Need some help troubleshooting spark knock

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Jan 7, 2004
Santa Rosa Valley, CA
My engine runs smooth. Timing is set to 7 degrees BTDC; I'm certain of this. The engine is stock, included the distributor. It's one of those weird 78 era dizzy's that has vacuum advance and I think also retard. It has two vacuum ports. Currently running 87 octane of California gold, which involves 10% ethanol. Running 33s on 4.11 gears. I'm near sea level. I'm pretty sure my vacuum lines are connected correctly, but I wouldn't bet my life on it. At idle, adding a little suction to the advance port does increase RPMs.

I can advance the timing and get a bit more power, but (as usual) at a certain point, I'll get predictable knocking. In this case, however, at 7 degrees BTDC the engine sounds and runs great through all gears, even under load. EXCEPT ... when it's doing 55+ mph while crusing in 4th. At that speed, even on flat ground, just a little throttle can generate what I'm pretty sure is a light knock. It's that telltale ticking/clicking sound. I probably inherited this noise from the PO. I've done so many things to the motor recently that it's a heck of a lot quieter. Now I just hear more problems.

So, what should I try first? I'd like to be able to not retard timing more than the BB. Could it possibly be plug gaps or could the dizzy be out of whack? Odd that it seems to work great at all gears/RPMs until that 55mph point. Meaning, I can take it up a hill in 3rd under heavy load and it be just fine. Same for 4th. I can rev it in 3rd going flat out and it also doesn't seem to have a negative effect.

Thanks in advance for any tips!
For what it's worth- Before I did anything at all I'd try to find some no ethanol fuel- in my state premium has none. I'd run the tank good and low and fill up with the good stuff... I've experienced ethanol fuels cause drivability issues even in "ethanol approved" vehicles.
Have you tried running premium or an octane booster to see if it still pings?
It's quite difficult to find e-free gas in CA. I know it can be had, and I think it's worth a shot just to see if it makes a difference. But it's not a practical fix as those stations are too few and far between. I'm hoping I can get this to work correctly on the readily available fuel supply.

I haven't as yet tried premium to see if it'll help with this newly discovered issue. When I have put premium in the tank, I felt kinda silly.
I’d see about disconnecting the vac advance too. Everything move freely in the distributor? (Advance weights). Usually a change in throttle position should drop the vacuum and the extra advance would fall off but maybe that isn’t happening? Are the lines to vacuum retard and vacuum advance switched?
I recently had the dizzy apart for a cleaning and it's in good working order. Great advice to mess with those vacuum parts. What has me confused is that they appear to be doing their thing at higher RPMs in the other gears and it's hard to understand why speed is a factor.
I'm pretty sure my vacuum lines are connected correctly
If you're not certain, post a pic so someone can say for certain.

It's that telltale ticking/clicking sound.
I wouldn't describe detonation (knock) as a ticking/clicking sound. Detonation/knocking/pinging has a lower pitch. (I've always thought "pinging" was a poor choice of words for it.) A vid with sound would help us help identifying. It might be something else.
That could be possible exhaust leak!!

It's an uninteresting video, but if you try really hard you can hear the noise.

I'm land cruising at freeway speeds on 33" mud terrains. That's the swarm of bees in the background. If you crank up the volume, at about 5 seconds and then again at 8 seconds, you'll hear the start of the noise. Crank it up. Every so often I'm rolling on the throttle and the sound picks up at that point... then dies down when I roll off. The thing that's got me scratching my head is that I can't hear this at slow speeds, regardless of rpms. It sounds like it's coming from in front of the firewall, but with all the road noise it's kinda hard to know for certain. So what's moving on my truck at 60mph that can make a rattle like that?

An exhaust leak, maybe? but you'd think I'd be able to hear it in my garage.
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I can hear the noise you’re describing. I think why it would happen at speed only in high gear is because you have less mechanical advantage with the higher gearing when under load. A quick way to see if it is spark knock, retard the timing by 5 degrees or so and try it again. If it doesn’t go away, I’d be thinking it’s not spark knock.
Going to try most of the things above this weekend and will report back. Thanks all.
Adding a little to Skreddy’s load explanation. Knocking indeed is prone to happen more or sooner while under load. You can also simulate a heaver load by starting out from a stop in a higher gear than first. Once you get it moving with the clutch fully out, give it some extra throttle. If it wants to knock it’ll do it there.
Some more results:

Checked over my vacuum lines and they’re all connecting to the right places, at least. Took it out for a spin to see if it was still happening… and it was. Pulled over and cursed a bit because I forgot my timing light. Retarded timing by ear. Drove back home and the issue was gone, but I was really missing some horsepower. I had brought timing back to near TDC. No bueno, but no knock. I set my timing for 5 degrees b4 TDC and went for another drive. At 5, the knock situation is much improved. I have to try pretty hard to get it to knock, and I think I can hear it at moments, but that’s only when smashing the pedal at 60mph while going uphill

91 octane might actually cure this, but it feels like something is off with my dizzy.

It’s one of those ‘78 electronic dizzy’s with 2 vacuum ports. If I apply vacuum (that reads better than “suck on”) the outside port, rpms go up. If I apply vacuum to the inside port (the one that‘s pointing upward) RPMs decrease. But somehow it’s not adjusting the timing correctly at 60mph. As I think about this I’ve realized I don’t know exactly how this thing works.
Tried a couple of things and double checked my vacuum hose routing. Retarding timing and 91 octane does improve the situation a bit. Retarding timing a lot eliminates the knock, and it also eliminates horsepower. At 7 degrees BTDC while running on 91 octane it's pretty hard to make the engine knock. It's do-able though. Freeway speeds and smashing the pedal while pushing it up an incline will generate a bit of knock. Is some knock under those heavy load conditions a fact of life or is it supposed to be a never event? I think I need to go to Distributor School. My air gap .... well, I should first say that about an hour ago I found out that A) an air gap is a thing, and B) the FSM has a spec for it .... was a little bit more than the max tolerance. I moved it back into spec but put everything up for the night. Not sure if that could cause erratic spark, which could lead to the knock.

One other odd thing I can't figure out is the 3-way vacuum connector in the pic below. One of the ports has a couple of extra holes, and the main hole is smaller. The factory emissions manual refers to an "orifice in 3-way connector" with these black triangles. Not sure if this is supposed to be a regular 3 way connector or the odd one in the pic. The tees on that loop in the vacuum tube head back to both ports on the distributor. Anyone familiar with that connector? Perhaps the end with different inner diameters helps keep different vacuum pressures on the dizzy's advance and retard ports.

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Are you setting idle timing with the vac advance hose removed from dizzy and plugged?
I crimp off both the advance and retard hoses, but they aren’t disconnected.
I think I've got it working about as good as can be, but it still knocks a bit under load. My brain is starting to blame the mechanical advance on the distributor. I'm not exactly clear on how it works, but perhaps my springs are worn. Check my comprehension here ...

My most likely 44 year old springs have weakened. Running above 55 is a decent amount of RPMs... not sure how much as I don't have a tachometer, but maybe 3500? At that RPM, the mechanical advance must be towards its limit. When the gas pedal is smashed vacuum drops and the dizzy is relying completely on the mechanical advance, and the engine is now under heave load. In this case, with hypothetically weaker springs, it's increasing the top threshold of the dizzy's total advance, and that's just a little too much for my engine. Do I have that about right?

Who's the current dizzy guru that's still paying attention to the 40 section on this forum? Might try to PM them if they're kind.
When you mash the gas up there, the vacuum advance should drop out, returning the engine to only the mechanical advance. I’m not farmiliar with the dual port distributors, but maybe there’s a vacuum signal or lack of, to retard at the same time the advance drops out to aid?
You can check your advance/retard and mechanical while installed. Pull the cap and suck on the vacuum hose to the advance side of the can; should move freely counter clockwise (I think…) then when vacuum released: “snap” back to neutral. Now do same with the retard side. You can check the mechanical advance by twisting And releasing the rotor clockwise.

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