Duraspark Vacuum Advance

[urchinhead]

I was playing around with a beat up 390, and mocked up a Duraspark ignition using a parts store 1976 Cardone rebuilt distributor (using a small cap) and an NOS Motorcraft coil and and NOS blue strain relief module.

The motor would not idle and backfired some unburned gas through the carb.

I went through every electronic part, connection and wire, triple checked TDC at compression stroke. I kept having to advance the timing to get the motor running longer.

Finally, I pulled the vacuum advance hose off and plugged it with a pencil before starting the motor. It fired up instantly and ran smooth. Could this be a bad vacuum advance?

Also, any clue what Ford/Motorcraft used for the Duraspark connectors? The 1976 manual recommends a silicone free dielectric grease, but the existing yellow-white paste looks more like a heat sink compound.

[1964Fastback]

Was the canister hooked to ported or manifold vacuum?  What is the initial timing set to?

Pat

[57 lima bean]

    This was posted awhile back  https://www.youtube.com/watch?v=WD_bhDq_8tY&t=286s

[urchinhead]

The vacuum was hooked to the ported vacuum on an Edelbrock carburetor.
I started off with 10 degrees BTDC, and advanced it from there.


I’ll check out that video. Thank you for the responses.

[1964Fastback]

The vacuum was hooked to the ported vacuum on an Edelbrock carburetor.
I started off with 10 degrees BTDC, and advanced it from there.

Huh.  Wouldn't have expected the vacuum advance to cause problems like that.  If the diaphragm had a hole in it, the carb could have been sucking a little air in through the hose.  Shouldn't be at idle, but if the blades were open a little too far or at fast idle, maybe.  But figure at worst, it would just be a little lean.  In that scenario, the timing wouldn't be affected with or without the hose because the advance wouldn't be working, just the A/F ratio would change.

If the diaphragm was okay but dialed in wrong so it was giving way more advance than it should be, maybe that could cause it?  Still, hooked to ported vacuum, there shouldn't be much of a signal at idle.

I would investigate a bit more.  Take the pencil out of the hose with it running and put your finger over it.  Strong suction at idle is probably not ideal.  If you have one of those little MityVac hand pumps, you could take the distributor cap off and hook the pump up to the advance and watch the operation, make sure it's not hanging up on something, etc.  That would also show if it had a leak and wouldn't hold vacuum.

Pat

[urchinhead]

I do have one of the MityVac pumps. I am going to test the advance today and see if there is any movement.

One last thing that crossed my mind - the hose that was formerly used for the brake booster was capped, but just with tape. I did thread a large bolt into that hose as I went over everything. If vacuum was pulling through this tape, it could also be a cause, but again, this was primarily run at idle . . .

[Chuck]

Drill a hole in an old distributor cap at number one. When it's running, I think you'll find the spark is too far away when it's not running correctly. The vac advance moves that relationship around and that's why it may run differently at times. That's how it works and I doubt anything is wrong there. Try switching the polarity of the pickup. I tried roll pin holes, curve slots, and I don't remember what else one day with a setup like that. Once I switched the wires, the spark was right on and problem solved. It was out of phase. As far as the grease, clean that out so you get a good connection. NYK corrosion compound works well for me in its place.

[Cyclone03]

To find the correct polarity of the pick up check it with a timing light. The connection that gives the most retarded timing is correct.