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Big Block Engine Idling Vibration

B

brit_trader2003

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Hi folks from the UK, I need help.

I have a 69 with a Custom built Big Block engine. After a couple of months since the engine has been tuned the engine develops excessive vibration when idling. This has happened 3 times subsequent to the Carb being retuned within 500 miles.

The engine is new, but run in. It is a rebored 454 with forged steel domed pistons to give a Compression Ratio of 9.5:1, Edelbrock Performer high performance Hydraulic Camshaft and Intake Manifold. Holley 750 CFM Carb with Vacuum Secondaries, Hooker Headers with Side Pipes, Hood Scoop for Cold Air Induction. I have not tested the engine but it was designed to produce 450bhp at the wheels and 500 ft/lbs torque

The problem is that within 500 miles of the carb being tuned the vibration becomes bad when idling. It rocks the car. I have had the carb retuned 3 times and the problem goes away only to return. When tuned the vibration is much reduced and the engine sounds sweet. The idling speed is correct throughout just the vibration. Once moving the engine performs as normal (mind you it does vibrate all the time plus with the sidepipes it is LOUD)

I have done some research and I have a theory that the Big Block engine vibrates moving the Idling Mixture screws which then puts the Idling out of balance, more vibration and the problem gets worse.

I am guessing that the Edelbrock Performer Camshaft makes the vibration problem worse being a 'wild cam'.

Has anybody else had this problem and what is the solution ? It is tedious putting the car into a garage every 2 months so do I have to reset the Carb when the problem manifests itself, if so how do you do it. I have a Haynes Manual for Holley Carbs but it is not as clear as I would like.

Any advice or help would be very much appreciated. Unfortunately high performance Big Block expertise is a little rare in the UK.
 

maxrevs85

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709
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Eaton Rapids, Mich
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!985 ,blue coupe, 4+3 Z51
With the mods you have , tuning can be like a bit of a problem until you satisfy whats required. The carb being the major part of the tune. Jets , airbleeds and power vavle all need to be right for engine. An out of the box carb of any kind on a modded engine will never be quite right strraight out of the box. The idle curcuit is only used when the engine is at idle and butterflies are closed. First make sure timing, initial and total advance, are what you want, probably about 36* total. Find your manifold vacuum and then you cant select the proper power valve.Your jets will probably be in the mid 80's range.

Another option which was my choice is to find a pro carb tuner and send carb to them with ALL specs on your engine , gearing and trans. Talk to the person doing the work directly. I have a guy here in Mich that is great with the carbs.
Good luck,you'll get there.
Dave
 

74bigblock

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Barrington, IL
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2008 Z06
I had that problem in my 454 which is .030 over. and an Edelbrock 425 kit...


Make sure none of your plugs or wires are touching each other, the frame, or the steering column. I highly doubt your issue is fuel delivery related.
 

Jack

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Florence, SC (Timmonsville SC)
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71 War Bonnet Yellow VERT 71 BH Blue CPE (SOLD)
You tell us your motor is "custom built" ... and it's a "454".
Something to consider:
396 & 427 are "internal balance" motors.
454 is an "external Balance" motors.
Internal vs External torsional dampers/harmonic balancers look alot alike but are balanced quite differently ... external damper has an eccentric mass and is intentionally "out of balance" ... but is matched to an intentionally out of balanced crank ... it is designed that way. If you have the wrong damper it'll cause the motor to shake/vibrate. For example, if you have a chevy 454 crank and someone put a damper on it for a 396/427 ... it'll vibrate ... it'll eventually break the crank.

The tuning theories are worth looking into ... but first I'd check to make sure the "custom" builder matched the correct type damper to the crank. This difference does get overlooked and does happen.
JACK:gap
 

74bigblock

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2008 Z06
Jack said:
You tell us your motor is "custom built" ... and it's a "454".
Something to consider:
396 & 427 are "internal balance" motors.
454 is an "external Balance" motors.
Internal vs External torsional dampers/harmonic balancers look alot alike but are balanced quite differently ... external damper has an eccentric mass and is intentionally "out of balance" ... but is matched to an intentionally out of balanced crank ... it is designed that way. If you have the wrong damper it'll cause the motor to shake/vibrate. For example, if you have a chevy 454 crank and someone put a damper on it for a 396/427 ... it'll vibrate ... it'll eventually break the crank.

The tuning theories are worth looking into ... but first I'd check to make sure the "custom" builder matched the correct type damper to the crank. This difference does get overlooked and does happen.
JACK:gap

The problem goes away... therefore it cannot be an issue with the motor components... or it would be constant (out of balance like a washing machine)

It really seems like a fuel burn issue (lack of spark)... not burning all the gas, or not getting enough fuel or air (Carb).

What carb do you have on this motor and what is the CFM?
 

gmjunkie

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Messages
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Location
Old Fort NC/Vero Beach FL
Corvette
03 Z-51,78 L82 Coupe
Jack said:
You tell us your motor is "custom built" ... and it's a "454".
Something to consider:
396 & 427 are "internal balance" motors.
454 is an "external Balance" motors.
Internal vs External torsional dampers/harmonic balancers look alot alike but are balanced quite differently ... external damper has an eccentric mass and is intentionally "out of balance" ... but is matched to an intentionally out of balanced crank ... it is designed that way. If you have the wrong damper it'll cause the motor to shake/vibrate. For example, if you have a chevy 454 crank and someone put a damper on it for a 396/427 ... it'll vibrate ... it'll eventually break the crank.

The tuning theories are worth looking into ... but first I'd check to make sure the "custom" builder matched the correct type damper to the crank. This difference does get overlooked and does happen.
JACK:gap
Fly wheel Too!!:D junk!!
 
B

brit_trader2003

Guest
Thanks for all your quick replies.

I have had the car for 2 years with no previous experience of Big Blocks. I am learning and am gradually being able to tell if the engine is right or wrong. The best it has ever sounded was after the last re-tune and it had very little vibration. It then sounded really sweet.

I know the 69 should have a 427 in it, however the previous engine was also a 454. I am not sure about the dampners and it will be hard to check here as there are very few Big Block specialists (possibly none) in the UK. How do I tell if I have the correct type of dampner ?

As 74BigBlock suggests the problem does go away after I get a garage to re-tune the Carb. The carb is a standard Holley 750 CFM 4 Barrelled Carb with Vacuum Secondaries. Having read the Haynes Manual for Holley Carbs it seems that it is simply a case of turning two screws until the engine sounds right and they are in sync, I guess it is an Idle Mixture setting for each Primary Barrel. It should be readily apparent if the problem goes away. The Idle Speed setting is good at about 650 to 700 revs and stays that way.
 

gmjunkie

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Old Fort NC/Vero Beach FL
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03 Z-51,78 L82 Coupe
brit_trader2003 said:
Thanks for all your quick replies.

I have had the car for 2 years with no previous experience of Big Blocks. I am learning and am gradually being able to tell if the engine is right or wrong. The best it has ever sounded was after the last re-tune and it had very little vibration. It then sounded really sweet.

I know the 69 should have a 427 in it, however the previous engine was also a 454. I am not sure about the dampners and it will be hard to check here as there are very few Big Block specialists (possibly none) in the UK. How do I tell if I have the correct type of dampner ?

As 74BigBlock suggests the problem does go away after I get a garage to re-tune the Carb. The carb is a standard Holley 750 CFM 4 Barrelled Carb with Vacuum Secondaries. Having read the Haynes Manual for Holley Carbs it seems that it is simply a case of turning two screws until the engine sounds right and they are in sync, I guess it is an Idle Mixture setting for each Primary Barrel. It should be readily apparent if the problem goes away. The Idle Speed setting is good at about 650 to 700 revs and stays that way.
Put a Carter AFB on it and it will be Just Fine!! Holley's Just don't stay in Tune for me and put Big Burned spots on Hood!!:D junk!!
 

Tepot

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Sep 13, 2005
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Moncton, NB, Canada
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1974 Metalic Blue L-48 Coupe 4 speed
Not sure what engine you had in your car before. But I had a similar problem with my truck.

I did an engine swap from a Inline 6 300 engine to a V8 302 engine in a 1994 Ford F-150 4x4 truck.

When they completed the job. I was ok when I was driving. But in Idle or between gears when I was shifting there was a very big vibration. I thought the truck would fall into peices.

So When I took it back. they realized it would take another clutch. mine was not compatible. Then the problem was fixed.

I don't know if your car is automatic or manual. But if it is a 4 speed manual transmission. It could be your clutch.
 

gmjunkie

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Old Fort NC/Vero Beach FL
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Tepot said:
Not sure what engine you had in your car before. But I had a similar problem with my truck.

I did an engine swap from a Inline 6 300 engine to a V8 302 engine in a 1994 Ford F-150 4x4 truck.

When they completed the job. I was ok when I was driving. But in Idle or between gears when I was shifting there was a very big vibration. I thought the truck would fall into peices.

So When I took it back. they realized it would take another clutch. mine was not compatible. Then the problem was fixed.

I don't know if your car is automatic or manual. But if it is a 4 speed manual transmission. It could be your clutch.
Naw thay put the 300 flywheel on the 302!! Hell yes it will Shake Heavy!! :upthumbs junk!!
 

Jack

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Most washing machines spin at a pre-set rpm.
But car tires/wheels don't ... neither do car cranks. An out of balance tire will vibrate like crazy at some RPM ... but increase the RPM and it seems like "it goes away" ... continue increasing the speed and vibration reappears. It never really goes away ... it's much more obvious at certain RPM than at others. Agree, the flywheel/clutch/pressure plate assembly could be a problem in same manner as the damper.

How do you tell an external balance damper from an internal type?
Both types have two metal castings ... an inner drive hub bonded w/ rubber to the outer inertia ring. Rotate the damper through 360° while looking closely at it. Either type has to viewed from front, side and rear. If it's an internal balance ... the inner hub and both inner & outer diameters of the outer ring will be concentric with NO eccentric casting depression(s). If it's external balance, some part of it will have a very obvious eccentricity. 396/427 do not have depression(s) and are internal ... 454 will have depression(s) and is external.
JACK:gap
-edit-I'm NOT a BBC expert. I'm NOT saying it's NOT the tune-up. What I'm trying to convey is this: Multiple tune-ups haven't solved this ... and ... The wrong damper is an easy mistake to make ... it's also very easy to locate, identify & troubleshoot ... should be able to determine right vs wrong in about five minutes.
 

gmjunkie

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Jack, You are Absolutly Right!! Harm.Balencer and Flywheel are Posable and Likely!! Put a 350 Flywheel on a 400 SB and it will idle Great,get it up to 4500 and it will shake the Finders off of it!! :D junk!!P.S. I spun the Balencer on my LS6 and it had a weard vib. I put a new one on it there not $90.00!!
 
V

vette-dude

Guest
It could also be that the primary idle air circuits are not large enough for the cam you installed and thus it is loading up at an idle. The carb would be fine after a tune then when the barometric change occurs the engine idles rougher. There is a small screw on the bottom plate on the primary butterfly that you can turn in and it will open the primary butterfly a little more to give you some more air. If that doesn't work, you can drill a small (optimum word is small) hole in each secondary butterfly to allow more air at an idle. Of course once you do all this the carb will have to be re-tuned.

Randy
 

maxrevs85

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Messages
709
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Eaton Rapids, Mich
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!985 ,blue coupe, 4+3 Z51
Randy, Thats basicly where I was headed with my post. If its definitly a vibration , then thats a different issue. I have a 396 Chevelle with 12:1 and an 830cfm 4150 HP. I have to run racing gas . The tuning is on going with the idle curcuits. Some of the 4150's have holes in the butterflies to aid this situation. When you cross the line with performance you will always have certain issues. Same with FI.
 
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It would be very enlightening to know exactly what the shop did to "re-tune the carb" that always makes the symptom disappear..... :eyerole
 

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