So ever since Ive had my truck the vac advance has been disconnected. Its been annoying me so Ive tried to get it to work, unsuccesfully.
The truck is an 82 F150 with a 302w, duraspark II.
Without the vac advance it runs perfectly, base timing is set at 10 deg BTDC, Ive verified the timing marks on the balancer, so theres no issue there. After hooking the vac advance up it starts to run rough, backfire and eventually stall. This happens with manifold or ported vac, although ported only starts to play up under light throttle with the same outcome. So to eliminate a vac leak I plugged the carby up and used a manual vac pump to advance the dizzy, its holds vac but after a certain point of advancing the same symptoms occur. Ive adjusted the can all the way in and out with the allen wrench and it doesnt make a great deal of difference, just slightly alters when the can advances. Going off the timing marks the vac advance adds 15 degrees of timing which seems right. Now Ive run out of things to check so hopefully someones been through this.

 

Still sounds like the balancer has slipped. How did you verify that 0° on the balancer and pointer is actually TDC.

 

Where did you connect the vacuum pump? Directly to the actuator? I’m wondering if the line between the carb and distributor is leaking, but it was never tested.

 

I checked TDC by marking where the piston stops either side of TDC with a plastic straw and splitting the difference. Bit of a crude method but its the best way I had available to me. Maybe ill have to check again, because its the only thing I could put it down to as well.

 

The vac line is brand new, but even if it was leaking it wouldnt cause and issue if the carb isnt involved as there isnt any vac leak as such. Thats why I ended up using the manual pump to eliminate that as a problem. Its got me perplexed.

 

When it's connected to the carb & acting up, are you getting pinging? How many vacuum connections are there on the vacuum advance can?

 

no pinging, just very rough idling, popping and stalling. Theres only one connection on the can, I dont think duraspark II had a retard connection.

 

No, that's right - I just wondered if an incorrect vacuum advance had been fitted by someone.

I'm clutching at straws here.......but is the earth (black wire) at the pickup coil/distributor body, attached & sound? Also, the wires to the pickup coil flex more with the can operating.........if you play with them while the engine is running, do you get any response? The flexing is inside the distributor but you have to start somewhere.

 

haha ive been clutching at straws for a while now. Visually the wires look fine but then again theyre close to 40 years old so who knows whats happening under the insulation. One of the pickup coil wires has been nicked but I dont think its all that bad.
I think at this stage im going to buy a piston stop tool and confirm TDC more accurately and then go from there.

 

Oooh, excellent theory, and easy to test. The two wires inside the distributor? They only move when the vacuum advance is actuated. If no vacuum advance, there's no movement at all.

Two possibilities for a fault: A wire could have a bare spot from rubbing against the inside of the distributor housing. This would short the signal to ground. Or a wire could have broken strands inside the insulation, but look fine on the outside. Either scenario could be intermittent, and only act up as the wires flex with movement of the vacuum advance system.

To test, disconnect the plug from the pickup coil. With an ohmmeter, measure the resistance of the pickup. I forget the specs, maybe somebody can chime in. Connect your vacuum pump and actuate the advance mechanism back and forth a few times. The resistance value should remain rock steady. This tests for an intermittent connection through the pickup and wiring.

If that was good, move one test lead to the distributor housing. Leave the other lead on one of the wires. Either wire is fine, as they are both connected via the pickup. The ohmmeter should show infinite resistance, meaning no connection whatsoever to ground. Cycle the advance mechanism again several times. As before, the reading should remain rock solid.

 

Good catch Karl, never seen it happen but anything is possible. Between the Purple and Orange, the value should be between 400 and 1300 Ω, between the Black, to either the Purple or Orange the value should be 70,000 Ω or more, from ground to either the Purple or Orange the value should be 70,000 Ω or more, black to ground should have continuity.

 

Yes good way to test Karl.
This was an issue when GM first came out with the HEI dist. in the mid 70's.
I remember coming across this when in shop in high school.
It would idle great but as soon as you gave it any gas and the vacuum advance kicked in the motor would cut off like you turned the key off but as soon as it lost vacuum the advance would go back to normal and the motor would run again.
At first you would think it was a bad accel pump in the carb.
Dave ----

 

Thanks for the detailed numbers. Remember, Ken is the guy who had the idea to check the wiring to the pickup coil. All I did was suggest how.

Turns out I was less correct than usual, as I was thinking there were only two wires, not three. Glad you caught that. So a slight edit: Check the THREE wires per your instructions, and make sure nothing changes while actuating the vacuum advance.

 

Never thought about testing it like that. Ill give that a go next chance I get to work on the truck. Ill report back once Ive gone over everything. Thanks for your help everyone