Engine shakes after 305 to 350 swap HELP! (HD video included!)

[bhw33191]

I'm basically begging for help, usually stay on the caprice forum but i've been flooding it with posts. I think i'll try a post here. I just can't go another week without my baby. ('83 Caprice)

HD Video of the problem: http://www.youtube.com/watch?v=BAuhzhVwhqY

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

It runs fair, idles ok, but when the RPMs are around 1200-1400 it shakes/shudders a lot. 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 (stock) 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!

-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

-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

NOTES:
-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.
-And also, the timing mark does seem jump around 2-4 degrees..don't know if that means anything
-AND FYI. The HEI (with vac adcance) dist. that came with the 350 apparently was fried, was not getting any spark at all so thats why we swapped it for the one that was in my 305

THANK YOU IN ADVANCE IF YOU CAN HELP!!

[silicon212]

Ok.

You CANNOT get a valid timing setting with the EST harness plugged in, no matter how much time you waste on it. You MUST unplug this cable to get the engine timed. That's the 4-wire cable.

If the engine will not idle with that unplugged, this leads to other issues. Your best bet is to get a vacuum gauge and check idle vacuum. It should be 15-19 in Hg on the test, steady. My car, with the worn out cylinder heads, got about 13 in Hg, but ran well. The new heads fixed that. The gauge should hold a steady reading.

If the gauge is reading steady but low, you have a vacuum leak, so start tracing. You can use WD40 to do this with - any change in engine idle when spraying near a suspect leak will verify the existence of said leak. Make certain you are using the proper carburetor base gasket - if the intake on your engine (looks like a Performer?) is spread-bore, you should be OK with the stock gasket as long as you're using the Q-jet. If it's square-bore, you will have leaks here with the Q-jet and the secondaries will be useless.

My bet is on carb/vacuum leak. Once you solve this problem, you should be able to idle your engine with the EST harness disconnected at the distributor, so you can make a proper timing adjustment.

[534BC]

I'm questioning if a third party took it for a test drive would they imediately conclude "hey, there's somthng bad wrong here" experienced or otherwise.

Anyways, if they would then I see two potential problems at first.

1 rumble,buzzing,vibrating reminds me of an exhaust pipe up against metal frame/x-member, ect.

2 same thing reminds me of a metal motor mount or one that the rubber is gone or beat out.

3 if niether of these is true then the engine may indeed require balancing.

[Ol'Jim]

Did you ever get your car figured out? I was wondering which flexplate you used on it? If it was one that came with the engine, could they have inadvertently bolted on a 400 flexplate? They are externally balanced, and would make it do exactly what you are experiencing... Let us know what you found out, please?

[bhw33191]

Originally Posted by Ol'Jim

Did you ever get your car figured out? I was wondering which flexplate you used on it? If it was one that came with the engine, could they have inadvertently bolted on a 400 flexplate? They are externally balanced, and would make it do exactly what you are experiencing... Let us know what you found out, please?

Yeah sorry about that i guess i forgot to update you guys on this thread. But it was actually the harmonic balancer was wrong. I'm told it was using a balancer from a 70's 350. It was twice the size and the weight was also offset on one side where the one thats on there now is completely symmetrical.

I could've replaced myself but i had a hard time believing something as essential as the harmonic balancer would be wrong so I just had to give it to my mechanic at that point.