The Stovebolt Site Forums The Shop Area Engine Shop Knock or rattle from engine

The engine seems to have a slight knock or rattle between about 2000 and 3000 RPM. It is louder on acceleration and when the engine is cold.

What do you suppose this is? Is it anything to be concerned about? Thanks.

Does the noise happen under a load, such as going up a steep hill or when you shift into third gear before you have enough road speed?
Carl

Shifting into third too soon there's no rattle, the rpms are low.

Going up a hill it rattles. Mind you the "hill" was a short underpass for a railway.

Retard the timing just a bit. Goes away? It is PING. no problem. If and this is a big if, it is a loose wrist pin, then maybe the "rattle" will still be there. I don't work on that many GMC motors so just guessing on the wrist pin. But timing too far advanced will cause a lot of rattle like noises on these inline engines. Try the timing first. Then get some help from the GMC folks. There are several on here. I am assuming, from your post...it is the stock engine in that 52 GMC in your post? It could very well have a "Chevy" engine in it.

where do you buy your gasoline? As a test try getting a quality brand higher octanes gas.Then drive the same route. If it is "ping" you will notice a change for the better.

Could it be the valves? According to the owners manual the valves should be adjusted with every tune up. I just adjusted the valves on my 235. Very easy procedure. 8 of the 12 valves were right were they should be. Four of them needed adjusted. this eliminated the rattle/knock I had at start up and acceleration. Here is a good procedure for adjusting them. This was my first time and I did it in about 30 minutes.

http://devestechnet.com/Home/ValveAdjust

I think the timing is very close - it was too far advanced last fall so I retarded it a bit this spring. Maybe it needs to go a bit further.

I have not yet adjusted the valves - it seems to me timing gets set before the valves. I'll double check the timing with a vacuum gauge, change the gas, set the valves and report back.

To be clear, this is a Chevy 216 which came stock in Canadian GMCs. Thanks to all.

Next step is to remove the pan, check big ends of rods, small ends, all other moving parts.

Good luck,

Ed

When you have the "ping" under load, it affects all of the cylinders, hence the rapid, continuous rattle. If it is a loose wrist pin, it is just one cylinder and the noise occurs with less frequency. Adjust your timing a smidge,test drive, repeat until the ping is gone or greatly reduced.
Carl

how have you made out,Stove?

Thanks for asking. I was out of town so didn't attack this yet. I added a half tank of high octane gas and it then became less noisy I think.

Does this indicate timing? I'll retard the timing just a bit further and see what happens.

A timing rattle (called detonation) sounds like a bunch of BB's rattling around in a tin can and usually happens while passing, pulling a hill, etc. Drop the timing back a few degrees and see what happens.
Jerry

Okay, problem solved! Thanks to all for your assistance.

The timing is now set with just a barely noticeable clatter between 2000 and 3000 rpms. Is this safe? Most people would never notice the noise so it probably can't do too much damage.

With timing turned back too far, the engine was sluggish, hesitating and backfiring. But no clatter.

There doesn't seem to be much wiggle room in between. The difference between the two extremes is maybe about 4 degrees. I wonder if this indicates a problem with the vacuum advance.

The vacuum advance only functions at light-throttle cruise. As soon as the throttle is opened even a little bit to accelerate or pass, pull hills, etc., the vacuum goes away. The timing drops back to the initial + centrifugal advance setting. Try moving the distributor rotor clockwise by hand and see if it moves smoothly and springs back to the same position every time it's moved. The weights and springs have a habit of getting gummed up and sticky and making the centrifugal advance unpredictable. You can access the weights and springs by removing the breaker plate. Do a good cleanup and be sure the springs are in good condition while you're in there.

One degree of over-advance on the timing robs as much horsepower as 5 degrees of retard from the "best-power" timing setting, so err on the side of "not enough" if possible. I have documented this on dyno runs repeatedly over the past 30-something years on all sorts of engines, not just stovebolts. Detonation has a habit of deflecting piston ring lands, breaking rings, and even burning holes in the tops of pistons, not to mention destroying spark plugs. Even a little detonation rattle on a pull is not your friend.
Jerry

Possibly try different octane gas and with/without alcohol (ethanol) to see if it makes a difference. I have a friend who has a runway down the road and uses 100 Low Lead in his truck which he claims fixed all that. You can still get AVGAS, but probably not worth the expense. Not much else we can do other than fuel additives.

Higher octane already didn't solve the problem. I would do what HRL suggests and dig into the distributor counter weights/springs.
Carl