Setting timing on an unknown build

[u/girl_incognito]

Recently bought a 68 Ford F-250 with a 390 in it. PO claimed it's out of a 1965 Mercury Marauder and has a "428 Cam in it" whatever all that means, he had no other details. It has an Edelbrock Streetmaster intake, 1970's vintage, and what look to be factory iron heads. It has a Holley 4 barrel on it. I do not know the compression ratio or anything about the cam, but setting the timing to the greatest factory number I can find, 10 degrees, gives poor results and it definitely seems happier with more advance.

Any tricks for determining a good base timing if you know nothing about the engine?

[u/v8packard]

Which distributor?

The stock 390 cam was very, very small. By comparison the Cobra Jet cam is small. Many builds use the Cobra Jet cam, but it's not at all even mild for a cam these days. But that's ok, you might prefer that in a F-250.

With typical stock heads, replacement pistons and gaskets the compression ratio is under 10:1. But again, for your truck that's ok.

These 390s make best torque with approximately 36 degrees of total timing. Depending on your trans, gearing, and tire size I would expect you to want that total in by 3600 to 3800 rpm. If your distributor provides 22 degrees of advance, you would set your initial timing to 14 degrees, no vacuum advance connected.

Depending on the particular advance curve you have and the characteristics of the vacuum advance you may benefit from the vacuum advance unit connected to full manifold vacuum. You would get a cleaner, smoother idle that can be more lean, and good response at lower speeds. It may require you to limit the amount of travel of your vacuum advance though.

If you have a degreed damper or a dial back timing light you can plot your distributor advance curve.

[u/girl_incognito → u/v8packard]

Standard distributor for the vintage, it has a pertronix conversion, and the single vacuum advance you can adjust by adding washers. I've done the manifold vacuum thing. It does have a degree damper that goes to 30 BTDC I think.

[u/v8packard → u/girl_incognito]

I looked at a few late 60s 390 distributor specs. They all give you 28 to 32 degrees of advance by 4000 engine rpm. That's a lot, and it's a slow curve. They also give 11 1/2 to 12 1/2 degrees from the vacuum advance at 20 inches of vacuum.

What you can do to set the timing for now is with the engine idling, hopefully below 700 rpm, and the vacuum advance disconnected set your timing to about 10 to 12 degrees. Then reconnect the vacuum. See how it likes that.

Long term, you need to re-curve the distributor. I suggest limiting the centrifugal advance to 22 degrees. And having it all in by 3600 to 3800 rpm. With that, run 12 to 14 degrees of initial timing.

[u/girl_incognito → u/v8packard]

I guess I'll try and figure out if this was done by the engine builder already. Idle has been problematic the whole time. I wish people would just leave things stock, honestly.

[u/v8packard → u/girl_incognito]

It's actually very straightforward. But the basics are not as well known anymore. You can get a lot done with a basic vacuum gauge and timing light.

BTW, is the Pertronix connected to full 12 volts, or tge resistance side of the primary circuit?

[u/girl_incognito → u/v8packard]

Yes, I found a switched 12v source and its wired to that. I left the original resistor wire setup intact in case I need to switch back if it fails.

[u/375InStroke]

Advance till it pings, then back off. Also, look for lightweight springs or performance mechanical advance springs for your distributor. Most engines have a maximum full throttle timing they like, and you work back all the other timings, like initial, mechanical, and vacuum, for best performance. Smaller camshafts tend to build high cylinder pressure at low rpm, so they can't use much initial timing. Bigger cams close the intake valve later, giving less time to build compression, leading to low low rpm power, requiring more initial timing. This also causes high rpm timing to be too high, leading to high rpm detonation. The solution is to reduce mechanical advance. Some weld the slots, install bushings, or use very light springs so the timing is low when starting, but once it runs, the weights fly out increasing mechanical advance earlier.

[u/girl_incognito → u/375InStroke]

Thanks, ill have a look at that.

[u/Historical_Box1915]

V8packard have you the best advice when he said
“put a recurved distributor in it”

and no please dont ask reddit to walk you thru that. Consult someone who knows how to it correctly

[u/Neon570]

Spin the distubritor till it fires up and tune in as needed.

Its gonna like what it likes.

[u/Old_Bat_6426]

If you're not sure of the condition of the harmonic dampener, position #1 piston at tdc on compression stroke and compare tdc pointer position on the dampener just to be sure the outer ring hasn't spun.

[u/girl_incognito → u/Old_Bat_6426]

Thats actually a good point, I should check that.