ChrisJP85

I don't think I've ever heard engine knock. I was doing setting the timing today and heard a new sound. It was then I realized I had no idea what engine knock sounded like.

tonyv123

At wide open throttle, it sounds like 4 marbles shaken violently in a coffee can.

mikejpss

engine: knock, knock
driver: who's there ?
engine: noise.
driver: noise who ?

and THAT my friend is what engine knock sounds like
Sorry man, I couldn't resist.

the 4 marbles in a coffee can sounds dead-on.

ChrisJP85

Ok thanks guys, haha.

My sound was more of a low bassy grumble so I'm not too worried

mikejpss

actually, Tony is being serious. If yours has a bassy sound, it may be bogging down. Timing not right, starving for fuel.... have you checked your timing, and your fuel filter?

ChrisJP85

I just replaced the fuel filter. And I was adjusting my timing today; I started hearing something of a very low pop. I am inexperienced so I wasn't sure what I was hearing; it's very tough for me to describe. When I rev the engine and then let it down it get a fairly loud rumbling sound, is that some sort of indication?

tonyv123

Chris,
Give us as much detail as you can about your engine (how many miles, any performance parts, stock exhaust, etc.), and what exactly you did when adjusting your timing (used timing light, changed plugs, adjust carb, etc.). Could be that's the way it's suppose to sound, but with more info, you can most likely get it figured out here.

ChrisJP85

Thanks Tony, what I have is a NOM 350 GM with a Rochester Quadrajet (I'll look up the part numbers in the morning, as far as engine mileage I'm not sure). The fuel pump is a 1970+ mechanical style, the type where the pressure regulation is on the pump. The fuel runs through a Wix inline fuel filter, pretty big (rated 1-2 gpm). I also replaced the fuel filter in the carburetor. Last fall I put in new spark plugs (AC Delcos), replaced my old points distributor (it had no vacuum advance) with a ~1971 dist with vacuum advance, and bought an ignition rebuild kit which was just a cap, points, condenser, and wire set. I also checked the float and the rest of the inside of the carburetor to make sure it was clean; didn't change anything other than the float which i measured to spec (forget the number but it was in a book i have and I verified it here on the forum)

For timing is use an Equus 5568 with dial back. Since I replaced the distributor I tried my best to start at what I thought would be 0 degrees then turned it till it started. Once it was started, with the weights on the distributor tired up, I made sure I was at about 8 degrees and then tuned the RPM down to about 700. After that I put my initial to 12. I then untied the weights to make sure I wasn't getting advance on idle. Once I was sure I throttled to about ~2500 rpm to make sure the weights were kicking in and then again to ~3000 rpm to make sure I wasn't exceeding 36 degrees (This was kind of hard, I don't know if its experience but I found the strobe more difficult to read at higher RPMs). It turned out that at ~3000 RPM I was getting ~35 degrees, after this I took it for a test drive. When I got back I hooked up the vacuum advance and made sure I wasn't exceeding 52 degrees, it didn't look like it so I took it for another test drive. So far as I could tell there was no knocking as described above. When I got back, with the engine off, I turned both the fuel mixture ***** all the way in and then back out 1 1/2 turns each. I started the car again and hooked up a vacuum gauge to a vacuum port on the carburetor. It was reading about 11 psi. I then proceeded to turn each **** 1/2 a turn until I got the highest reading on the gauge which was about 13 psi. Did it again just to double check, went on a test ride and that is where I left it.

Aside from being uncomfortable about certain sounds (mostly because of inexperience), there were a few times where it looked like the timing would jump at low rpms. I'm not 100% sure but I think I heard a change in the sound of the engine but it was a tough sound to describe. And I found the seeing the timing at high rpms a bit tough and I'm not sure if was jumping around at all. I did some reading and found a recommended way to diagnose a few ignition related problems is to attache a vacuum gauge and see what it does (holds steady, bounces around, etc). When I attached my vacuum gauge to do fuel mixture it held steady, so I'm not sure what to take away from that.

Thanks for the help, it was a good exercise to write down what I did and even better having someone to look it over.

Easy Mike

Are you hearing engine or exhaust?

ChrisJP85

To me it sounds like exhaust.

I forgot to say that my exhaust is stock manifold to sidepipes.

ptroxx

Video maybe ?

ChrisJP85

I can make a video. Do you just want to try and hear it or should I walk through all my components?

OzzyTom

You won't hear engine knock simply by revving the motor unloaded (in neutral).
You will hear it when the motor is loaded, such as in top gear under accelleration from lower rpm.... especially going up a hill.

As mentioned, it sounds like a loud rattle inside the engine.
It's also known as "pinging" or pre-detonation.

Unchecked / uncorrected it will result in damage to your engine.

It can also occur if your engine is tuned for higher octane fuel, and you fill up with a lower octane fuel.

low bassy sound doesn't sound like knocking.
Marbles in a can is a good analogy.

Indiancreek

Sounds like you are getting an MRI

dbguettl

Knocking sounds like a diesel engine at idle.