Rattling sound while accelerating?

[hambo]

So the past 5 days or so, my 87 big block k20 with an sm465 has been making a rattling sound when I accelerate. It happens in 2nd and 3rd gear but only a little bit in 4th. The rattling sound almost sounds like an old diesel. But it only happens when I accelerate. The previous owner welded the exhaust with two flowmasters, and I know there's a few small leaks, but I feel like it could be the exhaust, I just dont know how it could be. How it could suddenly change. Is there anything else that could be rattling like that?

 

[rich weyand]

Sounds like detonation. Careful, you can dynamite the motor if that's what it is.

One way for that to happen is if the distributor is not clamped down tight and moves to more advanced. Normally it will go retarded when it is loose, but the wires or some vacuum hoses or such could pull it advanced.

 

[hambo]

Sounds like detonation. Careful, you can dynamite the motor if that's what it is.

One way for that to happen is if the distributor is not clamped down tight and moves to more advanced. Normally it will go retarded when it is loose, but the wires or some vacuum hoses or such could pull it advanced.

What does advanced mean? What does it moving to more advanced mean?

 

[chengny]

Rotor turning clockwise: Rotate distributor counter clockwise to advance timing, clockwise to retard.


Ignition timing, in a spark ignition internal combustion engine (ICE), is the process of setting the angle relative to piston position and crankshaft angular velocity that a spark will occur in the combustion chamber near the end of the compression stroke. The need for timing the spark is because fuel does not completely burn the instant the spark fires, the combustion gasses take a period of time to expand, and the angular or rotational speed of the engine can lengthen or shorten the time frame in which the burning and expansion should occur. In a vast majority of cases, the angle will be described as a certain angle advanced before top dead center (BTDC). Advancing the spark BTDC means that the spark is energized prior to the point where the combustion chamber reaches its minimum size, since the purpose of the power stroke in an ICE the engine is to force the combustion chamber to expand. Sparks occurring after top dead center (ATDC) are usually counter-productive (producing wasted spark, back-fire, engine knock etc.) unless there is need for a supplemental or continuing spark prior to the exhaust stroke.

Setting the correct ignition timing is crucial in the performance of an engine. Sparks occurring too soon or too late in the engine cycle are often responsible for excessive vibrations and even engine damage. The ignition timing affects many variables including engine longevity, fuel economy, and engine power. Modern engines that are controlled in real time by an engine control unit use a computer to control the timing throughout the engine's RPM and load range. Older engines that use mechanical spark distributors rely on inertia (by using rotating weights and springs) and manifold vacuum in order to set the ignition timing throughout the engine's RPM and load range.

There are many factors that influence proper ignition timing for a given engine. These include the timing of the intake valve(s) or fuel injector(s), the type of ignition system used, the type and condition of the spark plugs, the contents and impurities of the fuel, fuel temperature and pressure, engine speed and load, air and engine temperature, turbo boost pressure or intake air pressure, the components used in the ignition system, and the settings of the ignition system components. Usually, any major engine changes or upgrades will require a change to the ignition timing settings of the engine.

 

[hambo]

Rotor turning clockwise: Rotate distributor counter clockwise to advance timing, clockwise to retard.


Ignition timing, in a spark ignition internal combustion engine (ICE), is the process of setting the angle relative to piston position and crankshaft angular velocity that a spark will occur in the combustion chamber near the end of the compression stroke. The need for timing the spark is because fuel does not completely burn the instant the spark fires, the combustion gasses take a period of time to expand, and the angular or rotational speed of the engine can lengthen or shorten the time frame in which the burning and expansion should occur. In a vast majority of cases, the angle will be described as a certain angle advanced before top dead center (BTDC). Advancing the spark BTDC means that the spark is energized prior to the point where the combustion chamber reaches its minimum size, since the purpose of the power stroke in an ICE the engine is to force the combustion chamber to expand. Sparks occurring after top dead center (ATDC) are usually counter-productive (producing wasted spark, back-fire, engine knock etc.) unless there is need for a supplemental or continuing spark prior to the exhaust stroke.

Setting the correct ignition timing is crucial in the performance of an engine. Sparks occurring too soon or too late in the engine cycle are often responsible for excessive vibrations and even engine damage. The ignition timing affects many variables including engine longevity, fuel economy, and engine power. Modern engines that are controlled in real time by an engine control unit use a computer to control the timing throughout the engine's RPM and load range. Older engines that use mechanical spark distributors rely on inertia (by using rotating weights and springs) and manifold vacuum in order to set the ignition timing throughout the engine's RPM and load range.

There are many factors that influence proper ignition timing for a given engine. These include the timing of the intake valve(s) or fuel injector(s), the type of ignition system used, the type and condition of the spark plugs, the contents and impurities of the fuel, fuel temperature and pressure, engine speed and load, air and engine temperature, turbo boost pressure or intake air pressure, the components used in the ignition system, and the settings of the ignition system components. Usually, any major engine changes or upgrades will require a change to the ignition timing settings of the engine.

Okay. Thank you for this information. That makes sense. But how would the engine suddenly have bad timing? It never made that noise before about 5 days ago. The previous owner put a new top end on the engine with an upgraded intake and headers, but I figured he corrected the timing because it never made this sound before.

 

[Skweegle89]

Okay. Thank you for this information. That makes sense. But how would the engine suddenly have bad timing? It never made that noise before about 5 days ago. The previous owner put a new top end on the engine with an upgraded intake and headers, but I figured he corrected the timing because it never made this sound before.

you wouldn't notice it over time. It always works until it doesn't anymore.


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[CSFJ]

And you should never assume the previous owner knew what they were doing. You could start a whole new website with those horror stories.

 

[Skweegle89]

I would certainly check out your distributor hold down and get a timing light on it. I would also check your exhaust out, especially at the collectors and make sure it's tight. If timing doesn't fix it, pour a little marvels mystery oil or 3in1 oil down the carb and it will quickly show you where your exhaust leaks are. Don't toy around with it before you check the timing though, as said before, detonation isn't something to take lightly and should be corrected ASAP.


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[CSFJ]

This is a piston

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This is a piston after prolonged detonation

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[hambo]

I would certainly check out your distributor hold down and get a timing light on it. I would also check your exhaust out, especially at the collectors and make sure it's tight. If timing doesn't fix it, pour a little marvels mystery oil or 3in1 oil down the carb and it will quickly show you where your exhaust leaks are. Don't toy around with it before you check the timing though, as said before, detonation isn't something to take lightly and should be corrected ASAP.


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Okay. Yeah I was planning on checking it with a timing light. How would I check the distributor hold down? Just feel the distributor and try to physically move it and see if it moves and is loose? And my 454 is a tbi so I'll just visually check the exhaust.

 

[chengny]

Generally speaking, if your ignition is so far advanced - to the point where running the engine risks the possibility of mechanical damage - you would probably have noticed any or all of these symptoms by now:

1. Excessively high idle

2. Backfire at engine shutdown

3. Kickback while cranking the engine to start

Ignition timing is changed by loosening the clamp and indexing the distributor:

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Unless the clamp bolt is loose, the timing will stay where it was set. (Sorry, I don't have any images of a BB distributor.)

If you suspect that your timing has "wandered", check for that possibility this way:

Without tightening /loosening the clamp bolt, grasp the distributor cap in both hands and attempt to rotate it in either direction. To test whether the distributor is securely clamped - twist it hard!

Even with the clamp loose, it is sometimes difficult to budge the distributor from it's present position. If you cannot move the distributor - it is unlikely that your timing has changed.

Whether it was set correctly in the first place is another matter.


To identify any exhaust leaks, listen to Zach when he says:

pour a little marvels mystery oil or 3in1 oil down the carb and it will quickly show you where your exhaust leaks are.

I don't know about 3 in 1 oil, but MMO dribbled into the carburetor throat of a running engine will cause streams of white smoke to issue from any active leaks in the exhaust system.

There are other - more serious - reasons that will cause an engine make the sounds you describe, but eliminate the simple stuff first.

A good first diagnostic would be to loosen the distributor clamp and retard the timing just a bit (turn the cap CW - looking down). If the noise diminishes, go a bit further. If that helps even more, stop - at that point you should really buy or borrow a timing light to determine where your timing is set.

 

[hambo]

Generally speaking, if your ignition is so far advanced - to the point where running the engine risks the possibility of mechanical damage - you would probably have noticed any or all of these symptoms by now:

1. Excessively high idle

2. Backfire at engine shutdown

3. Kickback while cranking the engine to start

Ignition timing is changed by loosening the clamp and indexing the distributor:

You must be registered for see images attach

Unless the clamp bolt is loose, the timing will stay where it was set. (Sorry, I don't have any images of a BB distributor.)

If you suspect that your timing has "wandered", check for that possibility this way:

Without tightening /loosening the clamp bolt, grasp the distributor cap in both hands and attempt to rotate it in either direction. To test whether the distributor is securely clamped - twist it hard!

Even with the clamp loose, it is sometimes difficult to budge the distributor from it's present position. If you cannot move the distributor - it is unlikely that your timing has changed.

Whether it was set correctly in the first place is another matter.


To identify any exhaust leaks, listen to Zach when he says:

pour a little marvels mystery oil or 3in1 oil down the carb and it will quickly show you where your exhaust leaks are.

I don't know about 3 in 1 oil, but MMO dribbled into the carburetor throat of a running engine will cause streams of white smoke to issue from any active leaks in the exhaust system.

There are other - more serious - reasons that will cause an engine make the sounds you describe, but eliminate the simple stuff first.

A good first diagnostic would be to loosen the distributor clamp and retard the timing just a bit (turn the cap CW - looking down). If the noise diminishes, go a bit further. If that helps even more, stop - at that point you should really buy or borrow a timing light to determine where your timing is set.

Okay. Awesome post. Thank you. The only thing I have noticed is the truck will idle around 1100rpm for maybe 15-20 seconds around startup a few times. But not everytime. Then it goes back to normal around 750-800rpm. Once it went low around 600rpm. But it doesnt do that once it warms up. And I dont have a carb, I have tbi, so where woukd I pour the mmo? Sorry, my mechanical knowledge is pretty limited. Just trying to learn. Haha.

 

[chengny]

That high/erratic idle problem is probably an idle control valve issue. If it settles out after a while (and is only intermittent) - it is not timing related.

To inject MMO into to a TBI system is basically the same as with a carb - just pour some down the throttle body throat.

 

[hambo]

That high/erratic idle problem is probably an idle control valve issue. If it settles out after a while (and is only intermittent) - it is not timing related.

To inject MMO into to a TBI system is basically the same as with a carb - just pour some down the throttle body throat.

Okay. Thanks for your help. Hopefully I should be able to get ahold of a timing light tomorrow and check it out.

 

[hambo]

Well I wasnt able to get ahold of a timing light today. And now I also notice it sounds like theres a knock in the engine while idling. And the rattling seems to come in the first little bit I press the accelerator at idle and then it goes away. At this point I might just take it to a mechanic and have him figure it out because im starting to worry with the knock that this might be more than just a timing issue