Just finished rebuild of mid 70's 350. Runs great, but have slight spark knock issue under light to medium acceleration. Rebuild consisted of the following:

Bored .030
rods & mains .020
valve job
heads & Block decked
flat top pistons
Edelbrock Performer Cam
h.v. oil pump
Flame Thrower Dist.
Professioal rebuild of stock 4 barrel carb
removed EGR from stock intake
stock exhaust dueled out of muffler


Currently running 93 octane pump gas. There is an improvement if use 104 octane boost, but problem still there. Base timimg should be around 20 degrees BTDC, but have lowered to 16 degrees BTDC with little improvement. Do not really want to retard timing any further and sacrafice power. Any Sugestions????????

 

Yes a bunch,

Any sharp edges on the heads or block? Did you use a true flatop piston, no dish at all? Compression may be too high with that cam, that's a very mild cam. What heads?

Could have a vacuum advance thats pulling too much too quickly, same with your mechanical advance. Need to check total advance amount and when it comes in.

You rebuilt the carb but did you rejet? All those mods may need more fuel, too lean of an A:F will tend to knock more.

Thermostat and cooling system? Good flow, correct thermostat?

Lastly, are you sure it's not an exhaust leak or valve clatter?

 

The first thing I would check is your vacuum advance on your distributor. Since it is an aftermarket distributor it probably has adjustable vacuum advance. I have a Summit HEI distributor in my car and it pinged a bit on acceleration until I turned my vacuum advance out a few times. All it takes is an allen wrench.

 

Yes a bunch,

Any sharp edges on the heads or block? Did you use a true flatop piston, no dish at all? Compression may be too high with that cam, that's a very mild cam. What heads? Heads and block looked good, pistons are ture flat top, heads are 7A


Could have a vacuum advance thats pulling too much too quickly, same with your mechanical advance. Need to check total advance amount and when it comes in. Flame Thrower dist. Vacume advance is set at 16 degrees , but is adjustable, I might have to back out a turn???

You rebuilt the carb but did you rejet? All those mods may need more fuel, too lean of an A:F will tend to knock more. I told the rebuilder what I was going to do, he regetted primary to .073 and secondary to .049

Thermostat and cooling system? Good flow, correct thermostat? New radiator and correct thermostst ( 190*)

Lastly, are you sure it's not an exhaust leak or valve clatter?[/quote]

Pretty sure there is no exhaust leaks- definately a pinging noise

 

I would try to adjust the vacuum advance down a few degrees, see if that helps. Also, do you have a water temperature gauge on the car to confirm the operating temp? You could also try to run a cooler heat range spark plug. If all else fails, you could put in a more aggressive cam to bleed off some cylinder head pressure and that usually alleviates pinging. Hope that helps.

 

I had my mild built 350 tuno tuned to deal with my knock. Tuning the carb under load with A/F ratios I believe was a big help. My final timing was 13 initial max out at 33 and I dont run vac advance. HP was 285 TQ was 345.

 

Pretty sure there is no exhaust leaks- definately a pinging noise[/quote]

From my experience 16 degrees of vacuum advance is waaaay too much. Try closer to 6-8 with that cam and compression combo. Start there.