevy 350 vibration

My buddy's 350 in his 39 has developed a bad vibration at about 1100 rpm. At idle and above 1100 rpm it smooths out. Seems to run great and gets ok gas mileage. We have done the following trying to fix it.
Replaced HEI distributor
Replaced spark plugs and wires
Timed it.
Checked vacuum...20 inHg
Checked booster on power brakes
Checked balancer
Checked all vacuum lines and PVC valve
Disconnected torque converter to flex plate. No joy
Checked motor mounts
Checked compression...around 170 to 180 psi on all cylinders
Looked for missing hardware on flex plate
What the heck are we missing?
Thanks.

No cracks in the flex plate where it bolts to the crank?

try it with the belts off, no telling what a bad pump or alternator might do

Bill

Ok, we tried the belts off things, no joy. Got the engine on #1 cylinder and looked at the damper, no joy. Looked for a possible partially blocked muffler by hooking a vacuum gauge to the intake and running at 200rpm for 30 seconds, no joy. Also looked very closely at the flex plate, no joy.
Anyone ever have this problem or have a possible solution. Thanks

Inspect the harmonic balancer again. Had one of those on a 2.8l that appeared to be good but the rubber was actually separated from the metal.

What year is the motor?? I remember back in the late 80's, early 90's our Chevy small block fleet was losing cam lobes left and right. Seems they made a bad batch of cams. Hows the needle on the vacuum gauge doing when at idle?

The block is older. Engine was rebuilt about 5 years ago and has 16,000 miles since then. Crank was turned and rest of rotating parts were new. Edlebrock aluminum heads.
We have already been through the ZDDP problem as I toasted a motor about six years ago. We both run "Brad Penn" oil now.
FriedGreenTmater...We got the motor up to TDC on #1 cylinder. The timing mark on the balancer was setting at the '0' mark on the timimg tab. The logic being...the inner hub can't move, and if the outer piece had moved the timing mark would be off, right?

No not necessarily, the rubber binding the two parts together could be broke on part of it causing it to be out of balance.

Try prying out on it in different areas around the circumference.

Everything FriedGreen says about balancer and how hard it is to detect is also true for motor mounts. If one is "sheared", the weight of engine will keep it looking good to the eye. Some component is broken and is reaching a certain natural frequency, then passing back out of that frequency (harmonic balancer, motor mount, maybe even a misfire caused shake) Hope your buddy did not just do some kind of work on engine like put in an externally balanced component or some other no joy.

I would second or third the checking of the balancer. If it is indeed good, then check the fly wheel closer.

Was the engine made in 1985 or earlier? Two piece rear main seal?

Or is it a one piece rear main seal? They take different fly wheels. Maybe a weight was tossed off the fly wheel.

The flywheel won't both up from one piece (external balance) to 2 piece (internal balance) engines.
However, used to see plenty of issues with small block 400 engine being external balanced uses differant flex plate & damper than all other 2 piece main seal small blocks but fit like a glove!

Along with the motor mount might want to look the transmission mount over real good. Seen small gravel get jamed into a mount & cause a ground out. Seen rocks jammed between crossmembers & transmission before.

Guys,
This engine doesn't have a flywheel, it has a flexplate. We pulled the three bolts that hook it to the tranny to eliminate it and the torque converter as a problem.
No two piece seal.
We will do the pry bar test on the balancer and look even harder at the mounts.
No changes were made to engine prior to problem appearing.
Will get back to you soon, and again thanks for all the help.

one more thing to check, make sure the key didn't shear and the balancer spun on the crank. Ive never seen it on a sbc, but newer Chrysler cars do it once and awhile. Cause's all kinds of wacky things to happen

Motor mounts and trans mount are easy to check. With the hood up, engine running AND YOUR FOOT ON THE BRAKE, put your trans in reverse and rev the motor. Look for the motor to swing from side to side. Then put the trans in drive and do the same. If that motor pops up and down during this procedure, you probably have a broken motor mount. If, during this procedure, you hear a good thump at the trans tunnel, you probably have a broken trans mount.

Oh no. Another thread left hanging.

I have what seems like the same issue as the OP (beltfed) had and have gone through many of the same steps with no change to the vibration. I was hoping that he would post back when he resolved his issue.

I'm gonna go ahead and post a reply -- knowing this thread is over a year old -- in the hopes that he might have set up email notification and will see this and come back with what he found and what he did to eliminate the vibration.

Howard
Seattle, WA

I must say I'm sorry for not reporting back as to the problem solver. We pulled the intake manifold and found it was leaking in a number of different places. He had used a Fel-pro gasket on original installation and it was not up to the task. He replaced it with a very expensive GM gasket (about $90.00). Problem solved.
This one had everyone scratching their heads

A vac leak caused this? I've had my flex plate crack into 3 pieces and cause this.

Like I said, it was a real head scratcher. While the problem was ongoing, we attended the NSRA Nat South. My buddy took the car to the Mich. Street Rod Assoc. repair shop at the show. They tried a little of everything and couldn't figure it out. Finally someone suggested putting gobs of RTV around the intake manifold as a 'why not try it' fix. Let the car set overnight to dry. It then ran slightly better. Then we knew we were on to something. When we pulled the intake it was leaking many places. He decided to go with the big dollar gasket to solve the problem once and for all. If and when I pull my 350 intake, I'm going to bite the same bullet and buy that gasket.
As a side bar. I have followed this car for many miles going to far away shows. The car after sitting at a red light would put oil smoke out the right side on pull away (not much smoke). We just returned from the NSRA Nationals. I noticed that the smoke is now gone. Don't know why the new gasket would fix that but the smoke is gone.

The underside of the gasket is exposed to the valve valley, which can draw oil into the intake ports. I remember seeing a NASCAR race at Talladega where one of the cars would blow a 100-foot long plume of oil smoke on caution laps, and then clear up at speed. Some sneaky crew chief had built in an intentional air leak to cheat on the restrictor plate rules. I did something similar on my claimer engines by drilling holes in the Allen bolts that held the carburetor adapter to the intake manifold, allowing a little air to bypass the 350 CFM 2-barrel Holley carb.
Jerry