The tick/rattle I referenced prior was bad spark knock! Never heard it so bad! I figured it out right after that last post but never put anything up. Timing was set to 15 so I backed it down to about 10 btdc and it was mostly gone except long hills. Disconnected vacuum advance as well and that cured it. I knew I’d come back to it but have been busy.
All that leads to this post. I finally got around to playing with it the other day and while I’m not done, I’m well on the way! After lots of reading here and checking both my spare distributors, I assumed my advance bushing was disintegrated, causing too much mechanical advance, thus my spark knock. I picked up a few aluminum #8 id 1/4”x1/4” bushings as I’ve seen many mention on here, then dove in.
To begin, I wanted to baseline where my distributor was so I increased rpm’s from idle in 250 rpm increments and used my dial back to note the advance until it stopped advancing. I know a dial back isn’t 100% accurate for this but it’ll be close enough for this experiment. What surprised me is I only got 9 total mechanical advance which should be stock from a graph I saw
@FJ40Jim post. I was assuming my bushing was gone (it was) and I should’ve seen about 5-6 more degrees full advance. 2 assumptions, the first is my idle is set to 750 rpm and maybe the mechanical advance starts coming in lower than that? Next assumption/reality further down….
Anyhow, advance curve plotted, I waited for it to cool and pulled the distributor. It was in worse shape inside than I expected! Besides being really dirty inside, there were 3 main issues I found.
1: The bearing plate the vacuum advance moves was super gummed up and “notchy”. I pulled some vacuum and it would slowly and not smoothly move it, then releasing vacuum; it would stay put for up to 10 seconds then twitch back to rest.
2: The advance weights were equally gummed up and would stick at different points that I put them at and need some input to get them to move further.
3: As expected, the bushing was disintegrated, allowing (if the weights moved freely) too much advance.
With these things, I think I found my bad spark knock. Mechanical advance staying wherever it felt, too much total advance and the vacuum advance staying all in well after it should’ve released.
So I cleaned and lubed the bearing plate the vacuum advance moves until it spun very freely, cleaned and lubed all the weights and springs and then installed the aluminum bushing.
I had to get it back together for work the next day so no time to graph the advance curve now, but I set initial timing to 15 degrees btdc and it is running way better and pulls much smoother through the whole rpm range. Seems to rev up much faster during normal driving and better accel from 2000-3000 where it used to be sluggish. Still need to hook up the vacuum advance and I’m going to play with advance springs a bit over the next few weeks as I have time. I’m not going to try the typical HEI springs as they are much shorter than the stock ones and I think it’ll be an excessive spring rate once stretched (I want advance all in by 2500-ish rpm).
What I found when I pulled distributor
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Gummed up mechanical advance
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Cleaned up and ready for install.
Bushing added for mechanical advance.
Green line is where I’m hoping I get my mechanical advance when I’m all done. Blue is supposed to be stockish advance curve.
