RazerRay

Well I finally got me a corvette.I always dreamed of having one My is a 1976 stingray I got it at a great deal because it has a vibration in the motor The guy I bought it from said it has a 383 stroker in it. It just don't run like a 383. I was told to look at the harmonic balancer and the
fly wheel it is a automatic I just really need some advice on how to trouble shoot the problem and how to fix it I going to try and fix it my self if possible That's rite im one of the cool kids now I own a corvette

jim-81

Congrats on the new Vette! Now the fun begins. Yes that could be an incorrect balancer/damper or flex plate. Depending on the internals of the engine both of these need to be weighted. You need to find out if the engine is internally balanced or not, need the specs on the engine... crank, manufacturer, details. If you don't have that info it's gonna be hard to determine the proper balancer and flex plate. By the way if it's an automatic then you have a flex plate on the back not a flywheel (terminology). For example my Blueprint 383 is EXTERNALLY balanced which means the balancing of the engine/crank is done with the front balancer/damper and rear flywheel (I have a stick). Higher end engines may be INTERNALLY balanced which means the balancer and flywheel/flex plate are NEUTRAL weighted.

If this is the problem... !

RazerRay

I will look at the flex plate and balancer an try an confirm the displacement and let you know what I find

jackson

Maybe wrong damper and / or flexplate ... maybe a dead / dying cylinder ... I'd first rule out a cylinder.

Up until about 1986, and with sole exception of sbc 400, all OE SBC are Internally Balanced, neutral balance front & rear; no counterweight front or rear ...
... OE sbc 400 is Externally Balanced and has counterweights both front & rear. Until about 1986, ALL of those motors have 2 piece rear main seal.

Then, about 1986, ALL OE SBC received One-piece rear main seal AND they all have a "hybrid" balance because the front is neutral but rear has a counterweight.
Sole exception is the aftermarket Chevy/GM Performance OE 383 which is Externally Balanced and requires counterweights both front & rear.

Regardless of size, ANY SBC can built entirely Internally / neutral balance.

If you can find your Block Casting Number, we can tell if it's One or Two piece RMS ... if it's One, it's near-certainty it requires a counterweighted flexplate.

To determine if it's a 383, I don't know how to tell without first pulling a head or oil pan and then measuring stroke. ...
... some regional racetracks' tech/scrutineering shed have a test "pump" that can determine cylinder volume ... takes about 10-15 minutes.
https://www.speedwaymotors.com/Speed...-Tool,703.html
https://static.speedwaymotors.com/pdf/550-3200.pdf

derekderek

google 400 sbc balancer to get a pic of the counterweight. the starter wheel (auto trans) has a hunk of metal welded on. get car up and get under. turn motor with wrench or bump starter to see which type. you will also be able to see back of crank to tell one or two piece seal. after removing dust cover of course. still doesn't tell you what stroke though. pan is pretty easy to drop on vettes. easier than head. cheaper gasket too...

derekderek

if wrong flex plate, engine or trans has to come out. 200 buck engine hoist is cheaper than paying somebody to do it. if pulling engine do compression test first. then consider pulling heads and cam and finding out just what this engine is.

jackson

Lotsa guys on here can spot from across the shop the business end of a crank and know if it's 1 or 2 piece rms ... Not all though ...
... there're surely some here who'd hardly know the back of a crank from back of their hand.

I've worked with brilliant 15 year old late model circletrackers who don't know a cam's heel from its nose ... they couldn't care less ... no matter; they're bad fast.

OldCarBum

My questions are;
What kind of vibration are you experiencing?
This could be a number of issues depending on what type of vibration.
When do you feel the vibration?
At idle, at a certain rpm, wide open throttle, When?
Does the vibration come and go as the rpm changes?
Better at some rpm, worse at others?
Constant?
Does the pulse of the vibration stay constant or does it increase or slow with rpm change.
Explain the vibration you feel.
Do you feel the vibration when you are in park, when driving, stopped, accelerating, decelerating?
Did the prior owner tell you the problem just started, or did it start when he swapped or built the engine?

RazerRay

I check the stroke and it is 3.75 so its a 383 because the block number matches the vin number an the internet also says its a 350 the fly wheel has no weight add and the damper is smooth all the way around so I think I need to change them what would be a good flex plate and damper to purchase and what diameter damper should I get and do you think i may be wrong because it has happened before if not what is a good brand to buy

RazerRay

the back of the crank has what looks like a small counter weight on one side made as part of the crank shaft

MelWff

before you start ripping into the motor, remove the engine fan belt and see if the vibration goes away when the fan isnt spinning.

OldCarBum

Don't just start dumping time and money into major components until you can isolate the vibration.

RazerRay

the fan was the first thing I eliminated I dropped the transmission to look at the crank and flexplate the back of the crank looks just like a 400 crank a buddy had The flexplate has no counter weight welded on and the harmonic balancer is smooth all the way around with no metal taken out

jim-81

I believe that makes it a one piece rear main seal (RMS) block. Still doesn't determine whether it's internally or externally balanced or not. Experts here will chime in. Not sure how you would find out! Unless you could get some documentation on the engine - receipts etc... Or I guess you could just throw a weighted balancer and flexplate on it and see what happens lol.

RazerRay

thank you all I need is a little guidance my wife wrote the number down wrong the first time sorry she ant to happy about my new toy so here it is the vin #1z37x6s433074
engine code on front of block #14r416309 T1119CMP

OldCarBum

Vin and engine code won't help too much if the engine has been rebuilt and stroked to 383 cu in.
Can you find out from the prior owner who built it or if it was a create engine who manufactured it?

jackson

OP
ALL OE sbc 400 have 2 piece RMS.

Older 2 pc RMS BLOCKS have crank with Six bolts at flexplate flange AND Larger 3.58" bolt circle.
typical 2 pc RMS flange
https://www.ebay.com/itm/FOR-CHEVY-S....c100005.m1851

Newer 1 pc RMS BLOCKS have crank with Six bolts at flexplate flange AND Smaller 3.00" bolt circle.
typical 1 pc RMS flange
https://www.ebay.com/itm/CHEVY-350-5...IAAOSwZ8ZW~D8q

Both OE 350 & OE 400 cranks' 2 pc RMS flanges look quite alike.

? What is your BLOCK CASTING NUMBER?

?How-where did you measure stroke?

?You have pics of your observations?

RazerRay

I finally found the number on the crank its 3951925 can any one tell me if it requires to be externally balanced with a balancer and flex plate

jackson

OP
? How about answer the questions post #7 ? ... Answers might help us to help you.

3951925 not within my lists ... perhaps in someone else's resources

Pics are very important in these situations.

RazerRay

Sorry I miss typed the number not really text savy I measured the stroke by rotating the crank an measured the bottom of rod an rotated 180 deg an the steel ruler said 1” an 3/8