Oh SBC gurus......Have an issue and need some suggestions. We're working on a 40 ford coupe for a friend of the family. this has been a prolonged project, but it's finally nearing completion. we're ready for final assembly and to finish the wiriring, but there's some issues with the motor. It's a crate SBC, slightly warmed over,4bbl nothing major with a 400 behind it. fires and runs great except for the wicked vibration that gets worse with RPM's. My first thought was balancer. The balancer was used so we picked up a new one. at the suggestion of a couple of guys locally we let it run sans balancer (the theory--however wierd it seems to me--is that it should not vibrate without a balancer--that that's the source of the problem. Still vibrates. Dad's going to put the new balancer on along with the accessories and see if that remedies the problems tonight. Assuming it doesn't what else should we look for? the other previailing theories are flex plate and torque converter (this is #2). any help or suggestions greatly appreciated!!!
Are you sure every cylinder is firing? And yeah, flexplate/converter could be causing that. Just check the simple stuff first.
Who's crate? Crate usually means new from GM, not some rebuilder. If GM crate, I'd suspect the torque converter first. If it was a 400 SBC flexplate by mistake, that would vibrate so bad at low rpm, that you would have shut it off..IMO How bad is bad? a moderate imbalance on the TQ can sometimes be cured by trial and error placement of washers at the bolts. It's a pain to do, but I've seen it done and I have done one myself years back, and it was perfect. I ended up with one washer on one, and 2 washers on an adjacent bolt.
This probably won't be much help, but it's a crate 290 hp/350 ordered from a dealer in Alabama who's since declared bankrupcy (sounds bad already). add to that we drug our feet and it's 3 months out of the 3 year/30k warrenty. we don't really know any build specifics beyond that--we ordered the stander 290 hp package. edelbrok performer intake and 600 carb. ebay coated headers, hei, aftermarket air package etc. like I said. no frills warmed over crate motor. tranny was gone through by a local shop. all cylinders are firing and appears to run smooth at idle. the shimmy picks up as you accelerate (while in park--can only assume it does it in motion as well).
..all bets are off if it is not GM crate. Could be anything including a bent end flange on the crank. I'd take the convertor bolts out, then rotate the motor by hand up front, while using a dial indicator on hopefully an unchewed side of the ring gear. Turn it from the front because the flex plate will flex too much if you turn it there. See if it's got mega run-out.
from what I recall is was a GM dealership selling GM crate build--but not purchased directly from GM--not an autozone motor--the dealership has since had financial issues which limited their willingness or ability to help for obvious reasons. sorry to be vague--it was 3 years ago and dad handled the initial purchase--I just came home to install the new crate motor--and it's past his bedtime
my opinion is, I would be looking to make sure you have the correct harmonic balancer for the year of the engine. I would also make sure you have the flex plate is also a match, rule that out before you dig into what might be a good engine. My thinking is, it may be an internal balanced engine and the external balancer is causing issues, or the flex plate is balanced or not and it should be.... does that make any sense? If you are sure of that, I had a buddy who develpoed a bad vibration after a few miles on it, it turned out his mains came loose. go figure
Agree with your logic, but dont think an "external balance" flexplate will bolt to an "internal balance" crank. Im thinking the balancer as you suggest. Could just be a loose accessory, such as an alternator resonating.
As suggested, you need to make sure you know what you have for a motor and are using the correct flexplate and damper for it. To eliminate the trans and TQ, just unbolt the TQ and push it back. Fire it up and see. Also make sure your motor mounts aren't loose or broken. Also make sure the valves are adjusted properly.
Are you sure about that? The reason I ask is that I put a 400 sbc in a 69 stick pickup in 1975 and could not find a used 400 stick flywheel. So I transferred the weight from the A/T 400 flexplate to the 69 stick flywheel. I have no clue about the newer motors though. By the way, I did start and idle the 400 without the weight, and it shakes so bad at very low rpms, that you know enough to shut it off
Could be a wobbly distributor shaft ,Just an idea ,My firebird had one ,It would idle fine until I drove it and it wouldnt break up .Just wouldnt go....
It will bot on just fine, but will cause carnage with vibrations. Rotate the engine by hand, if there is about a 1" weight there it is an external balanced flexplate. I have had it happen to me. I have also had people install the wrong harmonic dampner on them and ask me why they vibrated. Does the dampner on the front have a notch machined out of it? Is it a stock dampner or aftermarket? I have seen them boxed incorrectly. Last but not least I have seen used flexplates used that had been bent when they were on the old engine that was moved around the shop. I have used harmonic dampners from 283's on 350 and even a 377 stroker without problems that way. The timing tab needs to be correct for the year of the dampner, but that is not the issue at this time. Good Luck and let me know what you find. CJO13
Fwiw, ALL small blocks prior to like '86(???) can interchange all neutrally dampened flywheels/flexplates, early 265 and 283 damper "hubs" and later real dampers up to the 400 engine. The 400 engine has it's own externally scalloped damper and offset flywheel and flexplate counterweights. Hope you figure it out. Now the later 1-piece engines have regular dampers on them, but externally counterweighted flywheels and flex plates. pdq67
thanks for all the great suggestions--keep em coming and I'll pass them along (I'm 3 hours away and won't be back at the car for two weeks--but dads plugging away trying to figure this out). motor mounts new (replced the old hard mount with new rubber mounts) how do I tell what year the engine is with a crate?
Like alot of guys said, disconnect the converter and push it back then disconnect the belts to where it's just the engine, flex plate or blancer. Just a process of elimination. Then if that doesn't work do a compression check, vacuum check, and go through the ingnition. Do the simple stuff first. What kind of carb. are you using?
If it's a '86 or newer mill the flexplate is balanced, but can be put on wrong if there is no dowel in the extra hole. If there is no dowel just line up the extra hole in the rear crank flange.
Have you tried calling a different GM dealer (or GM tech center?) that still carries these motors? Just because the dealer went under doesn't mean that another GM performance parts dealer won't help you sort out the parts mis-match. This your motor? http://www.goodwrench.com/GMPerform...p?engId=350290&engine=350/290 HP&sku=12499529 Call the techs at Scoggin-Dickey. They'll be able to walk you through what the motor is and what it'll need for a balancer and flywheel. http://sdparts.com/contact The folks here are helpful at times too.... http://www.sallee-chevrolet.com/ There are many other GM performance part dealers out there. I'm sure there's somebody out there that'll help you sort out what you need. -Bigchief.
I wold believe that Hilrod is on to something here. You should be able to lay under the back of the engine and look up and see if it is 85 and earlier with the counterbalance on the back end of the crank or the 86 and up with no counterbalance piece on it. Chances are that as Hilrod mentioned the flexplate was put on a bolt hole or so off register and that is causing the vibration. If it is an early engine that is internally balanced and you have the external balance flexplate on it you can knock the counter weight off the flex plate if necessary. I had to do that on my brothers truck that he was positive had a 400 in it and the engine ended up being a Targetmaster 350 that he had the mechanic install a 400 flexplate on when he had the trans rebuilt.
If the engine has a 2 piece rear main seal its an '85 or older and would use the typical flexplate and damper for the internally balanced engines. "86 and newer have the one piece rear main seal and are externally balanced and you need the '86-up flexplate and damper.
In case anyone's wondering we finally located the problem--or at least most of it. Took it to the guy who built the tranny thinking there was a problem with the converter. He unbolted the exhaust and elminated like 85% of the shake. exhaust system too firmly mounted. Who'd a thunk.
Glad to hear it! Just in case you or anyone was wondering, the 290hp crate engine is based on the old style 350 block...2 piece rear main...so any old style small block flywheel/flexplate will bolt up (even the incorrect 400 sb one, which is externally balanced).
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