Some time ago, I had to have my carb rebuilt on my 1976 because the throttle plate bushing was leaking. After it was rebuilt, I put it on the car and took it for a test drive at my local proving grounds (State highway with very little traffic). When I turned onto my street after some fast driving, the engine starting to have a large knock. I pulled into the driveway and immediately shut it off.

I asked for some opinions on the forum. One of them was that the little screws that hold the secondaries on sometimes come loose and drop into the engine, especially after a rebuild. I checked the top of the carb and the screw was gone. I thought for sure that that was the problem. So, I took the top of the engine apart all the way down to the pistons. There was no trace of the screw, no knicks or scratches, or anything on the pistons, valves, or heads.

One of the other suggestions was a broken flex plat banging against the engine. I finally got around to checking the flex plate. I was not able to get the guard completely off to look at the entire flex plate, but was able to see about 2/3 of it, and it appears to be in good condition.

Any other suggestions, or did something happen in the bottom end?

Any suggestions would be greatly appreciated.

 

We had a 69 Camaro last week and a 70 Vette this week with an intermittant rattle down by the oil pan-torque converter area. You guessed it...wrong or loose torque converter bolts. Check them. Not visually, but with a wrench......

 

This is more of a knock than a rattle. Can the loose bolts still cause that?

 

Yes, and a cracked flex-plate can cause a loud knocking sound also.

 

Thanks. I will have to go and look at it again.

Any suggestions on how to get the flex plate guard off without removing the exhaust and transmission cooler line?

 

Put leather gloves on and PULL! You can temporarily deflect the trans. likes over a bit, then move them back when done. You will bend the converter cover, it will bend back easily to re-install.

 

check out your fan clutch.....look around the shaft and see if the beaing shows any signs of being oblong. also grab the fan and try and move it and see if it has any movement in it.

 

I will check that tonight also.

Thanks for all the help so far.

 

How much carbon buildup did you see on your pistons?
Years ago I did a tuneup on a 70 Nova and the guy that owned it never drove it over 50 mph and after I tuned it I ran it hard and it developed a knock. Ended up being carbon broke loose from a piston and lodged sideways between the piston and the head and caused a knock. If this is the case driving it hard will eliminate the carbon.

Just my $.02 worth

 

When I had the heads off, I wiped down the cylinder walls and everything looked clean. I did not see any debris at all in the cylinders.

 

O.K. I checked the flex plate. There are no cracks. Both bolts that I checked with the wrench would not budge. They were rock solid.

I also checked the fan clutch. Everything seems to be tight there.

Does anyone have any other suggestions before I conclude that it is the engine?

Thanks for all the advice so far.

 

My vote is the harmonic balancer.

 

What should I look for on the harmonic balancer? Will it actually separate?

 

Do the simple things first...carbon can cause a knock....as stated..so why not mist some water down the carb from a spray bottle..keep rpm's up while doing this,and mist water in ....hopefully,that is it...happened to me once..takes about 5 minutes or so....
Rich

 

Where would the carbon have to be to cause the knock? There was no carbon in the cylinders when I had the heads off.

 

The carbon build up would be on the top of your pistons and in the CC chamber of your heads. You would be able to see the carbon build up with the heads off, if the was any. When the engine is dead cold you can rev the motor slighty when first started up and hear if any rods are knocking before the engine gets to operating temp. What type of oil pressure did you actually have at the time of the knocking? If you have checked all the external pieces of the motor time to start looking on the inside of engine. Possibility of a bad oil pump as the pressure spring could be bad and starveing the engine of oil. Spun bearing on the crank somewhere either rods or mains is possible too. Never a good thing when you engine is knocking after some high rpm driving. One of the best things to burn carbon out of the combustion chambers and pistons heads is X66 a GM product. Available in spray or liquid it is called tune up in a can for a very good reason. I always used the liquid version. You rev the motor moderately as you pour it in to carb. When close to the end of the can you actually let it choke out the engine so it dies. Wait about 20 minutes and restart the car. Make sure you are not in a enclosed area when restarting. If you have any carbon buildup in combustion chambers it will have the crop duster effect till the carbon is burned off completely, houstonvett

 

That test would involve having the car running. I don't have it in running condition because after I came up empty when I looked in the cylinders, I did not finish hooking some of the items up because I just assumed that something in the bottom end gave out.

We are talking small items, like plug wires, valve covers, vacuum lines, etc. But, the dilemma I have now is, do I hook everything back up and start it again after having been out of commission for awhile. This would requiring tearing it all down again if I have to pull the engine.

Can I assume that there is no carbon knocking if there wasn't any in the cylinders, or is that assuming too much?

 

As long as you have it that far drop the oil and pull the oil pan...

Be looking for metallic flakes in the oil...

Could be a bad Rod or Main bearing....

 

Rip the oil filter open and check it and the bottom of the pan for metal shavings. Sounds like bearings to me. Good time for a rebuild and upgrades since the heads are off. You can justify the expense to your wife now since it "has" to be done.

 

I replaced the oil pan gasket some time ago on this thing and it wasn't easy maneuvering the pan down around the suspension.

Does anyone have any tips on how I can make this easy. Is there any certain suspension piece that I can unhook, etc.?