73-87 Chevy _ GMC Trucks > Engine/Drivetrain

Vacuum Advance on HEI 350

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I have a bone stock, all original '79 C10 with a 350 that has about 90k on it.  I've owned this truck for a long time, over 20 years and I've fought off and on with it pinging at a light load (e.g. going up a small hill or a lazy acceleration) from pretty much the get-go.  I've done all the usual things, checked/verified the timing (multiple times) last year I replaced all the vacuum hoses and the EGR valve, sprayed everything down to check for leaks and I have never been able to eliminate it.

The solution has always been 89 octane gas, 87 it pings, 89 it doesn't.  Well I just filled up with a tank of 89 that I suspect isn't really up to snuff so it's been pinging a bit.  For the fun of it, I disconnected the vacuum advanced canister and low and behold the pinging went away.

I know they make adjustable canisters but before I order one is that the route I should be chasing?  I'm sure the pinging could be the result of something in the old 350 being tired and I do plan to freshen it up in the next year or two but it always starts well and other than the ping runs well and you know what they say about fixing things that are not broken.  It just feels like that canister is adding at tad to much advance in low throttle situations. 


I'm going to reply to my own thread.

After doing some googling it kind of looks like I need something to limit the canister more than adjust it.

The first thing I found was the Crane 99600-1 kit but it looks like that it's NLA.

Doing a bit more digging it looks like the MSD Ignition 84281 HEI Vacuum Advance Stop Plate might be the ticket.  I'm sure any adjustable canister on top of this would really dial it in but I just want to pull some timing out a light throttle.


OK, I would look/google "vacuum advance stop plate". I don't know how much this forum likes linking. You can use pretty much anyone's adjustable vacuum advance kit. The crane kit was nice because it had almost everything. You had to do a little machine work but it wasn't bad.

The parts you'll need:
Adjustable vacuum advance
a limiter plate you'll have to make
some screws (you'll need the drill and tap for these)
the springs from one of those kits for adjusting mechanical advance (the weights are junk)
timing light

There's a guy that used to post on other forums called ignitionman that has the setup that you need in terms of initial, how fast you want to see the mechanical and then the additional timing for the vacuum advance.

Clear as mud?

You are going down a rabbit hole. You could simply retard the timing if it is over-advanced. Figure out why it is pinging:

What plugs are you running and how do they look?

What is your initial timing and total timing?

Do you have a borescope? Can you get a look in the cylinder to see if you have excessive carbon buildup? You can buy a cheap borescope for your phone for like $10 bucks on eBay.

Too bad water injections systems aren't on the market anymore.

I bought the last one around here back in '87, still have it  8)

I could run 87 in my 3+3 with a 454, fully loaded 5th wheel toyhouse, and 3 brats and cool as a cucumber!


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