Initial Timing to Total Timing

[Ken Caruso]

Hi All,

First real post here. I was inspired by a couple recent posts to float this one by the group.

I mostly solved a severe "pinging when hot" problem on my 66 GP 389 YE Block / 4BBL / Dual Exhaust by really going through the cooling system (boiled out radiator and a Flow Kooler Water Pump).  Reduced observed temps from the 215 - 225F range to the 180 - 200F range.  I also map some of the pinging to old gas. I typically get through only 1 or 2 tanks of premium gas a season. The car doesn't get driven between Dec 1 and May 1 (nearly half the year).

Other factors include an aftermarket Carter AFB 625 CFM installed 2001 (before I received the car) and a Pertronix pointless ignition and coil which I installed in 2014. At that time I discovered the advance weights were totally gummed up and resolved that problem so they move freely now.

Having said all of that, the engine will still ping slightly after a highway cruise (20-30 miles) / exiting and driving local backroads stop and go.

I checked the timing per factory instructions, I.E. disconnect vacuum and 500 RPM. It is right on at 6 degrees BTDC. I guess that would be the Initial Timing.

I reconnect the vacuum hose and the timing jumps immediately to around 22 - 24 BTDC at idle (estimate based on progression around pulley from timing marks).

I then rev the engine to maybe 3000 RPM and the timing will max out at around 32 BTDC. This makes sense, that would be the advance weights doing their intended function. I guess this would be Total Timing.

It's the fact that the timing immediately jumps to 22-24 BTDC at idle when the vacuum is connected that concerns me.

Does anyone have experience as to whether this is normal or not? Could the vacuum line need to be connected to a timed port rather than a full vacuum port?

Am I being fooled by the fact that in actual driving conditions the vacuum drops under acceleration and most of the timing advance is provided by the advance weights under load conditions?

Ken

 

 

 

[Last Indian]

Well I’m reading this quick so I may need to re-read this, but my first take is the vac line is connected to the wrong port. Vacuum advance works of Venturi vacuum. The opposite of manifold vacuum. When you connect the vac line at idle nothing should happen or at least no more than 1or 2 degrees. But really nothing, as you decrease manifold vacuum Venturi vacuum increases.

You can also use ported vacuum, I always preferred Venturi when I setup a car with vacuum advance. Although I really just prefer all mechanical, much mor reliable.

Edited November 25, 2017 by Last Indian

[Stripes]

Ignition timing has 4 components: initial, the rate of advance, total advance, vacuum advance.

Your initial is 6BTDC. Thats low enough. Your total is 32BTDC. Most say the range should be 32-36 depending on load, size, weight, gearing, etc.  Close enough.

What is your rate of ignition advance? At 3000 RPM, is all 32 degrees in? Is it all in at 1800 rpm? You need to test for pinging with the current set up with the vacuum advance disconnected. If it doesn't ping then, try hooking the vacuum can to a ported carb connection and see what it does. If it pings without the V advance connected, what heads and or compression ratio are you running? Some head combo show 10.75 to 1, too high for today's pump gas in most cars. 

Those vacuum cans have different rates of advance AND differing total advance. On my car I found the v advance coming in too soon and providing too much. Looks like your can is providing 18 degrees at full vacuum. Many feel 10 degrees is about right. Stay away from adjustable v advance cans, they typically only adjust the total advance and not the rate of advance.  It sounds like you need a new v advance can.