Intermittent pre-ignition 1979SC – Ideas on why?

[Jamie79SC]

The 1979 SC has an intermittent but serious knock accompanied by noticeable power loss under moderate load at or below 4,300 RPM or so. A down shift or backing off throttle eliminates audible knock and power loss.

The problem appears, lasts for a minute or two, and goes away. This has happened on four occasions, the first last summer and the last three in the past three weeks. There does not seem to be a correlation with ambient or engine temperature and it has happened with fuel from at least three sources. I do run 87 octane fuel but, really that should be fine for a stock 3.0. The distributor cap, rotor, and plugs have 7,500 to 10,000 miles.

Here are my questions:

1) Could a vacuum leak to the distributor cause this? (Seems opposite of what I’d expect, but I thought I’d ask since you guys are the experts.)

2) Is there something in the distributor that needs to be lubricated?

3) Is there a DIY way to test for proper return from high RPM position on the distributor?

4) Am I all wet, and looking in the wrong place altogether?

Thanks guys, you never fail to generate many (and sometimes correct) answers.

[smestas]

Kind of sounds like a broken head stud.

But Im no expert!

[Jamie79SC]

Oh, I really didn't want to hear that. I guess I'll pull the covers off during an upcoming oil change.

[Jamie79SC]

Come to think of it, I have adjusted the valves since the first incident and didn't find any little suprises then.

[Paulporsche]

Have you made sure your distrib is tightly clamped down and not rotating on its own?

You should pull the cap and go into the top of the dist. Take out the felt pad and drip oil down inside until full. Then dampen and wet the pad. This will lube it.

When you rotate the shaft using the rotor, does it snap back?

Have you done the usual checks for things like rotor, plugs and wires?
How about vacuum leaks and WUR function?
Furl flow, filter, injectors?
Mixture?

[Jamie79SC]

Good info on the distributor lubrication. I'm unclear on how fuel delivery could cause knocking. Could you elaborate?

-editing - Of course over lean condition would cause knocking... I'll check filter too.

[carreraplanes]

When I bought my SC some years ago it had sticky advance and retard weights. Penetrating oil soon eased the situation. Worth checking because the car was obviosly over advanced in the low to mid range.

K.

[tops911]

My 78 SC had a spring off on the advance weights in the distrubtor, I was getting something like 45 degrees advance at 4000 RPM. It would ping only (that I could hear) on hard acceleration. The timing was spot on at idle. Might be worth checking total advance

[oneblueyedog]

False air can also contribute to a lean pinging under acceleration in the upper ranges. Check all hoses and FI o-rings. The Fuel Injectors have an O-ring on top and an O-ring in the plastic insulators in the intake runners. Unlit butane torch can trace the leaks. Search for threads.

My distributor's advance plates were stuck I freed then up with solvent. Just make sure you keep the runoff of the cleaning fluid out of the bushing/ shaft area. Make sure you remove the distributor at #1 TDC. Careful with the green wire too.