random knock

[Rolling_Thunder]

Well just got a big block built and it has a random knock in it...    it is not a main bearing, or rod but still sounds rather severe...    usually onl happens when i let off the throttle after giving it alot of gas...     any ideas ?

[cudaken]

 RPM range you run it up to and what RPM's you hear it. Good luck as well. We thought mine was nothing.

                                Cuda Ken

[RD]

are all the flexplate bolts tightened to the appropriate lbs?

how long does the knock last on deceleration, and have you located the area where the knock is?

[Rolling_Thunder]

I'll run the engine up to maybe 5000rpm and let off the gas and it will knock...     once....      then it'll settle down to idle and run like a champ...     get into the gas and let off...    knock...       again only once...   which is why it is bothering me...    if it was constant i could figure it out...     but it is totally not the case...     any suggestions ?     I know nothing in the bottom end is loose...   flywheel is torqued...      so im thinking maybe something in the top end...     only place it can be....   

[Runner]

sounds like a drive train issue to me.  bad u joint or ring and pinion.   is it a manual tranny.  if so it could easily be a clutch issue.

[Runner]

also , dont forget to check motor mounts and a tranny mount.   it could easily be a motor mount.

[RD]

I agree with runner, sounds like the driver side motor mount.

[Rolling_Thunder]

not a driveline problem...   as it also does it if i am playing with the throttle by hand and in neutral...       It isnt the motor mount...   and it doesnt even knock everytime i romp the throttle...    maybe only 3 out of 4 times ?

[Rolling_Thunder]

anyone ?

[cudaken]

 Try rasing the RPM's slowely till you hear the noise. Might try dropping the belts as well. Motor mount would dent the headers or knock off paint on the inner fender maybe. I don't think its a mount, I have broke a lot of them.

What valve gear are you running, Mech or HYD. I don't think you will find it there, but some time you never know. What arms are you running?

Double check the bell housing bolts and starter bolts. Had the bell housing bolt back out on me one time and sounded like a rod. When I was yanking the 426 Maxie I found where thet had back out. Lose start bolts could do it as well. Nose of the starter bounces off the fly wheel. When you say flywheel I am 95 sure you did not mean flex plate. If you are running  727, lose converter bolt will do that as well.

Oh, check the crank free play. Bad thrust bearing could do it as well.

                                   Cuda Ken

[Rolling_Thunder]

thanks Ken - i'll start looking...    Bellhousing is tight...    flywheel is tight....       thrust bearing is good...    its a fresh engine....     

Valve gearing ?  ummmm   im running stock rocker arms and shafts....     if i raise the RPM's it doesnt happen...   but if i romp the throttle is will sometimes happen as the RPMs fall...    but it will only knock once...   

[firefighter3931]

but it will only knock once...     

It's difficult to diagnose without being there, but based on this info it doesn't appear to be engine related. Is it possible the headers are contacting the frame or torsion bar ? As you accelerate the engine will lift a little then when you let off the throttle it settles back down and something is clunking/knocking....just a theory.

Ron

[Rolling_Thunder]

but it will only knock once...     

It's difficult to diagnose without being there, but based on this info it doesn't appear to be engine related. Is it possible the headers are contacting the frame or torsion bar ? As you accelerate the engine will lift a little then when you let off the throttle it settles back down and something is clunking/knocking....just a theory.

Ron

doubtful....    engine doesnt move at all...    solid motor mounts   

[Chryco Psycho]

trans mount moving sideways

[Rolling_Thunder]

trans mount is tight...

[cudaken]

 Check the headers and exhaust pipes. If close to anything they could hit it just a little. Pipes are hung with rubber mounts, when backing off the could move even with solid mounts.

I am starting to think it not the engine either. With the car in the air get under it and have some one give her a little hell. See where the sound is coming from. My Hookers are real close to the front bar, when I but it in reverse it hits the front bar one time. I run rubber mounts and know that is not your problem, just a example.

You will track it down.

                                Cuda Ken

[Rolling_Thunder]

well does it help that it did the same thing on the engine run stand ?  so i pulled the heads and didnt find anything and put it all back together...     then it ran fine ---  now it started doing it again....   

[is_it_EVER_done?]

If the items already posted don't solve it, try these three things:

First, check the condition of your vibration damper. I once bought a car with the understanding that it had a bad, or blown rod (severe knock). Upon removing the engine for teardown, I discovered the damper rubber had slipped enough to let the outer ring hit the hub. another damper solved the knocking.

Second, try it with a couple gallons of race gas when the tank is near empty, or with a line from the fuel pump to the gas can. This should rule out or confirm spark knock.

Third, try your rev test with one plug wire disconnected. Work your way around the engine. If it's a wrist pin or rod bearing, the noise should (not always) dissapear when you disconnect the plug of the effected cylinder.

[Rolling_Thunder]

If the items already posted don't solve it, try these three things:

First, check the condition of your vibration damper. I once bought a car with the understanding that it had a bad, or blown rod (severe knock). Upon removing the engine for teardown, I discovered the damper rubber had slipped enough to let the outer ring hit the hub. another damper solved the knocking.

Second, try it with a couple gallons of race gas when the tank is near empty, or with a line from the fuel pump to the gas can. This should rule out or confirm spark knock.

Third, try your rev test with one plug wire disconnected. Work your way around the engine. If it's a wrist pin or rod bearing, the noise should (not always) dissapear when you disconnect the plug of the effected cylinder.

cool - thanks for the ideas...    the thing that gets me is it is not constant....   wouldnt that rulke out detonation and the dampener ?

[is_it_EVER_done?]

Darn it! I could just answer your question with NO, but instead I will explain as I’m always willing to help out the new generation that loves these cars. I apologize to everyone for the length.

It's probably not the damper, but since you've checked everything else, and it only takes about 10 seconds to check, I suggested it. Use a screwdriver between the block and damper (NOT RUNNING), pry on the ring to see if it is loose. At minimum it's one more thing you can rule out, and in the example I gave, it started out minor and intermittent and progressed to the point that 2 lifelong hardcore Mopar gearheads (the seller and myself) were fooled, and missed it.

I also doubt that it's a bad bearing or wrist pin (new engine), but if it's not spark knock, that's the next place I would start looking just to eliminate the possibility, because if it IS a bearing or pin, you can quickly go from an easily fixable problem, to catastrophic failure and irreparable damage by continuing to run it while checking for other causes   (don’t ask me how I know).

My first guess is spark knock, as your description sounds just like what used to be a common service complaint when unleaded fuel was starting to be phased in and these cars were still stock and under warrantee. Some Mopars would develop a very loud knock -- just one or two -- when the throttle was snapped closed from high RPM, or just free revved (quick stab of the throttle) in neutral. Yes it was intermittent, as temperature, humidity, barometric pressure/elevation, brand of fuel, operating temp, etc., all have an effect.

Though the reason was a slight decrease in octane, the cause was generally traced to improper distributor maintenance, aggravated by the design of the Carter carburetors.   Most people and garages were not keeping the wick under the rotor oiled, which would cause the centrifugal advance plate to be sluggish in returning from full advance. That coupled with the metering rod system of the Carter carburetors, which goes lean under vacuum (for gas mileage), and you can get a brief moment of detonation, usually in the #1 or #2 cylinder, as they run the hottest due to getting coolant last.

The common dealer fix was to oil the centrifugal advance plate shaft and wick, and sand a little off the diameter of the fat part of the metering rods. Unleaded gas quality improved quickly so it became less of a problem, and of course compression was reduced on newer models, so it was not a problem with post 1971 cars, but I still sometimes (rarely) see it with the resurgence of stock, near stock, and numbers matching rebuilds (you would be amazed at how many people don’t oil the distributor wick).

I have no idea if this is your problem as I can’t hear it, and I have no information what your rebuild entailed, but testing with high octane gas will prove or disprove if this is your problem, even if you have a different build combination.

Again, You said you have checked all the other posted suggestions and ruled out interference, loose bolt or bracket, motor mounts, or any other common and logical cause, so these things are easy and quick to check. If they don’t isolate it, you will need to start looking for a mechanical defect, which are much tougher and time consuming to locate.

[Rolling_Thunder]

Hey man thanks for typing that all up...     right now the engine is apart and i am swapping cams....      the dampener might actually be the problem...     as i tried to time it andthe timing mark was WAY off...     so i try to get it back to 0 degrees and it would die....        i have two other dampeners so i will check as you suggested....         if that is not the case i will be stumped....        The engine is running 9.0:1 compression and i am using 91 octane.....   heads are ported and polished 906's   i did have a problem tuning it to run correctly....     I couldnt set the timing so i have a stupid question...        what would happen if the engine was running lean ?   any problems with knocks ?

[Chryco Psycho]

not decelerating , only under load