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Rattling noise, conditionally

2K views 38 replies 8 participants last post by  DZA 
#1 ·
Vehicle: 1976 gmc 1500 w/350 & 350 trans

I get this rattling noise only when I am in gear and moreso once the engine is warm. It only occurs when accelerating and the noise is much louder up a hill.

The noise smooths out some when I give it more gas pedal. The noise is still there but is much quieter when i floor it.

I have revved the engine in neutral and no noise. I put it in drive with the break on and just giving some gas produces the noise.

The thing is running pretty rich. I smell fuel. The truck even diesels when I shut it off. If its a noise from detonation, wouldn't the rattling occur in and out of gear?

I don't see any cracks on the flex plate (or is it the flywheel that the starter contacts on start?

Could something just be loose?

Thanks!
 
#6 ·
I would want to verify that the timing marks are correct and that the marked outer part of the balancer has not slipped due to age. Start by finding actual TDC based on valve and piston position, then compare that to what the marks are telling you. Because if you are depending on the marks and they are incorrect, theres your trouble. Make a new mark for use only for the current day, then re-establish initial timing and re-check total.
 
#7 ·
I plugged the dist vacuum line and the dist that the vacuum line came from.

I checked initial timing and i was at about 10.

I checked timing at 2500-3000 rpms (by sound) and i was at 25 degrees.

I advanced to 34-35. When I checked initial timing after, it was also 34-35. How?

And if I advanced 10 degrees, wont that be more rich and more detonations?
 
#9 ·
You can not check total timing by sound. You need to be sure the rpm is above the mechanical advance engagement and steady. If your timing is not coming back down at idle, your advance is sticking or springs too light to return weights. Initial timing is checked at below 800rpm to keep the mechanical advance out of the equation.

If your balancer has slipped, replace it, no good will follow a bad balancer.
 
#13 ·
Just another log on the fire here...

Through the years, SBCs used different timing mark tab positions. If any swapping has been done, you can end up with something thats WAY off.

Its possible to make your own timng marks if need be.
 
#15 ·
i would with the ROTOR INSTALLED grap it and twist it against the return spring then release.. then verify it snapped back freely to the hard stop..


i have had so many distributors especially HEI's that the upper parts on the distributor shaft.. the part where the reluctor is seizes because the grease has gotten hard or has left all together..

i also totally agree with idrivejunk that the rotors can catch the advance weights..

you would also have the advance weight bushings that have failed and the weights carved into the pins..

if you decide to take the distributor out.. please preset the motor with the rotor pointing to the #1 position and fine tune that by turning the crank slightly in the normal direction of rotation till the rotor is pointing to #1 and the tips of the reluctor are aligned with the pickup coil .. then look at the damper timing marks.. that is base timing.. so you can easily reset it.. when installing the distributor.. make sure the advance is fully retracted.



i usually take the shaft out and disassemble the top to clean and grease the upper section.

huge tip... when you have knocked the gear off the shaft.. turn the housing bottom up... file the burr the roll pin made on the shaft smooth.. then fill the housing with carb cleaner... start turning and pulling then pushing the shaft back in.. working it more and more till it comes out.. do not leave it part way out as it gets tight.. you may have to refill the housing 2 or 3 times to do this.. you are working the varnish off the shaft so you don't destroy the upper bushing trying to get the shaft out..

i don't know about anybody else.. i use moly graphite assembly lube for the upper shaft .. think that black paste for cam lobes.. it goes on the lower shaft bushings also..
 
#18 ·
Yup, I got it wrong there.

Lots of good info here. I called in my mechanic neighbor. He said that I have an electronic dist and should self adjust? Anyways, he tuned the carb, idle and dist for the warmer weather. I was way lean. I have been using 87 octane. I was 1 gallon to empty so we added 93 octane. The engine itself is much quieter and the rattling is almost completely gone. We are going to recheck mid week.

Thanks for all of the info. I thought for sure it was the trans. You've all given me a lot to go google. I appreciate it!
 
#20 ·
Yeah... naw. No knock retard on that. Your mechanic... possibly. Set up properly, that thing should purr like a kitten on 89 or 91 octane with ethanol.

The good gas sure will help until you can study up and have a closer look.:thumbup:
 
#23 ·
When I was adjusting things (just for today, of course), I noticed that adjusting the degrees on the timing light didn't move the mark much. I changed the degrees on the light from 15 to 25 and the mark only moved a couple of inches. Is that normal? If not, maybe a symptom pointing at what is off?

The test drive went well. I took it out twice. The second time, mostly cooled down, I didn't hear any pings or knocks. At idle, I was getting some deep rumbles that I thought was the guy behind me with his bass thumping. Other than that, it's infinitely better.

Next is the testing as suggested... It just happens to by my daily driver. So hopefully the engine won't get too mad between now and Wednesday when I play with things next.
 
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#31 ·
Bring this thread back b/c the problem remains. The knocking has occurred for the entire time I owned the vehicle. It's been a couple years. I usually only hear it once warm and up hills. Now, I can hear it just riding down the road (mostly if I'm driving next to a wall or under an over pass). It still needs to be warm to hear, I believe, though.

I adjusted the timing a million times. I replaced the cap, rotor and coil. I don't think it's timing, like you said (unless there is something wrong with the rest of the distributors guts).

I disconnected the heater core and replaced the radiator cap and thermostat so the temp and coolant issues were resolved.

I am bleeding oil pretty bad. I didn't realize how bad. I was 3.5 quarts of oil low yesterday. I believe the oil is coming out of the rear main.

I had a bad exhaust leak at the header gasket. The engine smoothed out a lot. The knocking remains. Now the header gasket sounds like it blew again.

Here is the real question: How bad is it to drive on that motor? If the bearing is bad or gone and I'm just knocking real good, and I have no intention to rebuild, is it safe to drive on it? When will the rod/cap/piston just crumble from the unwanted movement? Is it just going to go boom one day? I know it sounds bad, but is that all that will happen? Just an annoying noise until boom? I give up on that engine, but I can't buy a replacement yet.

Thanks, again!
 
#26 ·
I am short a vehicle. I had to take the truck to work. The knocking is no where near as bad but still need to investigate. I didnt want to start a new thread bc this is related...

When I hit operating temp for the first time from being cold, the temp goes up to 185-190. I let off the gas and the temp quickly drops and stays at 180. This happens now and before richening out things this weekend. But now, the right side of the cabin fills with a little smoke. It seems odorless. Also, the right side of the windshield fogs up. Neither smoke or fog occurred before adjusting idle, carb and dist on Sunday.

Thoughts?

Also, at idle, there is this weird vibration that comes and goes like a bass heavy stereo. Its like the idle is thrusting. Its a much deeper tone than normal idle.

Thoughts?

Thanks!
 
#27 ·
could be the heater core leaking, fogging the windshield..
it should smell like antifreeze tho, check the pass side carpet, and the radiator level..


for the idle/running/rattling
i'd look at the mechanical advance weights and springs under the rotor,,
look at the weight pins and see if there wore down??
as said if the advance is sticking, it causes lots of problems..
 
#28 ·
Is there a point where I should consider a brand new distributor?

I do smell antifreeze when its first warmed up, sometimes. But the smoke i see isnt an antifreeze smell. When i park in the garage, i actually smell a burning. I can only assume its from the smoke I see. The smoke i see doesnt last very long at all though. The burning smell is something i am not familiar with. Maybe its oil. I have one or two oil leaks.

I think I would be surprised if the bottom end wasnt already damaged. I think I should just drive it til it doesnt anymore but have the other components ready for a crate replacement.
 
#33 ·
Your oil pressure is slightly low but not dangerous for a Chevy small block. If your knock sounds like marbles in a can it is probably pre-ignition knock (detonation) and not a rod or main bearing. Both can be replaced with the engine in the car. It's not easy or pleasant but it is possible.
If you have tried running on super unleaded gas and it still knocks it might be a combination of high compression and/or carbon build up.
If high octane gas doesn't stop it you can try a small amount of water poured into the carb while running at about 3000 rpm. There should be enough water to slow the rpm down but not enough to stall the engine. You will get a lot of steam and soot out of the exhaust so you may not want to have your neighbors witness the show. A combination of water and alcohol pured into the running engine should clean out most of the carbon. The proper way to do it is to tear the engine down and use a scraper and wire wheel but I have found that water and alcohol do a good enough job.
 
#34 ·
I did the water thing, but maybe not enough (?). I didn't see anything come out of the exhaust. No soot, steam or visible anything. The fan was blowing the water down the fuel side of the carb so I was trying to be careful. Should I try again with a little more water?

The truck seemed to run much smoother with a deeper note at idle after. But, I definitely still have the rattle. I can't tell if the noise is always there but it's only noticeable once warmed and accelerating (especially up a hill).
 
#36 ·
Just seeing this thread.

The way in which you measured your 'all in' timng is not correct. You need to rev it until it stop increasing timing to find your all in rpm. You mentioned you set it to 34ish degrees revving it to 2500-3000 rpm's. If your dizzy is stock it might not stop advancing until 3200-3800 implying that you might have the all in set too high - you think it's at 34, but it might be at 40. What you were describing back in April was just what I described above. Hopefully a summer full of driving hasn't lead to a bearing failure.

If I were you, then I'd recheck the all in timing and get to 32-34 degrees and then go drive it and make some WOT pulls upto 4000+ rpms with vac advance line plugged. If the noise is gone, then GREAT, but if not, then get the pan off and check the bearings. Keep in mind that if the bearings are whipped, then there is an excellent chance that the crank won't too special. Hopefully it's still salvageable.
 
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