22r detonation on highway

[gothmog]

I've got an 89 2wd pickup with the 22r motor in it and it has gradually developed a problem. I commute about 120 miles a day round trip though gently rolling terrain and my truck has gradually begun to detonate more and more over the past 5 months when climbing slight hills at 70 mph, the kind that require a slight application of throttle to maintain speed. About 2 months ago I retarded the timing by feel until it stopped detonating, and continued commuting... guess what came back, so I retarded it some more...it still does it. I finally got my timing light out and the timing was about 5 degrees AFTER TDC, so for yucks and grins, I set it to 8 deg BTDC, now it detonates damn near all the time. I'm suspicious that I've developed a weak or broken spring in the vacuum advance. The vac advance is the dual can type and the detonation only occurs when the engine is hot (that's when the innermost can is activated) Anyone have any thoughts on this? cuz it's annoying the stew out of me!

[Rycam]

For what it is worth, the inner diaphram on my carb/22r vac advance can has a hole in it. I have both hoses to the vacuum cans blocked off, and the outer can hooked up to a "permanent" vacuum manifold source. The idle is then tweaked lower to compensate for the rise @ idle. When smog-checks are needed, I temporarily hook up the system as stock to pass the visual exam. Pre-ignition has never been a problem even though the timing had been bumped 12*BTDC once.

I have never seen a broken advance spring, but I have seen a mechanical advance rusted in place. Anything is possible.
Have you looked at your plugs, or have reason to believe that carbon buildup hot-spots in the chambers are a problem?

[gothmog]

just replaced the plugs and wires today, no change, no excess carbon on the plugs, no holes in diaphrams, although when vac source #2 is connected to the inner diaphram, it sounds like it goes to full advance at idle, (there's a metalic click from the distributor.. ) I don't know if that's normal. I also don't know what the timing is supposed to set at because that field on my vehicle sticker under the hood is blank..."set timing to ___ degrees before top dead center with vac hoses disconnected" etc. I set it at 8 btdc 'cuz that's what my old '77 celica set at.