Too many unknowns. For me, pistons in the hole .067, with a .040 head gasket for a quench distance of .107, a DCR near 8.0, and the unknown cam timing, could be too much with iron heads. Maybe Sean could offer some insights.
Are you burning premium fuel? My stock 8.5 compression, 260 h.p. used to ping like crazy when leaded gas went to low lead, then no lead. Maybe you got a tank of crap fuel.
Always run 93. Consistent pings. Engine temperature same as before the rebuild. Maybe I should do a compression check?
I would be willing to try colder plugs if it might make a difference. I just filled up with 93 again and no change.
Idk. To take 10* out to stop ping, might be cam degree issue. Plugging vacuum did no change... Hope you find the problem.
I think you need to be SURE that your timing is not much more advanced then you think? Sorry if you have already done this? When you say "30 degrees in at 3000 rpm", does that just mean the mechanical advance? You said you have vacuum advance also? How much does that provide? Have you tried as Mart suggested above disconnecting and plugging the vacuum advance? I also agree with Fox's Den that I would try going overkill on the octane to see if that shuts it up...
Just so you know, my roller cam was ground 1 tooth off. To degree it with a 9 keyway gearset, I ended up with crank gear 4* retarded & cam gear 1 tooth off on the alignment dot. This gave me my 5-1/2* advance it's now set at. Was a real PITA to figure out....
Yes just mechanical 30* at 3000. If I retard it 20* from there it doesn't idle and runs like a pooch. But doesn't ping.
I wouldn't keep running it if it is pinging that bad, it is basically detonation and will beat your bearings to death. Get the race gas and see if that shuts it up 1st then go from there. I run 5 gal with about 15 gal of gas, put the race gas in then fill it up. I have done this since day 1 I have 10.6 comp with iron heads I did it for insurance and 28 yrs later I am still running. Take a picture of the top of the plugs so we can see where the line is at on top of "J" strap
I'll retard it again and see where the timing is again to get rid of detonation. Then I'll try race gas with correct timing.
Would a compression check tell anything? Also the detonation doesn't seem to make any change if the engine is just warm or at full operating temperature.
I'm also thinking of drilling the small holes in the bottom of the carb to increase idle circuit fuel delivery. Maybe would enrich mixture a bit.
