383 Vibration At Idle
Thread Starter
Racer
Joined: Jan 2020
Posts: 277
Likes: 69
From: Reno, NV
383 Vibration At Idle
Hi,
Have a 383 sbc in my 71 vette with th400. At idle, around 750-800 rpm, get a mild but annoying vibration. It’s smooth but then vibrates and the rpms drop a small amount (maybe 50) for a second or two, then rpms come back up and it smooths out. Does this repeatedly at a stoplight. Go into N, and the idle goes up to 900-ish and no more vibration. Any rpm above 900 the motor is quite smooth.
The 1-pc rear main motor was built with an Eagle internally balanced (cast crank) rotating assembly, with a neutral-balanced harmonic balancer, and Eagle supplied and neutrally balanced flex plate. This should be correct.
Cam is a Comp 224/230 at .050 with approx. .525/.530 lift with 1.6 ratio roller rockers.
Have replaced the harmonic balancer, distributor, plugs, heads, and torque converter (for a different stall). This motor has always had the same waves of idle vibration.
Some research suggests it’s just the resonance of the motor, and that I’ll just have to increase the idle a bit and live with it - which is what I’ve been doing.
Any ideas on what to look at next?
Have a 383 sbc in my 71 vette with th400. At idle, around 750-800 rpm, get a mild but annoying vibration. It’s smooth but then vibrates and the rpms drop a small amount (maybe 50) for a second or two, then rpms come back up and it smooths out. Does this repeatedly at a stoplight. Go into N, and the idle goes up to 900-ish and no more vibration. Any rpm above 900 the motor is quite smooth.
The 1-pc rear main motor was built with an Eagle internally balanced (cast crank) rotating assembly, with a neutral-balanced harmonic balancer, and Eagle supplied and neutrally balanced flex plate. This should be correct.
Cam is a Comp 224/230 at .050 with approx. .525/.530 lift with 1.6 ratio roller rockers.
Have replaced the harmonic balancer, distributor, plugs, heads, and torque converter (for a different stall). This motor has always had the same waves of idle vibration.
Some research suggests it’s just the resonance of the motor, and that I’ll just have to increase the idle a bit and live with it - which is what I’ve been doing.
Any ideas on what to look at next?
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
My money is on a weak cylinder rather than an internal rotating assembly.
One cylinder has a misfire. Is it a plug? Is it the plug wire? Is it the valve-lash?
Do you have a temp thermo gun?
Compare temps at the exhaust header or manifold.
A cylinder running 200* cooler than others is the guilty one.
One cylinder has a misfire. Is it a plug? Is it the plug wire? Is it the valve-lash?
Do you have a temp thermo gun?
Compare temps at the exhaust header or manifold.
A cylinder running 200* cooler than others is the guilty one.
Thread Starter
Racer
Joined: Jan 2020
Posts: 277
Likes: 69
From: Reno, NV
My money is on a weak cylinder rather than an internal rotating assembly.
One cylinder has a misfire. Is it a plug? Is it the plug wire? Is it the valve-lash?
Do you have a temp thermo gun?
Compare temps at the exhaust header or manifold.
A cylinder running 200* cooler than others is the guilty one.
One cylinder has a misfire. Is it a plug? Is it the plug wire? Is it the valve-lash?
Do you have a temp thermo gun?
Compare temps at the exhaust header or manifold.
A cylinder running 200* cooler than others is the guilty one.
1- 222
2- 220
3- 290
4- 260
5- 300
6- 335
7- 280
8- 380
However, found out that IR temp guns are inaccurate with ceramic coated and shiny headers, and mine are both which could've explained the significant variances.
Had also checked the resistance of my wires (MSD 8.5mm) in the past and all were in spec.. But, easy to check again.
Also, just replaced the plugs and the old ones were very similar in color. Would think a dead or intermittent cylinder would have a plug that looked different.
Checking valve lash could be on my to do list this week... Also, might go back to my 1.5 ratio roller rockers just to see if that changes anything.
Last edited by btwick; Oct 28, 2024 at 10:31 AM.
Team Owner
Joined: Jun 2001
Posts: 23,379
Likes: 1,067
From: Virginia USA
Get an old water bottle. Fill it up and poke a hole in the cap then squirt water on each exhaust outlet. It won't tell you the temp but if that cylinder is firing the water will boil off immediately. If it's not you'll see the difference.
Thread Starter
Racer
Joined: Jan 2020
Posts: 277
Likes: 69
From: Reno, NV
Timing steady during vibration, but normally check timing in Park and not Drive. HEI dist is set at 18* idle w/o vacuum advance, 18* centrifugal for 36* total at 3K rpm, and another 10* in vacuum.
Was thinking I will lower the idle in P to 700 and check timing where it vibrates and see what initial timing is doing.
Was thinking I will lower the idle in P to 700 and check timing where it vibrates and see what initial timing is doing.
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Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
BTwick,
I never heard of thermo guns inaccuracy with ceramic headers. Maybe.
I too have ceramics and shoot the laser dot at the flange right where the header tube is tacked welded to the flange.
If you take a reading about 8 -10 inches below that flange there is a significant drop in temps, but that often shows the gun is working properly.
You should not be concerned about super accuracy with a $30 toy, but instead focus on extreme ranges from one cyl to another.
In your case, #1 and # 2 are way lower than #8. That will signal a red flag to me when there is hundreds of degrees difference.
An exhaust valve that can not close completely (lash) will show a very hot header tube.
Then you have to figure why the valve is not seating. Weak spring? Valve seat itself? Lash too tight? Faulty lifter?
As far as reading the plugs, last yr I purchased 12 name brand plugs from a huge shipper, (name withheld) and had 3 out of 8 fail. Glad a bought extra. No color change indication, very low miles on the plugs. Only hint was a slight miss. Thermo gun showed cold cyl. Like yours, 200* cooler.
I never heard of thermo guns inaccuracy with ceramic headers. Maybe.
I too have ceramics and shoot the laser dot at the flange right where the header tube is tacked welded to the flange.
If you take a reading about 8 -10 inches below that flange there is a significant drop in temps, but that often shows the gun is working properly.
You should not be concerned about super accuracy with a $30 toy, but instead focus on extreme ranges from one cyl to another.
In your case, #1 and # 2 are way lower than #8. That will signal a red flag to me when there is hundreds of degrees difference.
An exhaust valve that can not close completely (lash) will show a very hot header tube.
Then you have to figure why the valve is not seating. Weak spring? Valve seat itself? Lash too tight? Faulty lifter?
As far as reading the plugs, last yr I purchased 12 name brand plugs from a huge shipper, (name withheld) and had 3 out of 8 fail. Glad a bought extra. No color change indication, very low miles on the plugs. Only hint was a slight miss. Thermo gun showed cold cyl. Like yours, 200* cooler.
Last edited by HeadsU.P.; Oct 28, 2024 at 07:56 PM.
Racer
Joined: Dec 2006
Posts: 417
Likes: 40
From: Swansea IL
My first thought was to check the vacuum at idle and see how that compares to the power valves in the carburetor. With higher idle rpm u are pulling more vacuum. The miss could be idle mixture.
Just a thought.
Just a thought.
Pro
Joined: Apr 2021
Posts: 571
Likes: 101
From: Texas
XR276hr
I guess we are on the same boat. I have the same cam from Comp Cams and my car shakes a little at Idle. I have done almost everything you have done except replace the Dizzy, but that was on my to do list. Anyway my car has a minor shake on it no matter at what Idle you put it. It does lighten up when you set it at about 900-950 rpms. I already tried taking it to professional mechanics and they tell me it is just the way the cams is. I have even called Comp Cams and spoke to Eric and he tells me it is normal for that cam to have a little shake. It is a little annoying to have to put up with that but at least now I know that there is someone else with this issue and it is normal.
Thread Starter
Racer
Joined: Jan 2020
Posts: 277
Likes: 69
From: Reno, NV
Vacuum is 12" in D and 14" in Park. That cam doesn't have much duration or overlap, so vacuum is good. Holley says divide it in half so running a 6.5 PV which should work OK. Have an AFR gauge and at idle I'm now sitting at around 13.5.
Thread Starter
Racer
Joined: Jan 2020
Posts: 277
Likes: 69
From: Reno, NV
I guess we are on the same boat. I have the same cam from Comp Cams and my car shakes a little at Idle. I have done almost everything you have done except replace the Dizzy, but that was on my to do list. Anyway my car has a minor shake on it no matter at what Idle you put it. It does lighten up when you set it at about 900-950 rpms. I already tried taking it to professional mechanics and they tell me it is just the way the cams is. I have even called Comp Cams and spoke to Eric and he tells me it is normal for that cam to have a little shake. It is a little annoying to have to put up with that but at least now I know that there is someone else with this issue and it is normal.
Thread Starter
Racer
Joined: Jan 2020
Posts: 277
Likes: 69
From: Reno, NV
BTwick,
In your case, #1 and # 2 are way lower than #8. That will signal a red flag to me when there is hundreds of degrees difference. An exhaust valve that can not close completely (lash) will show a very hot header tube.
Then you have to figure why the valve is not seating. Weak spring? Valve seat itself? Lash too tight? Faulty lifter?
In your case, #1 and # 2 are way lower than #8. That will signal a red flag to me when there is hundreds of degrees difference. An exhaust valve that can not close completely (lash) will show a very hot header tube.
Then you have to figure why the valve is not seating. Weak spring? Valve seat itself? Lash too tight? Faulty lifter?
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Just noticed a previous post about your 1.6 rockers.
In some rare cases, that change in geometry can cause an interference (rubbing) of the pushrod in the cyl head.
But that should be an issue with all 16 and not just one pushrod.
Unless its bent.
Certain brands of aluminum aftermarket heads Do Not Allow 1.6 rockers without binding.
In some rare cases, that change in geometry can cause an interference (rubbing) of the pushrod in the cyl head.
But that should be an issue with all 16 and not just one pushrod.
Unless its bent.
Certain brands of aluminum aftermarket heads Do Not Allow 1.6 rockers without binding.
Last edited by HeadsU.P.; Oct 28, 2024 at 09:06 PM.
Pro
Joined: Apr 2021
Posts: 571
Likes: 101
From: Texas
I have the 1.5s on mine. My engine is the l48 350 short block with the e street cylinder heads.(5089) Hopefully it is the pcv. My vacuum pressure is at about 12 to 13ish at 900 rpms.
Pro
Joined: Apr 2021
Posts: 571
Likes: 101
From: Texas
Good info.. I do have a bit of a sound when cold that could be a noisy lifter, and will be look at that, though the sound appears to come from the driver's side cylinder bank. Heads are newer AFR Eliminators with 800 miles (know that doesn't mean they can't be faulty in the ways you mentioned). The temp on #8 is concerning and will recheck the lash on that exhaust valve later this week. Thanks.
New oil pump and rear main seal.