Small Block LA Vibration @2500 RPM

[Not Jonathan]

Hey there everybody,

I'm seeking advice regarding an engine vibration in my 73 Challenger. 360 LA, tried running without accessories, finished replacing the balancer this morning and still no luck. The vibration happens in neutral or with the clutch in. Also happens while driving. So I would think this has to be something in the engine. Any thoughts? Motor mounts look fine. It seems like I'm just going to be living with it for now, but I think I can remember a time when this wasn't an issue. Don't think much has changed since then so I'm wondering what wear parts could cause this. Could a change in the induction setup cause an issue that occurs repeatedly at the same rev range? I have replaced the carb. Other than this, the balancer, and a new alternator, I haven't changed anything. Thanks for any ideas.

 

[Not Jonathan]

Also, I do have the proper counterweight in the balancer.

 

[Chryco Psycho]

360s can be a problem , I doubt it is a wear issue , I can't think of anything that can wear that would cause that kind of vibration unless the clutch disc broke as in chunks of the disc disintegrated which I have had happen !!
360s are external balance , but the LA & magnum use different external balance + a different piston weight from stock changes the balance , did they change pistons with the rebuild & was it rebalanced correctly after ? Often when rebuilding a 360 a rebuilder will use a lighter piston , lighten the rods & add weight to the crank so it is internally balanced requiring neutral balance flywheel & damper . Do you have any info on the build at all ?
I ran into this once with a poorly built crate engine , it had a massive vibration as well as numerous other problems , the only fix was disassemble it & have it balanced , at 3000 rpm the dash would be bouncing 3-4 inches vertically !!

 

[Not Jonathan]

360s can be a problem , I doubt it is a wear issue , I can't think of anything that can wear that would cause that kind of vibration unless the clutch disc broke as in chunks of the disc disintegrated which I have had happen !!
360s are external balance , but the LA & magnum use different external balance + a different piston weight from stock changes the balance , did they change pistons with the rebuild & was it rebalanced correctly after ? Often when rebuilding a 360 a rebuilder will use a lighter piston , lighten the rods & add weight to the crank so it is internally balanced requiring neutral balance flywheel & damper . Do you have any info on the build at all ?
I ran into this once with a poorly built crate engine , it had a massive vibration as well as numerous other problems , the only fix was disassemble it & have it balanced , at 3000 rpm the dash would be bouncing 3-4 inches vertically !!

The vibration isn't quite that bad. From 2500-2800 it's very noticeable, but it smooths out above that. Absolutely no vibration below. Unfortunately, I have no information on the build or the history of the motor at all. I know this was a 318 car and the 360 got swapped in along with a (at the time) keisler 5 speed kit. I'm wondering if somehow the flywheel isn't right for the car. Long term, I do want to build/re-build the engine, but I am hoping I can find the cause of this soon. I have only removed the passenger side motor mount to inspect. The driver's side looks fine but maybe there could be some sort of issue? Would solid mounts only worsen the problem?

One other thing regarding mounts: I'm a bit under-read on the subject, but I know the 360 takes a different driver side mount than the 318. It looks like a previous owner had taken a 318 mount and used washers to create a spacer to make it work. Could this cause some weird alignment/vibration issue?

I appreciate any advice

 

[Chryco Psycho]

Something has to cause the vibration , so a bad mount will not cause a vibration , solid mounts would transmit more of the vibration to the chassis . Obviously the builder cut corners nit using the correct mount for the drivers side mount but it will not cause the vibration .
The Flywheel could be the problem & as I said there are different flywheels on different 360s .

 

[moparleo]

Did you have the problem before the carb replacement ? Did you remove the distributor when you did the carb swap ? Timing change ?
To eliminate everything external ( pulleys, fan, pumps, accessory bearings etc...) remove all belts and then start it again. You can run it for a few minutes at a time without worrying about overheating.
If the vibration lessens or goes away, reattach the belts one accessory at a time until the vibration worsens and you have at least found part of your problem.
Try hooking up ma timing light and while someone slowly brings the rpms up see if there are any problems with the timing moving around at the balancer marks. Miss firing ?
Diagnosis can be a long process of elimination.
But really it is just a guessing game if you don't know the history of this engine. Ignition system. A complete rotating assembly teardown and inspection may be your only hope to resolve the problem.

 

[NoCar340]

The fact that it's at such a specific RPM range leads me to believe the engine/flywheel/damper balance is OK. Generally with a major imbalance like that you'll have problems throughout the RPM range, but they'll be noticeably worse at certain RPM. It sounds to me like when you hit a particular harmonic, somethin's either shakin' or potentially misfiring. If you already ran it 100% without accessories (no belts a'tall), those are out. If you seem to remember a time when this didn't happen, that tells me something is either worn or loose and this crept up on you over time. We know it's not the damper, nor is the crank pulley loose, but is that pulley straight? Is everything nice and tight on/in the bellhousing? Did you check the transmission mount too? Are all your exhaust hangers present and functional? It's surprising how bad an unsupported pipe can shake at certain RPM. All vacuum hoses present and uncracked? Move the throttle shaft to and fro (wiggling, not rotating). Check timing-chain slop by watching the distributor rotor while hand-rotating the engine back and forth. Have a look-see at the distributor bushings by trying to wobble the rotor in all directions, and while you're in there check for rotational slop in the distributor drive's tang by hand-turning the rotor back and forth.

Those last few suggestions come from a 2nd-owner (he bought it in '77) '71 Cuda 340 brought to our shop about a month back. 4,000 miles since the '92 rebuild but it just ran terribly--misses, weird vibrations, etc. We found an amazing amount of wear in the distributor bushings. We took the shaft out to replace those, and the shaft itself was worn badly enough to warrant replacement. It was worse than the bushings, and the drive tang looked like it had been beveled with a file. The drive slot in the oil pump drive was similarly hammered--it got replaced too. All this was after we'd corrected ~18° of timing-chain slop with a new high-end Cloyes set. We fixed everything we could, but the idle is still inconsistent and at it misses or vibrates at cruise--regardless of RPM. That's due to the almost 1/8" of play in the factory Thermo-Quad's primary throttle shaft. We're not set up to bush a throttle shaft (everyday auto shop, just with some cool customers & cars). We told him to send the carb out or find another, better '71 340 TQ throttle body. If you're wondering, yes, the timing set was replaced 4,000 miles ago with a Cloyes double-roller--the cheapest one they make. The distributor was the untouched original, as was the oil-pump shaft. The owner was ecstatic that the car runs so much better than it did, but the situation crept up on him, too. He rarely beats on it anymore, but the cruise behavior was worrisome. Then he hammered on it one day this summer, and land sakes, this thing runs like crap!

Sweat the petty stuff. You'll find it.

 

[70chall440]

My first guess is that your flywheel is loose or possibly either not seated correctly or worse somehow misshapen. I might be balanced correctly but not on the crankshaft correctly. Now, if that proves not to be the case then I would start checking all bolts that you can see/find. Worse could be something loose internally.

 

[Not Jonathan]

The fact that it's at such a specific RPM range leads me to believe the engine/flywheel/damper balance is OK. Generally with a major imbalance like that you'll have problems throughout the RPM range, but they'll be noticeably worse at certain RPM. It sounds to me like when you hit a particular harmonic, somethin's either shakin' or potentially misfiring. If you already ran it 100% without accessories (no belts a'tall), those are out. If you seem to remember a time when this didn't happen, that tells me something is either worn or loose and this crept up on you over time. We know it's not the damper, nor is the crank pulley loose, but is that pulley straight? Is everything nice and tight on/in the bellhousing? Did you check the transmission mount too? Are all your exhaust hangers present and functional? It's surprising how bad an unsupported pipe can shake at certain RPM. All vacuum hoses present and uncracked? Move the throttle shaft to and fro (wiggling, not rotating). Check timing-chain slop by watching the distributor rotor while hand-rotating the engine back and forth. Have a look-see at the distributor bushings by trying to wobble the rotor in all directions, and while you're in there check for rotational slop in the distributor drive's tang by hand-turning the rotor back and forth.

Those last few suggestions come from a 2nd-owner (he bought it in '77) '71 Cuda 340 brought to our shop about a month back. 4,000 miles since the '92 rebuild but it just ran terribly--misses, weird vibrations, etc. We found an amazing amount of wear in the distributor bushings. We took the shaft out to replace those, and the shaft itself was worn badly enough to warrant replacement. It was worse than the bushings, and the drive tang looked like it had been beveled with a file. The drive slot in the oil pump drive was similarly hammered--it got replaced too. All this was after we'd corrected ~18° of timing-chain slop with a new high-end Cloyes set. We fixed everything we could, but the idle is still inconsistent and at it misses or vibrates at cruise--regardless of RPM. That's due to the almost 1/8" of play in the factory Thermo-Quad's primary throttle shaft. We're not set up to bush a throttle shaft (everyday auto shop, just with some cool customers & cars). We told him to send the carb out or find another, better '71 340 TQ throttle body. If you're wondering, yes, the timing set was replaced 4,000 miles ago with a Cloyes double-roller--the cheapest one they make. The distributor was the untouched original, as was the oil-pump shaft. The owner was ecstatic that the car runs so much better than it did, but the situation crept up on him, too. He rarely beats on it anymore, but the cruise behavior was worrisome. Then he hammered on it one day this summer, and land sakes, this thing runs like crap!

Sweat the petty stuff. You'll find it.

Hey NoCar, thanks for the tall response. I'll go ahead and take a look at the chain slop via the method you mentioned. I oughta get the car up and do a bolt check of anything I can see in the bellhousing, too. I had actually been suspecting a misfire issue so I swapped the mopar box for an MSD I found, but no resolution there. So if the timing events are messed up it must be from the dizzy or the chain being loose. This dizzy is an original mopar one, I assume the one that was factory with this 360. If there are signs of wear then maybe I should consider a replacement? Could the pickup be bad? Does the pickup ever need adjusting?

One thing worth mentioning is that the passenger side exhaust pipe is almost touching against the bellhousing. I've been suspicious of this since the day I bought the car but haven't gotten around to doing anything about it. It's close enough that when I shift the trans with the engine off, I can hear reverberations in the exhaust pipe. I'll tackle this regardless next week and report back, maybe that is worsening the situation but not causing it. Should only require a minimal amount of welding/fab.

I'm wondering if a cylinder or two could be going lean at that rpm and be causing problems? I haven't pulled the plugs lately so I'll go ahead and do that today and see if there are any signs for alarm. Maybe I'll snap some pics of the exhaust/bellhousing situation and whatever slop I can find in the dizzy. Many thanks!

 

[moparleo]

Again, have you always had tis vibration problem or is it something recent and if so did you do any mods that may have caused this problem to show up or was it running fine and just started to have this problem out of the blue. More info = better diagnosis. The reason that I recommended that you hook up a timing light and slowly bring the RPM's up while checking the timing is that it will show any Distributor problems like spark scatter, a loose timing chain will have erratic timing. It will move around if the chain is loose.
Diagnostics need to be a step by step process. Majority of the time it will turn out to be the simplest, obvious thing that goes wrong. We tend to make things more complicated by not thinking of the simple stuff and instead think about more obscure things.

 

[Chryco Psycho]

The factory dist should have .008 between the reluctor & pick up .
But a misfire is a very different sensation from a vibration , usually a misfire will get worse under load & tends not to change with RPM ,you can look at the plugs to see if they are lean .
The engine I had problems with was exactly like yours , it had a very specific RPM around 2500-3000 where the vibration was very bad , it was smoother below & above .

 

[Not Jonathan]

Again, have you always had tis vibration problem or is it something recent and if so did you do any mods that may have caused this problem to show up or was it running fine and just started to have this problem out of the blue. More info = better diagnosis. The reason that I recommended that you hook up a timing light and slowly bring the RPM's up while checking the timing is that it will show any Distributor problems like spark scatter, a loose timing chain will have erratic timing. It will move around if the chain is loose.
Diagnostics need to be a step by step process. Majority of the time it will turn out to be the simplest, obvious thing that goes wrong. We tend to make things more complicated by not thinking of the simple stuff and instead think about more obscure things.

I remember the vibration beginning after I changed a broken transmission mount and exchanged a leaf in each spring to fix them sagging. For the longest time I thought this was a pinion angle/ driveshaft balance thing but the vibration occurs in neutral or with the clutch in, not moving or going down the road. The RPM range is repeatable as mentioned, about 2500-3000. I agree it's probably something obvious. Could the transmission be positioned in a way to cause this vibration? The broken mount let it sag much lower.

The clearance between the reluctor and pickup checks out. The rotor shaft doesn't have any play. I do notice that when I grab the rotor and try to advance it, it turns a few degrees. I'm assuming this is the vacuum advance movement and is normal. However I do notice that it is kind of sluggish to return after I let it go. Not sure if this is normal or if a spring is worn out or needs lubrication. I would think that this play would be a non-issue when everything is rotating, but I'm not so sure. I'll go ahead and put a light on it in the coming days and carefully watch for any bouncing in the timing. Thanks for all the help!

 

[Chryco Psycho]

As I said before the mounts can transmit more vibration but not cause it .
It could not hurt to lube the dist , there is a small c clip under the felt at the top of the dist , once you remove the 4 screws for the advance & plate , remove the c clip & the advance part of the shaft will slide off the top to clean & lube it ..

 

[PlumCraZRT]

If the vibration is mechanical, the vibration should also excite at double the RPM... does the vibration also noticeably peak at 5000-5600 RPM?

Some 360 flywheels have bolt-on weights, so it might be worth checking to see if that (or its Mallory metal) fell out. You havent checked the oil pan for any loose weights either, have you?

 

[Not Jonathan]

If the vibration is mechanical, the vibration should also excite at double the RPM... does the vibration also noticeably peak at 5000-5600 RPM?

Some 360 flywheels have bolt-on weights, so it might be worth checking to see if that (or its Mallory metal) fell out. You haven't checked the oil pan for any loose weights either, have you?

The vibration doesn't start again at 2x the RPM even a little. I'm getting suspicious of the distributor. Today I noticed that while trying to bring the revs up with a light on it, the timing seems to jump to be very advanced. I also notice that sometimes after revving and letting the throttle close, the idle will stay sped up for a bit and then return to normal. Could either of the advances be sticking? Admittedly, I'm using a "dial back" timing light set to 0*. It reads fine at idle but I wonder if the MSD messes with it when revving.

I checked the pan somewhat recently when I replaced the oil pump, within the last 500 miles. Nothing to report in there. I'll get the inspection cover off soon and see if there's anything loose in there. I'll inspect what I can of the flywheel.

 

[Not Jonathan]

I've been doing some more reading and wracking my brain to figure out what this is. I'm fixated on the trans mount because this is the only other thing I can think of that I've really done that could relate to this issue. The trans mount I installed is a poly energy suspension mount. I have the old rubber engine mounts. Could this maybe be what's causing or otherwise just transmitting the vibration? Maybe the rubber used to do a good job of soaking it up... Then again, the rubber trans mount was shredded when I bought the car so maybe the answer is that the poly mount transmits more vibration but there has been a vibrational issue for a long time prior to my ownership.

 

[Chryco Psycho]

As I said earlier a mount will not cause a vibration just transmit it .

 

[plymouth67]

So...I just went through this with a 360. It ended up being the front pump in the trans was pushing the converter forward and it wore out the thrust bearing. The crank was moving back and forth causing a vibration very similar to yours. It really doesn't take much to be out of balance to get this vibration. The rotating assembly is heavy so a little bit can cause a massive vibration. You say it has a Tremec in it? Did you measure the input shaft to ensure that it isn't too long causing some issue there?

 

[Not Jonathan]

So...I just went through this with a 360. It ended up being the front pump in the trans was pushing the converter forward and it wore out the thrust bearing. The crank was moving back and forth causing a vibration very similar to yours. It really doesn't take much to be out of balance to get this vibration. The rotating assembly is heavy so a little bit can cause a massive vibration. You say it has a Tremec in it? Did you measure the input shaft to ensure that it isn't too long causing some issue there?

Yeah it has a tko 500. I haven't measured as you stated, but this vibration didn't exist when the car was new (to me), so I must have introduced some vibration or some part must have worn. Whenever I eventually tear it down, I intend on taking all these measurements and also checking the concentricity of the bellhousing to the block. I have a receipt showing that this was all put together by some shop in 2004. It wouldn't surprise me if something about it was done incorrectly in the past.