noise under mid-hard acceleration 71 340 cuda

[culp71]

ok, I this is a stretch but at this point I am baffled.

I have a 71 340 cuda that I completely restored and finishes a couple of years ago.
I have been driving it very very gently and adjusting/tweaking the few odd things along the way. I have almost every thing worked out but yesterday a new noise caught my attention. it was also the first time since completing restoration that I actually put the pedal to the floor.

It seems no matter the gear (727) or starting from a stop or while rolling any time I put the pedal down fairly quickly or to the floor, the power is insane and its handles beautifully but every time I get a sound that sound like a dense "rattle", but its not as often as I would think if something was rotating.
so I went under the car and started try to shift anything and every thing, tapping my fist against everything to try to find this noise. I did find one of the trans cooler lines close to the K member engine mount area had been touching, so I bent this slightly and added a piece od rubber hose over it where it looked to be touching, temporarily. I took it for a drive, the same sound was there. I also checked over every bolt all connections and everything I could etc. 
everything is tight and appears correct.
it definitely is coming from under the hood somewhere...

any input on to what this noise could be coming from?
it doesn't do it when in N or P.
any direction or suggestions would be appreciated.

[RUNCHARGER]

It could be a loose torque convertor bolt rubbing against the back of the engine block, Or it could be detonation from too much timing in the engine for the fuel you are using.

[torredcuda]

Sounds like it`s possibly detonation which means fuel is combusting too early, it is not good for the engine. I would check your timing curve as you may have too much advance coming in at higher rpms.

[culp71]

ok, I just checked a few videos of ping, knock, detonation, and I can say that that is exactly what I am hearing, super scary...

so best to just retard timing via rotating distributor?

OR will this be adjustment or elimination of mechanical or vacuum advance?

(my distributor currently has internal mechanical advance degree limit set at 24 degrees)
(engine is essentially a stock 340 w 4 barrel and electronic ignition system)

[torredcuda]

All depends on engine combo and what initial and vauum advance there is now. Usually there is too much vacuum advance or it is coming in too quickly, heavier springs allow for a slower curve (coming in later/slower), welding up slots in plate will limit total vacuum advance. First thing is to map your curve with either an advance timing light or a timing tape on your harmonic balancer to see how much and when it comes in.

[7212Mopar]

What fuel are you running? What compression ratio is your engine?

[Bullitt-]

Check the fan shroud for contact marks from the fan if you have one. May be a clearance issue or a bad motor mount.. Also ck that the cooler lines are not to close to pulleys or belts.

[kathyscuda]

im with bullitt. you guys are confusing the hell out of this guy.
run your fingers around  between the fan and shroud to see if you have an even space all the way around.
you will find the noise eventually.

[dodj]

ok, I just checked a few videos of ping, knock, detonation, and I can say that that is exactly what I am hearing, super scary...

so best to just retard timing via rotating distributor?

If you believe it is detonation, retard the timing by rotating the dizzy. Go for a test run. If it goes away you found your problem. 

[anlauto]

ok, I just checked a few videos of ping, knock, detonation, and I can say that that is exactly what I am hearing, super scary...

so best to just retard timing via rotating distributor?

OR will this be adjustment or elimination of mechanical or vacuum advance?

(my distributor currently has internal mechanical advance degree limit set at 24 degrees)
(engine is essentially a stock 340 w 4 barrel and electronic ignition system)

Before you mess with timing, try an octane booster

[73_Cuda_4_Me]

Just venturing a guess, but I'll wager you are running an Edelbrock carb... mebbee a 1406 Performer???

They are notoriously lean... I had lots of pinging on my 70 340 that is temporarily in my 73...

Tried limiting advance, disconnecting vacuum advance, octane booster, etc...

Still had pinging under light to moderate throttle around 30-40 mph...

I upsized the jets and rods to get more fuel flow, and problem went away... runs so much better now... more fuel equated to cooler combustion chamber under light to moderate load for me...

Even able to reconnect the vacuum advance now for cruise mode mileage and exhaust temp reduction (better burn before entering exhaust headers)

[Katfish]

ok, I just checked a few videos of ping, knock, detonation, and I can say that that is exactly what I am hearing, super scary...

so best to just retard timing via rotating distributor?

If you believe it is detonation, retard the timing by rotating the dizzy. Go for a test run. If it goes away you found your problem. 

Easiest route, why make this hard?
Retard the timing and see what happens.

[73_Cuda_4_Me]

  with @Katfish - try retarding the timing a little to see if it goes away... not sure what cam you have, or what timing is at now...?

Try it, and see how it runs...

Once you get some feedback on it, can go on from there...

[culp71]

all helpful stuff guys thanks, so I backed off the timing via rotating distributor, took on test drive but the noise remained, it also would not restart well and would barely turn over, and idled noticeably rougher. so I set it back to where it was. so now im thinking its not timing related....
Part of me is extremely relieved as it seems it may not be engine timing related, as I would be devastate if I caused any major damage. still equally frustrating as I yet been able to pinpoint it, would not even be a concern if I know what it was and didn't fear it is something that could be detrimental to the mechanical functionality. and it was something simple and dumb as I don't ever drive it like that, but after 10 years of work and 1000 miles, I figured its all broken in, and we don't do these types of cars to drive them like a minivan.

I do have a 1406 Edelbrock, with which I have already upsized the jets and rods.

I checked the fan, and can find nothing that can hit or move. I thought it might have been the heat riser in the exhaust manifold so I removed it. but noise still remains. I am questioning the brake lines that have the "spring" wrap on them, as the noise seems to maybe not be so much with the engine load or rpms but a certain vibration caused by engine when rpms hit that generate that certain frequency that causes something to make this noise? so next im going to put some slotted rubber fuel hose over the brake line springs to mute these. try to determine if its these making the noise...

timing is almost exactly at 36 all in, 11 initial and 24 mechanical advance, no vacuum advance connected.
runs perfectly, it seems.
Compression is 10.25 :1 , I did try running up to 93 octane fuel with no change either. 
so even worth it to try a octane booster fuel additive?


on the up side I installed a MeterMatch Fuel Gauge Interface Module
From Tanks, Inc. and got my fuel gauge to actually reflect the amount that's in the fuel tank!

[73_Cuda_4_Me]

Just out of curiosity, what is your vacuum at idle?

It may be that your step up springs on the metering rods are too light, so it's not richening up the mixture with mild throttle increase when vacuum drops slightly...

I also had one step up piston that was stuck in the down position... so check to make sure both operate freely...