Detonation vs Preignition? 62 84hp

[66vairguy]

I'm not having any issues where a main jet would help. This is at idle and just off idle.
New info: Mechanical timing does not advance too quickly. It actually advances much slower than it should. No mechanical advance where the knock occurs. It does have vacuum advance on just opening the throttle as you would expect. I'm thinking it needs a restriction in the vacuum line to slow down the advance along with a check valve that lets the vacuum dump quickly so it can back off at normal speed. Anyone tried that?
I tried a tank of 89 octane and the problem returns but not as bad.
Price premium is .80 on 93 and .40 on 89.

Just disconnect the vacuum advance and plug hose and see what happens. Vacuum advance mainly improves fuel mileage under light engine loads and of course at idle there should be no ported vacuum to the advance can.

If the problem goes away, then yes a restriction will help slow it down as the throttle is opened off idle. Or maybe it is just a vacuum can with a weak spring.

[azdave]

I'm not having any issues where a main jet would help. This is at idle and just off idle.

My thoughts were that if you were running too lean and went up a jet size, then the engine would run cooler overall and would be less prone to rattle. With your latest post, it sounds like you will be able to tune it out by some vacuum advance tweaking.

[joelsplace]

I'm guessing that it isn't too lean since it doesn't detonate with higher throttle openings and load.
I need to check the total advance from the vacuum can and where it comes in to see if it is in spec.
Disconnecting the vacuum is a good idea for testing to see if my problem goes away.
Thanks!
I still need to check the heat range on the plugs and maybe try some cooler ones. I'm pretty sure it has 44s in it.
According to one chart I found an AC 44 is equivalent to an NGK 5.

[jimbrandberg]

The flip side of richer jets is possibly having carbon form which isn't good for pinging.

I've wondered about 84 HP engines, I think only available with a PG. 80 HP cam with more compression from 102 heads. Is this a good combination? What about a LM equivalent with a 95 HP 889 cam with 110 heads? Or Isky 260 which should have a similar 104 degree Intake centerline.
Jim Brandberg
Isanti, MN
CorvairRepair.com

[joelsplace]

I've wondered why they didn't just go with the 102. I would think the only reason for the low performance engine would be fuel savings and you have to use expensive fuel in the 84.

[jimbrandberg]

The 80 HP cam should have it's peak power at a lower RPM than the 102 cam.
I wonder how the performance is different between the 80 and the 84.
I suppose the 102 heads also get you the better valve springs.
Still curious about a LM 95 HP with 110 heads.
Jim Brandberg
Isanti, MN
CorvairRepair.com

[Nashfan]

I have been running 87 octane in my 62 A/C PG 84hp in the cold weather with factory ignition timing. No issues.
It hit 80 degrees yesterday and it started sounding like a rod knock from idle to about 1300 rpm. Above that no issues even at full throttle. I backed off the timing and put almost 8 gallons of 93 octane in it. The timing helped and the fuel cured it.
I've never had this issue before. Octane/timing issues have always shown up in high throttle situations.
How do I pull some timing out only at low RPM? Different weights or is this a overactive vacuum advance?

It might be that what is happening is that the vaccum advance, for whatever reason is "coming on" too strong, too early. If that is the case, maybe the simple cure is to put an orifice in the vacuum line. I've done this in the past on my car, for somewhat different reasons having to do with a "noisey" vaccum source and a hypersensitive manifold air pressure sensor. For my purposes, putting a .03" orifice in the vacuum line calmed things down a lot!
Another way to to modify the vaccum signal is the use of a vaccum delay valve, which basically slows down how fast its allowed to "turn on" OR turn off https://en.wikipedia.org/wiki/Vacuum_delay_valve. It might also be that the combination of a vacuum delay valve and orfice is all you need. Of course you can go down a huge rabbit hole trying to figure out WHY it suddenly works better when modified, but it could be subtle changes in weather, octane, fuel air ratio and a VA that was better suited for the kind of gas that was available in the 60's. Might be kind of fun to at least hack in a .03" orifice and see if the problem is beneficially affected!

[joelsplace]

If I determine that it is an overly zealous vacuum advance I would think it needs an orifice with a bypass check valve so it will come on slow at high vacuum but drop off fast when it goes low.