If nothing has changed, I'd suspect bad fuel too.

Does the engine have an EGR valve? A stuck closed EGR will do it too.

No EGR on this engine. I just fueled it up and burned off a quarter tank playing with the timing and carb adjustment today to make sure it was set up right. I guess I will take it for an easy drive and burn off some fuel to see if it makes a difference.

 

Funny you should mention it...

Recently my sbc began to ping under the slightest load. Out came the timing light & my timing had advanced on its own accord!

I loosened the HEI and began to retard it back to 26 degrees. Once there, I could not get the timing mark to quit wandering back & forth and hold steady!

I popped the top & rotor off the HEI and found that one of the springs had come loose from the centrifugal advance weights.

It must be the season for timing issues

 

Certainly wouldn't hurt to take a small sampling of your present fuel (perhaps enough to fill a small glass jelly jar about 1/2 way) and let it stand for a day or two, just for observation purposes. Also, a bit of carbon buildup can cause hotspots in the combustion chamber(s) and contribute to spark knock as well. Perhaps a "preventative" SeaFoam treatment might not be a bad idea as well.-Jim

 

retard the timeing. check the dwell.

 

Ayuh,...

Change the Fuel Filter,...

A Lean condition can cause detonation too...

 

Did some messing around last night after speaking to friend. He suggested moving the vacuum advance from ported to manifold vacuum, reset the timing to 12 degrees BTDC and take it for a run. Did as he suggested and the pinging has gone away. Whats up with that?
My friend is the one that built the engine so he is familiar with the set up. I had someone else do some tuning on it in the spring and they must have moved it from manifold vacuum to ported.
One thing i noticed after making the change is the engine seems to have lost some of its lopey idle......it seems smoother.

 

Chevy engines are supposed to use manifold vacuum. ported vacuum makes them lazy and sluggish. The idle is higher due to more advance. The manifold vacuum makes it have around 24 degrees advance at idle.

 

Chevy engines are supposed to use manifold vacuum. ported vacuum makes them lazy and sluggish. The idle is higher due to more advance. The manifold vacuum makes it have around 24 degrees advance at idle.

Is it commom for an engine to lose some of its lope at idle when switching to manifold vacuum vs. ported?

 

Out of the blue my sbc began to ping slightly at mid throttle. Car has run fine all year and have never heard it before. Checked the timing and it is at 10 degrees initial and 34 all in at 3000 rpm. Always run premium fuel which in my area is 91 octane.
Any advice on what has changed would be helpful. I'm thinking maybe some bad fuel. Your thought are welcome and appreciated

Beagle

Your probably seeing the change to winter time fuel. For cold weather starting they increase the amount top end high volatiles in the fuel. These are ping prone, so unless the weather turned cold at the same time the new winter fuel came out, your likely to get some ping.

I hear it in my headers, in Seattle they switched the mix a couple weeks ago, then the weather warmed up. When that combination happens, I can hear a light ringing sound off the headers when the engine is working hard that isn't there on summer mix fuels. When they switch back to summer mix in late April, early May that sound goes away. It's been like that for quite a few years.

 

That seems like a likely cause as well Bogie. All things being the same from a timing aspect etc.
I had it out today and it was 68 degrees and I could hear just the slightest ping. I don't imagine such a slight ping could do any damage.......could it?

Beagle

 

I came across this thread and glad I did since I just moved my vacuum line to to manifold vacuum and yes, it did smooth the idle from the more lopey idle on my car as well. I am running a Summit Ready to Run dizzy and it has the 21 degree advance bushing installed in it and my initial timing is set at 14-15 degrees. I have no pinging thus far but it is too early to tell. The car starts fine when hot and I re-adjusted the carb and the idle. I admit that these things are a little fuzzy to me and I am trying to learn how they interact with one another. Everyday is a learning experience.

 

woah lemme get this straight here... if u have a Chevy engine they should me manifold vacuum instead of port..... ****! well looks like i need to do some tinkering!

 

woah lemme get this straight here... if u have a Chevy engine they should me manifold vacuum instead of port..... ****! well looks like i need to do some tinkering!

brands have little to do with the preference of port to manifold vacuum... I recomend you do a LOT of research on the subject. It can vary depending on how your engine is built.

 

brands have little to do with the preference of port to manifold vacuum... I recomend you do a LOT of research on the subject. It can vary depending on how your engine is built.

This is what I would consider a must read for anyone owning, building or wanting to build a SBC engine. I believe it was written by a GM engineer.

http://www.corvette-restoration.com/resources/technical_papers/Timing101.pdf

 

carbon build up will cause pinging etc.
try some seafoam itll clean out the carbon