340 marbles rattling around 2200rpm

[Rockboyz]

Guys,
Brought this up before and still chasing this noise among other things. So it almost sounds like a couple marbles rattling hard in a glass jar. 340/727/3:91. Cruising along under light throttle between 2000- 2300 rpm, this noise appears. It only happens under very light throttle / load. once I leave off the pedal, noise is gone. gently hit the pedal again, noise comes right back. No noise below 2000 and no noise above 2300. If I hit the pedal a little harder when cruising between 2-2300, the noise goes away. Only happens under very light pedal or throttle. When cruising, I can play with it, light throttle on and off, noise comes and goes, and only in that rpm range. One other thing to note, It wont do it in the garage or driveway in park or neutral, has to be on the road under load. Engine was rebuilt in 1984, maybe has 10k miles on it for reference.   

[Katfish]

You're sure it's not detonation?

[torredcuda]

Knocking? What initial timing, how fast does advance come in and total timing? It might be too much advance coming in too soon. Pull a couple spark plugs and check them.
"Here's a breakdown of what detonation can look like on a spark plug:
"Fly specks" or pepper-like black spots:
These are the earliest signs of detonation and appear as tiny black specks on the ceramic insulator tip of the spark plug. They are caused by carbon particles being rattled loose in the combustion chamber by the detonation shockwaves.
Bright "diamond-like" spots:
If detonation is severe enough to cause aluminum from the piston to be blown off and deposit on the spark plug, you might see small, bright, diamond-like spots on the porcelain. This is a sign of significant damage and requires immediate attention.
White deposits or blisters:
While not exclusive to detonation, severe overheating that can accompany detonation can also lead to very white insulator tips or even blisters on the insulator."

[Rockboyz]

I'll pull a couple plugs and see what they look like. Thanks!

[Rockboyz]

Pulled two plugs. 1 & 2 to be exact. They look fine. If anything a little on the rich side. So I decided to start over and go through the steps from what I know. I'm not an expert my any means. Car was hot, pulled the vacuum hose off the distributor, plugged the hose, and at roughly 1050 rpm timing is at 18 degrees. ran the rpms up to 2000 and timing is at 22 degrees, hooked the vacuum hose back up, set the idle back at 2000 rpm, and played with the distributor just a tad. I'm reading 37 degrees total. Went through the carb settings again with a vacuum gauge, 1-1/8 turns out seems to be the spot. However, with this setting and idle at 1050, in gear the rpms drop to 650ish and it wants to stall. If I go to 1.5 turns out, it doesn't drop as much and will idle in gear better. but running very rich Drove the car and the marbles are still there, but not quite as bad.  Im missing something, i just don't know what.

[Katfish]

37 is a lot, I could never run my 340 above 30 total ( no vacuum) when hot.

[Bullitt-]

 It sounds to me you may have multiple issues .. I would work with the vacuum advance disabled until it runs well  then dial that in.

I'm guessing your timing is a little to advanced. I only have experience with a stock '73 340. I followed CP's method (link below) & ended up backing off a couple of degrees to get mine to idle around 850RPM. Heavily modified could want something else I suppose.

 https://forum.e-bodies.org/reference-material/18/setting-timing/213/msg2055#msg2055

 The stalling when put in gear sounds like the idle transition slots are over exposed because the idle speed screw is turned to far in.  Below is a video that explains the issue but some of the solutions applies only to modified motors
 I'm running a stock Thermoquad which the base mixture screw setting is 2.5 turns out, I think my final setting is closer to 2.75
 Different carburetors have different base settings.

 https://youtu.be/DpU7pV4Li3A?si=uqPfWmt-d1kj0xrp&t=318

[tparker]

You could try removing one sparkplug wire at a time and test to see if it is related to a single cylinder, or if it changes behavior. Perhaps something with the valve train?  Maybe there is something just loose enough or worn enough that it makes noise at certain RPMs. The next thing I would look at is adjacent parts that might be rattling. Probably not,  but worth a check.

[Cudajason]

Pulled two plugs. 1 & 2 to be exact. They look fine. If anything a little on the rich side. So I decided to start over and go through the steps from what I know. I'm not an expert my any means. Car was hot, pulled the vacuum hose off the distributor, plugged the hose, and at roughly 1050 rpm timing is at 18 degrees. ran the rpms up to 2000 and timing is at 22 degrees, hooked the vacuum hose back up, set the idle back at 2000 rpm, and played with the distributor just a tad. I'm reading 37 degrees total. Went through the carb settings again with a vacuum gauge, 1-1/8 turns out seems to be the spot. However, with this setting and idle at 1050, in gear the rpms drop to 650ish and it wants to stall. If I go to 1.5 turns out, it doesn't drop as much and will idle in gear better. but running very rich Drove the car and the marbles are still there, but not quite as bad.  Im missing something, i just don't know what.

WAIT---STOP....ok DO I have your attention.  Are you saying your doing all the tweaking with the car in Neutral / Park?

If this is an automatic, which I assume it is, you should be doing all your tuning and tweaking with the car in Drive / the engine under load.  I usually set the parking brake put the car in drive and et it idle away. 

Any adjustments you do with the car in neutral or "idling" at 2000 rpm will change one you idle it down.

[Rockboyz]

It sounds to me you may have multiple issues .. I would work with the vacuum advance disabled until it runs well  then dial that in.

I'm guessing your timing is a little to advanced. I only have experience with a stock '73 340. I followed CP's method (link below) & ended up backing off a couple of degrees to get mine to idle around 850RPM. Heavily modified could want something else I suppose.

 https://forum.e-bodies.org/reference-material/18/setting-timing/213/msg2055#msg2055

 The stalling when put in gear sounds like the idle transition slots are over exposed because the idle speed screw is turned to far in.  Below is a video that explains the issue but some of the solutions applies only to modified motors
 I'm running a stock Thermoquad which the base mixture screw setting is 2.5 turns out, I think my final setting is closer to 2.75
 Different carburetors have different base settings.

 https://youtu.be/DpU7pV4Li3A?si=uqPfWmt-d1kj0xrp&t=318

So I decided to try Chryco's method. went back and forth 3 times, turning the vacuum allen screw out 2 full turns each time, for a total of 6 full turns. At idle rpms kept going up by advancing the distributor. So I stopped. Felt like I was getting nowhere. What I decided to do next is run it back up to 2500 rpms and slowly back it down to idle, I could hear the marbles at about 1850, so I retarded the distributor till the noise went away. Back up to 2500 and now Im at 29 degrees. Brought it slowley back down to idle, no noise.  Now at 950 idle I can pull it in gear and it only drops 100 rpm, doesn't want to stall. Took it for a short drive and tried to find the marbles...seems like they are gone. the motor is hot for sure, puked a bit in the driveway. Ill probably park it for the night and take it for a ride early morning, see if the marbles are still there. Not sure what may have fixed it, one or the other or both, but I guess each motor has its own personality. I appreciate everyone's input! thank you! 

Well guys, looks like I may have jinxed myself, warmed her up, took her for a ride, can still hear the marbles...although not as bad as before. just around that 2200 lite throttle mark...maybe retard the timing a tad bit more?

[torredcuda]

What do you have for a carburetor? I had an issue with mine dropping too much rpm when putting it in gear and after a lot of trouble shooting and messing with carburetor and timing settings a friend said to check to make sure my advance wasn't coming in too soon, like at idle. Sure enough the slight bounce in timing at idle was not due to my lumpy cam but the advance coming in at 1000 rpm, once I  changed one advance spring to the next heavier one the issue went away. It may also be the secondary transition slots as someone else mentioned.

[Rockboyz]

What do you have for a carburetor? I had an issue with mine dropping too much rpm when putting it in gear and after a lot of trouble shooting and messing with carburetor and timing settings a friend said to check to make sure my advance wasn't coming in too soon, like at idle. Sure enough the slight bounce in timing at idle was not due to my lumpy cam but the advance coming in at 1000 rpm, once I  changed one advance spring to the next heavier one the issue went away. It may also be the secondary transition slots as someone else mentioned.

Its a Holly 650 vacuum secondary. 2 years old. Went after it again today. Retarded the timing a degree, played with the carb again, drove it and all seems good at this point. no marbles. idles 950rpm, in gear 850rpm. Doesn't seem to have any hesitation. So far pretty pleased with it. Now I have to work of the vapor lock. That's my next project along with chasing fluid leaks... does it ever end? lol! Thanks for the input!

[Rockboyz]

So marbles are still there gents. after lots of reading, I'm figuring this is detonation. Over the last several weeks I have tried a lot of different things. Higher octane fuel, New plugs Autolite 63, however old plugs looked fine. Seafoam game in the carb. Played with the vacuum advance canister. Put a pump on it. Diaphragm holds air at 18 turned all the way in. Tried adjusting the canister full in, halfway out, all the way out. adjusted the carb 20 times. switched between manifold, ported vacuum. Manifold vacuum is right around 14 at idle. where the car performs the best right now is initial timing set around 16 degrees. Manifold vacuum not ported. Smooth idle, snappy response, and still starts easy when operating temp (hot). 1/4, 1/2, or WOT car runs strong. no noise. That being said, cruising 2200-2600 rpms, under very light load, just touching the throttle rattle knock, rattle knock. Im coming up with the mechanical advance is coming in to soon. Thinking about trying heavier springs in the distributor, slow the advance down. Feedback is greatly appreciated!    

[pschlosser]

you're making progress.  it sounds like you've determined your engine timing is advancing too far and causing detonation.

knowing this, I would immediately retard the timing enough, so you cannot hear the marbles even under heavy acceleration at any RPM.  let us not forget, detonation can cause harm to your engine and melt pistons.

only after you no longer have detonation, would I begin to try to improve performance, and advance the timing, again.

if you suspect your distributor, or its vacuum and mechanical advance systems are out of spec, I recommend you have your distributor tested to show if the mechanical advance is out of spec.  Only then, would I begin changing springs and making alterations to the distributor.  

I understand the springs change WHEN (in the RPM curve) the mechanical advance is applied.  but your issue does NOT seem to be WHEN the advance is applied, but that TOO MUCH advance is being applied.

[Rockboyz]

you're making progress.  it sounds like you've determined your engine timing is advancing too far and causing detonation.

knowing this, I would immediately retard the timing enough, so you cannot hear the marbles even under heavy acceleration at any RPM.  let us not forget, detonation can cause harm to your engine and melt pistons.

only after you no longer have detonation, would I begin to try to improve performance, and advance the timing, again.

if you suspect your distributor, or its vacuum and mechanical advance systems are out of spec, I recommend you have your distributor tested to show if the mechanical advance is out of spec.  Only then, would I begin changing springs and making alterations to the distributor. 

I understand the springs change WHEN (in the RPM curve) the mechanical advance is applied.  but your issue does NOT seem to be WHEN the advance is applied, but that TOO MUCH advance is being applied.

Well, I've spent a lot of time, and made a lot of progress in learning, but not solving the issue. I have gone down the list of possible options. Fuel, plugs, timing, ported, manifold, playing with the vacuum canister. Spoke to Dana at 4secondsflat, tried the FBO plate, diff springs, I can rebuild a distributor in minuets now. At the end of the day, the engine wants 18 degrees at initial / idle, all in at 34. That's where it starts and runs the best, however the noise is still there. Im coming to the final conclusion that its a rod knocking. Must be a bearing.