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this one is stumping me, so much as to make me finally make an account here
so i have a 77 440 that gives a bad resonant vibration at 1800-2000rpm
but only off throttle, engine braking, free revving whatever. it only does it as it slows with zero throttle input across the aformentioned rpms.
on throttle is smooth, all other rpm is smooth, it even does it with the ignition off.
so far ive eliminated the drive train, this happens in neutral sitting still, clutch in or out makes no difference
pulled belts and fan, no change.
the vibration doesnt seem to come through the shifters much at all for the amount i feel it in the cab and seat.
the engine is a fresh ground up mild truck build, changed to a forged crank and neutral balance with a 143 tooth fly and centerforce clutch
all balance and machine work was done by a good reputable shop
new engine mounts etc as this was a BB conversion. i dont recall if this is getting worse or i just notice it more now that ive gotten other issues resolved like the bad dizzy and lack of exhaust.
any ideas what i might be missing?
id be inclined to think that too, but shouldnt i feel something under power or at more than one narrow rpm range and only under negative load?
it idles nice and smooth and is buttery smooth all the way up to redline, its only when its coming down with no throttle input through that one range.
how often do flywheels ship out badly out of balance? thats the only thing that didnt get balanced yet and i dread pulling the damn thing
IF your engine builder is confident that the engine internals are sound. I think you have a couple of choices at this point.
Dampner, Flywheel, Clutch asm,
Or like Iowan has passed on. Could be a problem that is being over looked. Pilot Bushing and issues back from that point might be an issue.
But yes... We have seen issues only on Decell when the engine is not under load, Per say.
Keep us posted...
I've heard of the pilot bushing having enough wear and or the front bearing in the trans causing problems, try depressing the clutch and letting it back out during the vibration and see if there is change.
yes, brand new, entire engine is ground up built
clutch in or out doesnt seem to affect it
Originally Posted by RacerHog
IF your engine builder is confident that the engine internals are sound. I think you have a couple of choices at this point.
Dampner, Flywheel, Clutch asm,
Or like Iowan has passed on. Could be a problem that is being over looked. Pilot Bushing and issues back from that point might be an issue.
But yes... We have seen issues only on Decell when the engine is not under load, Per say.
Keep us posted...
from what i know so far things are pointing to the flywheel/clutch, harmonic is a solid disc. but again, if its out of balance shouldnt it vibrate under some other condition as well?
i also did a compression test to make sure i didnt have a bad cylinder and they all came back good as well just the #1 and #7 look a little rich on the plugs
still waiting to talk to the machinist that has been building these for 40 years since hes out sick right now
Not so much.... You have already come up with the area/condition at where this happens. Now its just what is the culprit. Now it becomes a prosses of elimination.
Like: remove the trans and see if the problem is still there..... remove the pressure plate and clutch disc and see if its still there.... So on and so forth, or at that point it you just pull the motor down and send it back for recheck on the balance of the components?
But lets make note, we are assuming this is a balance issue in the engine and not the trans sending the vibration.
Not so much.... You have already come up with the area/condition at where this happens. Now its just what is the culprit. Now it becomes a prosses of elimination.
Like: remove the trans and see if the problem is still there..... remove the pressure plate and clutch disc and see if its still there.... So on and so forth, or at that point it you just pull the motor down and send it back for recheck on the balance of the components?
But lets make note, we are assuming this is a balance issue in the engine and not the trans sending the vibration.
is it not a fairly safe assumption that its not the trans since it still happens regardless of speed or being stopped and clutch in or out?
ill talk to the machinist that did the work monday since hes back in and go from there
Could be a safe assumption. Just don't totally discard it until you know for sure. Keep it in the back of your mind just incase you have to go back and Verifie it...
alright heard back from my machinist and he did some poking around and put me on the right track.
sounds like the centerforce clutch is the cause of the shakes, found some similar issues online then called CF they initally claimed the clutch is balanced and that wouldnt happen, then copped to them not being balanced precisely and the weights tend to move and will move to the heavy side if there is one and exacerbate any vibrations already there.
got underneath and pulled the cover... lo and behold the weights were cocked all to one side, poked them around to what was as centered as i could see and fired it up and it spun smoother than ever, a couple revs however shifted them again and the resonance returned.
but now that the weights are in a slightly different spot i can feel it from 2500-4k holding revs and the off power shudder has moved up to about 21-2300revs
for anyone else dealing with irregular balancing CF stated a 10% loss in overall holding force at 5k rpm if you remove the weights to correct the balance.
so next weekend ill be yarding the mess out and getting it spun with the weights to see just how bad it was then chopping them off and getting it balanced properly.
so hopefully ill have a full conclusion by feb since im limited to every other weekend for wrenching
conclusion.
pulled the clutch and flywheel out and cut the weights off. took it to the machine shop and turns out it was pretty far out but due to the pressure plate bolt holes being over bored it let the plate mount non concentrically and created a dynamic 20gram imbalance that moved around each time the plate was reattached
they ended up having to drill and pin the plate with indexing dowels to keep it centered and allow it to be balanced properly. so a lot of frustration on the machine shops part i got the mess back and got it all back in the truck.
got it fired up and the off power resonance is a lot weaker but still present but now there is no vibration at any speed under any amount of power.
so it was definitely a large factor but not the sole cause of the issue. ill snag some new transmission mounts next and beyond that im not sure if i care enough to worry about it since its not rattling everything so bad anymore.
as you can see in the pics the spring wire holding the weights in place is far from a perfect circle so that would be issue #1 for them getting out of center
and then the glob of steel to balance it out after staking it with the pins as close to balanced as it could get.
alright, addendum:
final source of vibrations were from the exhaust hangers, when hot the expansion got just that tiny bit of extra tension on them to pull the dampers tight and the shop that made it up used the cab supports to bolt them to.
once disconnected and out of the way, everything is smooth now.
the whole mess did inspire me to soundline the whole cab which makes for a very nice and quiet ride now, the process of pulling the seat is when i found the hanger situation.
01-08-2019, 09:42 PM
