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I have a fairly bad engine vibration in the 2000-2500 RPM range. This occurs in park. 69. The engine is a 350ci 78 truck motor with HEI. I think it has a mild cam, but I'm not sure as it was in there there when I acquired the vehicle. The plugs and wires are fairly new.

A timing light shows a slight blip on #1 cylinder and the cylinder directly adjacent to it (I forget the #) at idle. Blip seems to disappear at higher RPM. I pulled the wires, regapped plugs, cap and rotor were cleaned and look good.

Where do I start to diagnose? I've searched and see it could be a number of issues. Going to check tranny mount and flexplate tonight, but wanted to gauge some other thoughts from you guys.

Above or below that RPM range, it runs great, although I can hear the blip at idle discussed above. Maybe it's just a misfire? There appears to be more condensate on this side of the exhaust when warming up.

Thanks!
:beers:
 

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You could do a compression check with throttle wide open to check all are the same across the board.
Once done, turn up idle screw to vibration and disconnect one plug wire at a time and note decrease of rpm. The one cylinder having the least amount of drop would be suspect. Kind of like a dynamic compression check.
Another thought would be to swap in another distributor, even if its a point dist to troubleshoot.
 

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Ha. I'm of no help but I've also got a '78 350 truck block in mine that does the same thing in neutral. Same guy must've built ours. It's coming out after Christmas so I'm not spending too much time diagnosing it.
 

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Ha. I'm of no help but I've also got a '78 350 truck block in mine that does the same thing in neutral. Same guy must've built ours. It's coming out after Christmas so I'm not spending too much time diagnosing it.
Ha, I actually found the car in SC! May have been the same dude with 70's blocks a-plenty.

I like the idea of the dynamic wire check. I'll give that a shot. Would a simple miss give this much vibe? It's not teeth jarring, but definitely rattles the entire vehicle. I'd think it'd be something out of balance.

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Yes. A missing cylinder will sorta make the speedo hard to read.
An out of balance engine gives a high vibration throughout the car and depending upon the amount of imbalance, the accessories will vibrate about their brackets all the way to the speedo not being able to read.
 

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Ha!....That's weird!....I'm also trying to chase down an engine vibration in my freshly rebuilt 1967 327 motor!....The motor was rebuilt before I purchased the car so it wasn't me!....I do know however that a bent fuel pump pushrod was installed which I had to replace and a partially cracked oil filter adapter was installed which I had to replace as it sprung an oil leak messing up the driveway with oil!....Now, I'm noticing that the crank pulley appears to have a wobble and I'm wondering if it is out of round, resulting in my engine vibration!....The power steering pump is also not mounted straight, the P/S pulley is not straight in relation to the crank pulley and I'm wondering if this could be causing my engine vibration!....It's supposed to be warmer over the weekend so I'll remove the fan belts to see if the vibration goes away!
 

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The wobbling crank pulley sounds like a harmonic damper issue which would definitely cause engine vibration.

alan
 

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The wobbling crank pulley sounds like a harmonic damper issue which would definitely cause engine vibration.

alan

Yes!:yes:, I was wondering that also, Alan!....That harmonic balancer is also a used part!....Why would they go through rebuilding the motor and re-install used, junk engine parts?:confused:....It doesn't make sense and it's not something I would do!:noway:
 

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Also would cause belt slippage on hard acceleration, or at high RPM's.

Now that's weird!....One time I revved the motor high because I was pissed and I heard that belt squeal noise!....Now I'm wondering if I should even be running this motor for fear of damage!
 

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Now that's weird!....One time I revved the motor high because I was pissed and I heard that belt squeal noise!....Now I'm wondering if I should even be running this motor for fear of damage!
I guess what I was referring to was that the belt could slip off the pully, and not just squeal. It would happen to me on the highway when I would get on it. It was either a bad harmonic balancer or pulley, maybe both.
I always kept a spare fan belt in the trunk, just in case it slipped and the fan cut it. Went with a March pulley set up and different balancer, so far no problems, except at 7500 rpms, it gets a little squirrely and wants to slip off. :yes:
You mentioned alignment issues with you're set up, I'd work on that for sure. Remove the crank pulley and inspect it as well as the harmonic for seperation at the rubber.

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Yes!:yes:, I was wondering that also, Alan!....That harmonic balancer is also a used part!....Why would they go through rebuilding the motor and re-install used, junk engine parts?:confused:....It doesn't make sense and it's not something I would do!:noway:
Well, that's what separates the reputable from the non-reputable people, no? The less than reputable person would say "I bet I can use that balancer that's been sitting on the bench for 5 years and save a few bucks". The reputable guy has ethics and values his reputation. The guy who is doing it for himself only wants to do it once.

Take all the belts off . Fire the engine for a few seconds, if the balancer wobbles and the bolt is tight on the crank snout, replace the balancer.

alan
 

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You mentioned alignment issues with you're set up, I'd work on that for sure. Remove the crank pulley and inspect it as well as the harmonic for seperation at the rubber.

Will do!....I just hope it's not something serious!....Thanks!:thumbsup:
 

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Well, that's what separates the reputable from the non-reputable people, no? The less than reputable person would say "I bet I can use that balancer that's been sitting on the bench for 5 years and save a few bucks". The reputable guy has ethics and values his reputation. The guy who is doing it for himself only wants to do it once.

Take all the belts off . Fire the engine for a few seconds, if the balancer wobbles and the bolt is tight on the crank snout, replace the balancer.

alan

That makes sense, Alan!....My God!, this seems serious now!....I hope that balancer doesn't decide to wobble off the crank snout while I'm looking at it!....I know the bolt is tightened down as I just done an L-79 cam installation a couple months ago!....I never even thought to examine that balancer carefully at that time!:mad:
 

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Here is one I thought was weird for causing a bad engine vibration.Motor vibrated bad revving from idle up to maybe 4000 rpm.Tried all kinds of troubleshooting and solutions and turned out to be the stock clutch fan. It somehow just went bad. Swapped in a flex blade and all was good.
 

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Ron, make sure to see if it is the crank pulley or the damper that is wobbling.

alan

Will do!....My first thought, when I had the engine running last week, was the crank pulley!....It was a sunny day, so there was plenty of light shining down there, and I could clearly see the crank pulley wobbling but now I'm wondering if it could be the balancer or the fact that I have a power steering mis-alignment problem that could be causing the engine vibration!....I don't know, I've never had a problem like this before....We'll see what happens after I remove the belts!

P.S. Did you ever get rid of those pesky gophers yet?.....And, did you gas them out like I suggested?hehehe
 

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Discussion Starter #19
Ah-ha! Think I found the problem. After a bit of digging I see that both of the bellhousing bolts on the pass side of the tranny to engine had loosened up. One is actually gone! :confused: Yikes! So tightened up the one that was still in there and that did the trick. :hurray::beers:

Now just to figure out what size those bolts are to replace the one that's missing. Anyone know off the top of their head? Used a 1/2 socket so I'm thinking 5/16-18 x ? long. Looks like they have a flanged head to them.
 
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