305 to 350 engine swap problem, it shakes - Hot Rod Forum : Hotrodders Bulletin Board
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Old 06-14-2010, 11:30 PM
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305 to 350 engine swap problem, it shakes

I'm new here..but here's the problem thats stumping me, my mechanic, and my motorhead friends!


Bear with me on this BIG post, I'm just trying to help you guys out with as much info as possible. Here is the low-down after days of testing to no avail

Its an 83 Chevy Caprice. It runs fair, idles ok, but when the RPMs are around 1200-1400 it shakes/shudders a lot and shakes the dash. It doesn't backfire or sound like its missing either. After the RPMs pass about 1400 the shaking seems to balance or cancel out but something is still noticeably abnormal. Not seeing any excessive movement on the motor mounts either.

Heres the ENDLESS list of things i've replaced/checked and rechecked:
-disconnected torque converter..still shook but a little less. i think thats because when it was connected the vibrations amplified through the trans.

-The guy i bought the engine from had only put 10k miles on it, so we know the harmonic balancer should be correct, and we know the engine should technically be in good shape altogether!

-Vac advance dist. that came on the 350 was fried-->Using the HEI distributor (no vac advance, just computer) from my 305 b/c i knew that worked..the coil cap and rotor are only a year old too.

-There is no knocking, ticking, or pinging-->which leads me to hopefully believe there is nothing internally wrong

-Did not find any vacuum leaks

-New plugs (.045 gap) and wires..made sure they weren't crossed or anything

-Even put on the QJet carb from the old engine which i also knew was in working order..but still nothing

-Swapped the ECU (as primitive a computer as it is), MAF, and some weird relay on the firewall behind the dist. from my 2nd caprice...still no change

NOTE:
-A little confused on timing, heres why: I'm told when you time an HEI ignition like this apparently you're suppose to unplug the 4prong plug from the distributor that goes to the ECU, set the timing, then plug it back in. However, it barely idles when we unplug it and no matter which way we turn the dist. it doesn't get any better. So basically we had to keep it plugged in and set the timing that way.
THAT BEING SAID..i'm guessing this means the 350 isn't taking well to this computer advance crap..or something got messed up during the distributor swap?? So maybe i should go buy a working vacuum advance distributor? If i spend the $100 on that and it STILL shakes, then I..am going to be pissed to say the least. Just can't go another week without my baby

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Last edited by Lead Sled 83; 06-14-2010 at 11:36 PM.
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Old 06-15-2010, 09:06 AM
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Does your flex plate have a weight on it? If so, that's your problem. I've come across this in the past with this swap. I was able to just remove the weight without removing the engine or trans. for the fix. On one job the weight just would not come off so we ended up putting in a new flex plate for the 350.
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Old 06-15-2010, 09:20 AM
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i did a motor swap on my chevy blazer and shook like crazy when the lockup converter engaged. turns out the bellhousing bolts were never tightened. oops!
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Old 06-15-2010, 09:24 AM
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Are you sure you got a 350 not a 400?
Because either you have the wrong harmonic balancer or flexplate or you got a 400 with 350 harmonic balancer or flexplate.

Last edited by lg1969; 06-15-2010 at 09:30 AM.
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Old 06-15-2010, 11:36 AM
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Look at the passenger front of the block where the head meets the block. There will be a small machined pad there w/numbers and letters. Carefully copy and post these.

At the rear of the block, back by the distributor, there will be a couple of cast in (raised up, not stamped into the metal) numbers. One will be the casting number of the block and will look like "330817" or "3956618" (the last three digits are all that's needed, but be sure they're accurate- casting numbers are notoriously hard to read correctly, that's why all the numbers are usually asked for to narrow down errors), the other shorter one will be a date code, it'll look like G139 or B94. Post those as well.

You can in the meantime, try removing one plug wire at a time while the engine is running at the speed that it's vibrating. See if the removal of any plug wire causes a significant change in the vibration.

To properly time the engine, that 4-wire connector has to be disconnected. The procedure for timing the engine is HERE, about half way down the page. If the engine won't run w/it disconnected, you have ignition problems, like a bad miss- could be a carbon track in the cap, dirty cap terminals (in the cap or the outside terminals the wires hook to), bad plug wires, bad plugs, p-u coil, ignition coil, module (last 3 less likely, but still possible), etc. But it sounds like an ignition problem if it won't run w/the 4-wires disconnected.

Using a non feedback HEI w/a CCC controlled carb will cause havoc w/the ECM. It might run but at the very least it will throw codes. Which brings me to ask what coses have set? You only need a paperclip to jump the ALDL, then the codeS will display as flashes of the MIL. Procedure shown HERE.
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Old 06-15-2010, 11:45 AM
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I forgot, Check the rocker arm if not adjusted correctly.
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Old 06-15-2010, 11:56 AM
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Years ago I had a 79 Camaro Z28. It had a problem very similiar to this only it occured a little higher up in the RPM band. When I did the rebuild, it didn`t shake or have any vibration, the vibration formed in time and it didn`t matter what I seemed to do it didn`t stop it. Inspection of the flexplate with the transmission out showed it was fine, the harmonic balancer was correct and checked out good so I just kept driving it like that. Later on I sold the car to a friend of mine. The flexplate later developed a crack so he replaced it, afterwards the vibration was gone. So I learned then looks can be decieving when it comes to flexplates, they look good, no cracks, no warpage, but still something happens to them and it causes vibration problems that are hard to track down.
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Old 06-15-2010, 12:09 PM
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OP- do check the damper to be sure it is correct for a 350, if it's got a scallop removed from it, it's for a 400:


The idea in checking the engine numbers above is to tell what the engine started life as- a 350 or a 400.

If the motor mounts are metal on metal (worn out) or broken, this will impart a lot of vibration into the car.
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Old 06-15-2010, 12:21 PM
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Just a note: All 350 small blocks are external balance from 1986 on.
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Old 06-15-2010, 12:27 PM
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Quote:
Originally Posted by 406 bug
Just a note: All 350 small blocks are external balance from 1986 on.
Only on the flexplate/flywheel- the damper is still neutral balanced. The 1-piece rear main seal flexplate won't fit a 2-piece RMS crankshaft, they have different bolt patterns.
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Old 06-15-2010, 12:37 PM
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Quote:
Originally Posted by cobalt327
Only on the flexplate/flywheel- the damper is still neutral balanced. The 1-piece rear main seal flexplate won't fit a 2-piece RMS crankshaft, they have different bolt patterns.
I think the balance is on the inner hub.
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Old 06-15-2010, 12:46 PM
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Quote:
Originally Posted by 406 bug
I think the balance is on the inner hub.
Sorry, that would be incorrect. Maybe you're thinking of a 454 damper.

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Old 06-15-2010, 12:54 PM
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urr right, guess I am.
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Old 06-17-2010, 10:58 PM
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400 and 454 are ext balanced dampner in front

all others are just dampners not balancers

is your driveshaft or a u joint not messed up

the dash shaking thing is what my s-10 was doing when i ha da froze rusted up u joint that bound up and bent the rear half of my rear driveshaft on my pickup


id be checking that and also the tranny mount
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