[daveoxide]

After I my 69 351W pushed a connecting rod through the side of the block, i purchased a rebuilt 351W short block to replace it. I was able to reuse lots of the old parts; heads, intake, carb, etc, but did end up on purchasing lots of new parts... the clutch and flywheel being the most relevant to this discussion. After nearly a year since the original engine failure, I finally put the engine back in my Mustang (along with the rebuilt T5 transmission) and fired it up yesterday. I was able to break in the cam and engine, but had a rough running engine that shook more than normal. The vibrations shake the stick shift and steering wheel when just moving it through the RPM's on the driveway, feels like a buzzing vibration that changes intensity with RPM. I took it for a short test drive around the block and the vibrations are more continuous under load, and the vibration is more noticeable the higher the RPM's go. Just putting around near idle it vibrates more than it should for sure, but it is less intense.

My findings and troubleshooting efforts thus far:
1. Pulled and checked all spark plugs for good color, all plugs are consistent with one another.
2. Plugs, wires, cap, and rotor are all new and there are no obvious defects.
3. Distributor is from the old motor, which was previously tuned (mechanical and vacuum advance curves) and worked fine before hand, and I gave it a once over before installing it on the new engine.
4. I replaced the ignition module, thinking I was chasing a weird ignition issue, but that didn't change anything.
5. I found a vacuum leak under the carb. Replaced the gasket which fixed the vacuum leak but its still shaking.
6. I have a wideband O2 installed, 12-14 is the range I see it flopping around when playing with it on the driveway, so mixture should be close enough and its not leaning way out.
7. Timing is set at 14 initial, timing advances normally.
8. Quadruple checked firing order and its setup for a 351W (1,3,7,2,6,5,4,8).
9. Popped the valve covers off and reset all the lifter preloads (hydraulic lifters). Nothing was found outta whack, so it didn't change after putting it back together.

Now, I have two theories of what could be going on:
First theory... I used the old harmonic balancer from my old engine and purchased a new flywheel and clutch as the old one was toast (CF clutch disk had a cracked hub which twisted some and then hot spotted the flywheel). I purchased a "lightweight" steel flywheel, with the bolt on balance weight. I made sure I installed the smaller of the two weights as my new engine should be setup for 28oz imbalance (as all 351W should be). The flywheel bolted on only one way, like normal, no issues there. My concern now, is the aftermarket flywheel with the bolt on weight somehow not clocked correctly or is the weight not the right amount?

Second theory... My old harmonic balancer that I reused is one of the fancy fluid damper type balancers. It worked great on the last engine, but I remembered that during the rebuild, I dropped it at some point and it may have every so slightly dented the outer casing (can't seem to find the dent now). After reading up on how these dampers work and the internal clearances, I am wondering if the inner ring is now stuck and I am no longer getting any harmonic dampening.

If anybody has any experience with engine vibrations and is willing to share, I am all ears. The last thing I need is to snap the crankshaft!

 

[JohnW63]

I have read that the balancer and the flywheel to have to match. If I am not mistaken, I don't think you can put the flywheel on in the wrong place rotation. I think the holes only line up one way. At least that is how the 289 works. If the vibration increases with RPMs, and you are not hearing any internal engine noises, I would verify the balancer and flywheel combination is correct.

 

[gsxrken]

Sounds like you’re right to zero in on the dampener and flywheel. I’ve seen where there is two places to bolt in the same weight, so if yours was like that, make sure you’re in the one that you want. Do you know the history of the short block- might it have been setup for internal balance?
The other longer shot worth mentioning is the firing order. You mention that you verified it for a 351w, but since you didn’t build the short block, someone may have installed a 289/302 cam. I assume if you adjusted the valves yourself this would have become evident but I’ll throw it out there.

 

[GypsyR]

Third theory. I have no faith in the average engine "rebuilder". I've seen seven bores honed and fitted with standard pistons and the eight bore 40 over. Nationally known name brand rebuilder.
Mismatched rods, so the pistons had different heights.
For a real budget job I sourced a set of pistons locally. Three different brands were in the box. Six were cast and two were (much lighter) hypereutectic. They acted like it was no big and promised to fix me up. I demanded a refund as there was no way in Hades I would ever trust them to hand me parts again.
In short, there is some clobbered together stuff out there that shouldn't even be running done by people who are supposed to know what they are doing. Most rebuilders do a good job, but ....I have seen what I have seen.

 

[daveoxide]

Third theory. I have no faith in the average engine "rebuilder". I've seen seven bores honed and fitted with standard pistons and the eight bore 40 over. Nationally known name brand rebuilder.
Mismatched rods, so the pistons had different heights.
For a real budget job I sourced a set of pistons locally. Three different brands were in the box. Six were cast and two were (much lighter) hypereutectic. They acted like it was no big and promised to fix me up. I demanded a refund as there was no way in Hades I would ever trust them to hand me parts again.
In short, there is some clobbered together stuff out there that shouldn't even be running done by people who are supposed to know what they are doing. Most rebuilders do a good job, but ....I have seen what I have seen.

Fair assumption. The automotive world is ripe with half arsed rebuilders, and engines are no exception. That is why I have severe trust issues ever taking any of my vehicles to a shop... that and I worked as a mechanic for a few years in my youth. If I buy the parts and do the work, I at least know when I cut corners to get something done, and if it breaks prematurely its my own fault. With all that being said, just after I received the short block the first thing I did was rip it apart and start measuring! Everything looked pretty good, except the pistons were too low a compression (~9:1) for what I wanted out of the motor. I had a shop install the correct pistons on the rods, and then I slapped it all back together. So its had a good and thorough once over.

I haven't found any leftover pistons or connecting rod caps laying around the garage, so hopefully the mechanical bits inside the engine are ok. If not, I'll have egg on my face!

 

[Wayne clark]

Verify the cam is indeed for 351 and not a 302. Also I'll throw this out there does it have a pilot bearing in the crank?

 

[daveoxide]

Verify the cam is indeed for 351 and not a 302. Also I'll throw this out there does it have a pilot bearing in the crank?

Good thoughts! I double checked the catalog for the cam, assuming it is indeed the part number it says it is, it should be good (Howard’s cam, I trust it being a match). And yes, pilot bearing is installed.

 

[daveoxide]

Here is a short video of the engine running. It’s really hard to see the vibrations, but the battery actually shakes, and the engine jiggles more than it should at idle. I think I might have a small exhaust leak somewhere too. 😁

 

[kenash]

Since the damper is suspect, I would replace it with a a less fancy one, to rule out or confirm. It seems the easiest of the alternatives.

 

[Huskinhano]

It appears you've covered all your basics. You could buy a cheap balancer and throw it on just to see. Check the firing order as it costs nothing. I had a vibration issue, not bad but annoyed me. I bought a billet flywheel with a bolt on weight. Luckily I had the flexplate that came with my motor and had a shop exactly match the new flywheel to the old. That helped a lot. I was going to suggest the same then remembered you have a different motor.

My gut feeling, it's the motor. I use to work in a Chevy dealer in the mid to late 70's. The height of Vegas. If you remember, they had tin plated pistons riding in aluminum bores. They had a lot of cylinder scuffing issues. GM would only authorize the cylinders that were scuffed to be over bored.

 

[daveoxide]

Since the damper is suspect, I would replace it with a a less fancy one, to rule out or confirm. It seems the easiest of the alternatives.

I’ve got a new damper on order since the local auto parts store doesn’t have one on the shelf. When it arrives and I get it installed I will post the results.

 

[Desert66]

I did the same thing on accident, had to pull my T5 and change the flywheel due to weights. Hard lesson learned.

 

[daveoxide]

I did the same thing on accident, had to pull my T5 and change the flywheel due to weights. Hard lesson learned.

So wait, did you install the wrong weight or wang doodle your balancer? I have found only one or two forum postings with folks experiencing a similar problem I am seeing that DOESN’T involve them installing the wrong flywheel from the get go. Curious as to the details of your experience.

 

[ElConductor]

I'm assuming you've already checked upon installation but how are your motor mounts and motor mount brackets? If one is cracked or broken it can definitely make your engine vibrate or shake.

 

[daveoxide]

I'm assuming you've already checked upon installation but how are your motor mounts and motor mount brackets? If one is cracked or broken it can definitely make your engine vibrate or shake.

Good question. I did install new motor mounts as the old ones were, well, old. I ordered a new trans mount but got the wrong one, so I had to reuse the old one. It isn’t torn or anything like that, it was a “while I’m at it” sorta thing.

I also pressure washed and painted the engine bay while the engine was out and didn’t see any cracks on the frame mounts or in the general vicinity. I did take a small grinding wheel to all the factory spot welds that are sharp as hell to avoid cutting my hands all up again when working on the engine in the car!

 

[ElConductor]

Good question. I did install new motor mounts as the old ones were, well, old. I ordered a new trans mount but got the wrong one, so I had to reuse the old one. It isn’t torn or anything like that, it was a “while I’m at it” sorta thing.

I also pressure washed and painted the engine bay while the engine was out and didn’t see any cracks on the frame mounts or in the general vicinity. I did take a small grinding wheel to all the factory spot welds that are sharp as hell to avoid cutting my hands all up again when working on the engine in the car!

What kind of new motor mounts did you get? I know if you get the polyurethane motor mounts as opposed to the rubber ones that they can cause a bit of vibration, especially when driving, although I don't think it would be anywhere near what you're having now. Are you able to throw the old ones back on and see if it changes anything?

 

[daveoxide]

Quick update...

Got the new harmonic balancer installed, and it’s better, but I think I have multiple problems causing an ill running engine.

Upon initial startup in the garage, it went through its normal sequence of easy startup, high idle due to choke, a few throttle blips to bring rpm down after it warmed up and it was idling ok (not perfect, but better than before). After playing with the idle speed a bit, it was warmed up and I went to pull it out of the garage and it died. It was struggling to stay running and the rpms started falling and it wouldn’t respond well to more throttle.

After it died, then it would not restart. It was acting like it was flooded. So I left it for a few minutes and came back to it and I finally coaxed it to start and run again. Smelled like raw fuel when it fired back up but it cleared up and was idling again.

I took it for a short 5 mile test drive and pulled over about halfway through and found the header pipe for cylinders 8, 6, and 3 glowing red. Cylinder 4 spark plug boot cooked on the header pipe and was sparking through the boot to the header. When I got home the number 8 wire popped off the spark plug (it was still attached at the halfway point). Needless to say it was running like crap!

I limped it back home and into the garage and there it sits. There is so much going on I’m gonna have to step back and go at this a bit more methodically I think.

 

[DIHLON]

Stuff like this is what I am dreading about putting my motor back together. I hope you figure it out soon. Good luck!

 

[daveoxide]

Stuff like this is what I am dreading about putting my motor back together. I hope you figure it out soon. Good luck!

Lol! You are not alone my friend! That is partly why this rebuild took me nearly a year to complete. You fret over all the tiny details for months and it all comes down to the first few minutes of operation and it turns into a poop show!

 

[gsxrken]

Glowing headers is usually retarded timing. Are you sure that your timing mark pointer / balancer combo indicate a true TDC? Something to check. I still would think about trying to wire the spark plug order for the 302 order, just for the helluva it. It’s free, and it’s odd only some cylinders are that hot in a carbureted engine.
Lean cylinders can glow too but you said your wide band wasn’t too far off.