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Pontiac 350 Motor Knocking

13K views 119 replies 13 participants last post by  off2wildblue 
#1 ·
I finished my 69 Firebird today. At least I thought I did.

I have a numbers matching car so this is why I put the 350 motor back in. It is rebuild .30 over and a mild cam. It has ran about 20 miles.

Now, I know most guys will say you should have done the 400 motor. Well, I don't really feel like hearing that right now. What I should have done was probably an LS motor.

Anyway, my emergency is that I went out with my wife for our first test drive. We drove about 5 miles and everything was great. I just got the wheels aligned today and also charged the Vintage Air System (which ended up with smoke pouring out of the clutch on the compressor and the thing got hot as hell). I still don't know what is wrong with that. But to save the day and try and get over being upset about that damn AC, we drove it with the top down and the AC off.

We were going maybe 10 mph around a corner and the motor started knocking. I pulled over and then it ended up dieing. Needless to say, I had it towed home and now I pulled the valve covers and started it. I didn't see anything too crazy. Some of the rods might be loose but the knocking is a mystery. Can you guys watch my video and let me know what your thoughts are? I am pissed as hell and my wife is really worried it will cost a ton to fix.

What the hell is the knocking?

https://www.youtube.com/watch?v=QQsqyQkxJjg

I also included a pic of the car...just because.

Thanks,

SAM
 

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#2 ·
It does not sound good. You are getting oil to the top end. You can also hear a tinny sound in the background. It could end up being something simple. I wouldn't run it anymore until you tear it back down and find out what the problem is. What was done to the engine beside a rebuild?
 
#3 ·
Nothing was done to the engine. A pretty simple rebuild. .030 over, mild comp cam, that is about it. Even stock exhaust manifolds and no real upgrade on anything else.

I am not sure where to start. I thought this car was done. I wasn't planning on tearing it all the way down again. This blows!

SAM
 
#4 ·
Sam.
Try the old style mechanic way of trying to isolate a noise. I had a old broom stick that I cut off a wore out broom. You can use a crow bar if it is long enough, a piece of wood, closet clothes hanger rod.

Put the end of the rod,pole or whatever you use up against parts of the engine and you should be able to find the source of your knock. Check every where. You should be able to start right away to hear the noise, move around and it will get louder or guieter. N

Not rocker arms,you can here them. It did sound a little louder on the passenger side to me. If your engine idles(just a example) a bad rod or rotating assembly you hear it at 800 times a minute. Not hearing that. Valve train moves twice as fast. Not that. Something slower. Check the engine accessories. What did you for a timing chain assembly?

In listening to the video a few times, move around slower please, but check back by the distributor. I think the noise was louder back there. Loose cap? Use the broom stick way and you will find it.

HTH
Nice car. Old Pontaic mechanic.
 
#5 ·
Maybe it's the cam. Did you put in new valve springs? Pontiacs are known to spin rod bearings. Is there a factory windage tray in it? Are the oil galley plugs in it? I would still tear it down and inspect everything, it might end up being something you missed.
 
#7 ·
I have heard old style Chevys with a forged crank that was twisted in the forging process from the tranny side being held and the blank being twisted. It made the 7-8 journal at the main bearing was weak and it would snap in the main cap and it would slip a little every few revolutions. It would idle like that all day long and stay together. Even run down the road for a while.

You have a Pontiac and I never saw one like that. Did you build the engine or did a shop build it? They could have missed something.
 
#11 ·
It's something you can fix. There is not much to it if it's the pushrod. You can put your fingers on it and if it wobbles around loose than that is probably your problem. You can loosen up the rocker nut and pull it strait up. Roll it on a flat surface to see if it's round. It just drops right back in to replace it.
 
#14 ·
Plain as day, something is wrong with cylinder #4 exhaust valve, looks to be stuck open and either piston is hitting it or the backfire resulting from it is what is making the noise.

From the looks of the rocker stud threads showing the same adjustment as the one next to it, valve is seized in the valveguide/or valve is bent. Rocker is not moving and appears to be in the wide open position, pushrod is hugely loose and just jumping around in the pushrod guide slot, hammering away at the rocker arm cup.

Definitely bad things wrong, and going to have to tear it down to see how bad it is.

Are those Edelbrock Aluminum Pontiac heads, or a different brand??
 
#15 ·
Original heads that were machined and painted. Now how would you start a tear down? I really don't want to go through pulling it from the car. If it ends up fixable without doing that, I would be sorry I did. My wife would flip out after paying for the rebuild and all.

Maybe I should have done the LS conversion after all.

SAM
 
#17 ·
I couldn't sleep so I went back to the car and tried to diagnose it. Major break through but I am not sure how bad it is or what to do next. Need more help.

I have included pics.

I first went out and disconnected the spark plug wire from the distributor for the cylinder I believed to have the really loose rod or lifter. Started the car and the loud knock was gone. Great news if I want a 7 cylinder car. But I prefer 8. I then tried to tighten the rocker and it wouldn't tighten. So I turned the car off and reconnected the plug wire and the loud knocking was still there. I then took the rocker off and found it had tore itself apart. I pulled the rod out and rolled it in a smooth service and it seems straight.

I am not sure what to check or do next. Suggestions?

SAM
 

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#38 ·
I couldn't sleep so I went back to the car and tried to diagnose it. Major break through but I am not sure how bad it is or what to do next. Need more help.

I have included pics.

I first went out and disconnected the spark plug wire from the distributor for the cylinder I believed to have the really loose rod or lifter. Started the car and the loud knock was gone. Great news if I want a 7 cylinder car. But I prefer 8. I then tried to tighten the rocker and it wouldn't tighten. So I turned the car off and reconnected the plug wire and the loud knocking was still there. I then took the rocker off and found it had tore itself apart. I pulled the rod out and rolled it in a smooth service and it seems straight.

I am not sure what to check or do next. Suggestions?

SAM
Picture number 3 here clearly shows the valve retainers have moved up and are above the stem of the valve.
I would bet you have too much lift and the stock retainer hit the guide top, shearing the retainer lands.

Ericnova treed everyone here.IMHO
 
#18 ·
.
. Your rocker arms may not have a long enough stud slot for the lift of the cam you are using... they may also be weak Made in China rocker arms...

. The sound is the intake valve is letting fuel/air into #4 cylinder, but exhaust valve is not opening and letting fire/exhaust out... so when intake valve opens again, the hot exhaust goes into the intake manifold exploding the fuel/air mix in there... next time around it starts all over again and happens again...
 
#19 ·
So what would be the next step to getting repairing it?

Can I start it now with the plug wire pulled off the distributor or will I spew oil everywhere. I don't think I would but I am not a motor guy. I just want to see if it idles without any crazy noises.

Also, could the rocker have been tighter down too far?

Thanks again,

SAM
 
#20 · (Edited)
A rocker stud did not pull out, you have heads on there with screw in studs. First try and replacing the rocker arm, it should not cost more than a few dollars. If that does not fix the problem then it's time to remove the head. You can pull the head off your car with the engine still in it. Remove the intake manifold and the exhaust manifold. Loosen up all the rockers and remove the pushrods and then remove the head bolts and spark plug wires and anything else that is attached to the head. The head should just lift out. Once you have the head removed, take some more pictures.
 
#21 ·
I had the guy that built it come by. He said he had only built 3 Pontiacs. But he has built many Ford, Chevy etc. Since the studs are still good and the exhaust valve seems good and the rod is still straight he wants to put in a new rocker.

The only things he is concerned about is that he thought he maybe should have ground down the heads more to make room for the screw in studs. He said he measured it but the fact the stud has the base that looks like a nut he thought he should have ground it more. He also thinks this because the nuts that screw down the lifters are almost bottomed out.

What is the best way to test the motor and ensure some type of reliability?

He thought to let it run for a couple hours and rev it every now and then just to work the motor better.

Lastly, I have a 3.55 rear end with posi in the car but no stall converter, but it doesn't have much low end. It won't even come close to burning the tires. My old worn out Firebird I used to have in high school (with a 350 as well) would at least burn out. By the way, this is a TH350 automatic car.

Could lifter adjustment or rod length affect this and would a 2500 stall converter help this?

Thanks again,

SAM
 
#22 ·
If he used Pontiac screw in studs, the studs would not have to be ground down. You have to grind down the ends on BBC screw in studs to use them in a Pontiac. Used rocker arms get fatigued and that might be why it broke. The standard rocker arms should be available at most auto parts stores.
 
#49 ·
The standard rocker arms should be available at most auto parts stores.
. LOL! You're living 40 years in the past like me... 15 years ago I ran around on a Saturday trying to find a SBC stock rocker arm for the Ford engine I mentioned above (didn't expect to find a Ford one) so I could keep running my boat for the weekend... I mean, what could be more common... never did find one, had to wait till Monday for shops to open or steal one from another motor...
 
#23 ·
They were all new. I bought all new stuff for the motor. Hopefully, it is just a bad rocker. He has one at the shop that we will try and install.

Any idea on why my 350 is still a pig on the low end, besides being a 350?

It was terrible before but the 3.55 rear end helped a bunch, would a 2500 stall converter make a noticeable difference?

SAM
 
#25 ·
Could just be random failure of that new rocker, as it happened so soon in it's life cycle, I could tend to believe that. Parts like that either break soon if there is a problem, or they last a long time and are fine.

Be glad it isn't a bent or hung valve, and just a more simple fix. Replace the rocker arm and give it a go.

Valve adjustment could be too tight, but I'm not there to see how it was done...when converted to adjustable using chevy studs, they should be adjusted just like a chevy.

What is the exact size of the cam?? Post all the specs, so we can see if there are any possible incompatibility issues, or so we can look to tuning issues as to why it has less low-end power than stock. Nothing wrong with a Pontiac 350, should make decent power.

What are your initial and total mechanical ignition timing figures, and how much vacuum advance is being added??

What carb is being used??

Don't just jump into a stall converter as the next needed "fix", until the rest of these things have been addressed.
 
#26 ·
I hope this is the case. My brand new AC compressor from Vintage Air seized up within 2 miles of using the new system yesterday before this happened. They are sending a new compressor and drier under warranty.

I will have to get the other figures but I am using the Edelbrock 650 with an Edelbrock RPM intake. Not sure about the timing figures, it has the Pertronix added to the ignition and not sure about the vacuum advance. The cam is a Comp Cam and I will post the spec sheet.

I can look at the other stuff on Monday after I get the parts to put it back together. I will be out of town over the weekend so I have to do it on Monday.

SAM
 

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