This is in an '86 truck with 160,000 miles on it and a quadrajet. The timing tab is missing so have been doing it by vaccuum, it is at 15lbs right now but it seems like 17 is optimum but it pings too bad. I have backed it down far enough now that it only seems to detonate right when it is started and at a mid acceleration with a slight drag like going up a slight incline.

Should I keep backing off the timing until I lose all the ping? I heard somewhere here that a little is OK. I am running the lowest octane gas right now and would like to get away with that. If my engine is just plain tired would that contribute to this?

Thank you!

Carbon build up on the pistons or combustion chamber do contribute to detonation. Leaving hot spots that ignite the fuel before the piston gets there TDC.

I'm with Bob. Try dribbling some water into the carb while the engine is running. Might take a couple times to clean out the carbon.

Darren,
The EGR valve will also cause major pinging in those engines if it is not working properly. The egr passages in the intake manifold can also get clogged up, causing the same thing. Try reaching up under the egr valve with the engine idling and pull the valve open. It should change the idle a lot. If not, either the valve or the passages in the intake are clogged up. Be careful, it will be hot !

I actually poured about a cup of water down the carb last spring when I first started having problems with it. Just before that I made a trip to Chicago and back that broke some carbon loose to the extent that a couple of plugs had their tips bent in! Is that EGR valve on the intake, passenger side just behind the carb?

Darren,
I am almost certain that is where the egr is on that year. By the way, I think that there is a connector that needs to be unplugged to properly set the timing. I doubt that the timing tab is missing. Shut the engine off and climb up on the front bumper. Look straight down behind the water pump...that is where the timing pointer is on most of those years 350's and 305's.
Hope this helps,

Very good information, thank you. I will have to wait until Friday to actually look at it.

A coworker of mine ditched the ESC dist and put a regular HEI dist in with a vacuum advance and that took care of his pinging, low power complaint and his low fuel mileage, but check the EGR performance first.