Another 390 pinging thread. Carb, timing, or both?
 

I picked up a rebuilt engine from a guy who stopped his project truck. The engine is a 390 4v intake with Edelbrock 1406 600cfm carb. For the ignition I have a Pertronix kit installed with factory coil. The engine was rebuilt with a performance cam but I have no idea what it is nor do I know the compression ratio.
Now my problem. I put the Duraspark 1 series distributor in but had terrible ping no matter what I did so I went back to the points distributor and put the Pertronix in.
I went ahead and picked up a rebuilt distributor. I have my choice between the 10/15 mechanical advance or the 13/18 advance. I started with the 18 and got ping. Then to 13, then 10. I've moved my initial all over the place from 6-14 and still have ping. With the 10L slot setting and initial around 8-10 has been the best so far. With this combination I could press the accelerator past the ping when the secondaries opened up. This led me to believe the carburetor needed tuning. It was already in a stage 2 setup. I ended up using the .101" metering jet with the .070"x.037" rod. This combination has the least amount of ping.
With the engine warm I can accelerate until just before secondaries open up before I get ping. Press on down even to WOT and no ping.

What do I need to do at this point to get the last of the ping out? I've tried running mid grade octane and even tossed some octane boost in but the boost didn't make any difference that I could tell.

Bear and a few others on here will tell you that you need to know the total mechanical advance, as in the number where it stops rising as the rpms go up. You want it to be between 34-38degrees if i remember right. This is checked with the vacuum advance disconnected.

If I were you i'd use the 13L slot, and set the initial to 10btdc, and check what your total is after that to make sure what it is.

If using the 13L slot and set at 10btdc it should be 36 degrees total advance.

You may have to run 92 or 93.. That octane boost is pretty much worthless. I'm running 9.85CR with a 280 comp cams, with 36 total, and 14 initial with the spring kit that puts it all in by 2600rpm and i havn't heard it ping yet. But i'm running 93

2 things i didn't see mentioned- the distributor springs, and the vacuum advance.

if you're running light springs, the mechanical advance could be coming in too soon.
if your vac advance is set too high, it could be bringing in too much advance at part throttle (which would explain why you were able to 'accelerate' past the ping.)

Stock springs, stock vacuum advance. I've moved the vac advance line from manifold to timed vac and the only difference it makes is helping my idle when connected to manifold.

At the point that it starts pinging I would think the manifold pressure has dropped low enough that the vac advance would have stopped. I will try and drive it without the advance hooked up. If it makes a difference then I'll get an adjustable unit.

depending on which type of advance canister you have, it may still be adjustable.

to get an idea of how much advance your can is adding, try this:
disconnect the vac advance. start the engine, check the timing.
hook the vac advance to manifold vacuum, check the timing again.
the difference is how much vacuum advance you're getting at max vacuum.

if you want to get an even better idea of your exact trouble spot:
step 1: hook a vacuum gauge up, go for a spin. determine the amount of vacuum you're pulling when the engine is pinging.
step 2: hook up the timing light again, disconnect the vac advance.
using a hand vacuum pump, apply the amount of vacuum determined in step 1.
compare the difference, then adjust the advance can to dial that back a bit.

your current setting of 10L and initial around 8-10 is fairly conservative. (i'm running 10L and 15 initial with no ping, although it doesn't want to restart when warm.) my guess is that you'll find the vac advance is pulling more timing than expected. or your air/fuel mixture could be lean.

I reset my initial back at 10btdc, left the vac advance unplugged and no ping. You were correct rusty, and you were correct about it being adjustable. Two turns counter clockwise and no more ping. I'm letting the engine cool down a bit right now so I can set my jetting back like it was then I'll go back to timing. If I don't get any ping and I get the initial too high on hot starts I'll swap back out to the 13L slot and test again.
I've never known much about timing but I'm about to get it all learned tuned right.

right on!

let us know how you get on with the jetting changes.

Radiator sprung a leak so I've got to take time to get that fixed before I continue on with the jetting.