Vacuum advance trouble . . .

I have a stock distributor on my 331. Right now it's set at 14 and total timing comes in near 3K to about 34 degrees. When I hook up the vacuum I get 52 degrees of timing total at 2700 rpm. I tried adjusting the vacuum module with an allen wrench but doesn't really make much of a difference. The vacuum module is actually a new piece since I had same trouble with the older one.

When driving with the vac advance I get detonation in 4th gear around 3K. So I disconnected it. But my gas mileage is better with the vac connected. My MSD allows me to lock my distributor and make my own curve but I haven't fiddled with it yet.

 

With the old original vacuum cans you could adjust the amount of advance by changing the spacer inside the diaphram spring. Maybe with yours you could drill a hole in the arm near the can and pin it to limit the travel.

 

You will get all kinds of numbers with the vacuum line hooked up. The vacuum changes with engine load. It's best to go by set of pants and ears when adjusting the vacuum canister.

 

Or use a vacuum pump to actuate the can.

 

i use a vac gauge. set at the highest vac and go

 

If you are getting detonation with the vacuum advance connected then you need to slow down your mechanical advance or delay the vacuum advance with more spring pressure. Having the power valve open at the same time your vacuum advance begins allows a richer transition which helps prevent detonation too.
I have my vacuum advance set to start at 10" Hg and it goes to 14 degrees at 20" Hg. The mechanical advance starts at 1000 rpm and is all in (in two steps) by 3500 rpm. My power valve opens at 10"Hg so I am getting a bit of enrichment as the vacuum starts coming in.

 

Who's fuel (brand) and what octane are you running ?

 

how do you adjust when the vac. advance starts?
the only adjustment i know of is the small allen screw in the vac port. i believe that adjusts the total amount of vac advance that is possible. now i am not sure if that is in fact what that adjusts.

 

I am using 87 octane, usually mobile or shell. I tried 89 and 91 octane and still had detonation.

 

this is what my vac advance looks like



I don't see any springs that I could adjust.

 

When you turn the wrench it will adjust the spring that is inside the canister

 

I did that but when I was testing it with the timing light there was no difference. Are you saying that I won't see any changes with the test light and should just make an adjustment and drive it to see if I get detonation?

 

Yes, you can't simulate the vacuum, the engine has to be under load.

 

That's really interesting. What's the reasoning behind it? Sorry, I know it's an annoying question but it's so cool that you guys know this stuff. Ok, will play with the vacuum and report back. I'll do it today during lunch

 

You need to drive the car and have the engine under load to see whats happening. Sitting still with the hood open ain't gonna tell you nothing. Road test the car by accelerating at different rates, using part throttle in high gear. (High gear is suggested because lacking the torque multiplication of the lower gears, engine load is highest). If the engine does not ping or surge, insert the hex wrench in the canister nipple and rotate the adjusting screw in the clockwise direction. Road test the car again. If detonation is still not evident, repeat the adjustment/road test procedure until it is. Then slow down the vacuum advance rate by rotating the adjusting screw counter-clockwise, one turn at a time, until evidence of surge disappears.