ported vacuum on 3x2's??

[w ogden]

So, I have been messing with the timing on the 348 tri power and have it timed at 12 initial and 36 centrifugal all in by 3000 using a MSD distributor with the blue and silver springs. My issue is getting a ported vacuum to establish the vacuum advance for cruising which I have learned from other threads should be in the 40 + range.
If I use the hollow stud on the back carb it throws the advance in immediatlely. The center primary carb has a port like in the pic below

so I was wondering if I could use this or is it not a ported source? I did not have a spare base to check where this ends up but of course it does go to the base plate and I believe that it is above the throttle plates.
I did not want to pull the center carb off unless I have to so I was hoping someone can enlighten me on this port or add some advice as to overcome the issue I have.

Secondly, in checking the timing I notice that the harmonic balancer has two marks on it, an A and a O. Using my dial back timing light I can adjust the timing to 12 degrees which using the pull back light brings it to the A. Does this seem correct? Guess we are just too use to the more current timing tabs that show - and + degrees from the O mark.
Appreciate some input to straighten us out here.

 

[61BUBBLE348]

The A is advance and the O should be zero or tdc, others will chime in but I thought the vacuum for the dist adv was pulled of the manifold, had a carb stud that was hollow.
happy to stand corrected.

 

[Don Jacks]

The factory used manifold vacuum,not ported vacuum,the thinking being that as the throttle is opened as the load increases,the amount of additional advance would back off.Ported vacuum doesn't drop off fast enough when using most of the vacuum cans available.I don't know how fast the MSD cans respond.If your build is relatively stock,with vacuum advance you'll want in the 50-55 degree range.With your dial back light,dial it back to zero to determine your amount of advance,A will be off by 10 or more degrees.

 

[w ogden]

The confusing part of this is that at idle we are at 12 degrees dialed back to the A. When you dial back to the O we are at 20. This is at idle (620 rpm's) with no vacuum attached to the dist. At TDC on cyl 1 the timing mark on the balancer sets right at the O. When you run it up to 3000 rpm's the dial back to the A is at 36 degrees. Does this, for starters, sound correct?
Cause it sure is confusing me

 

[Jim Sullivan]

The "A" is stamped on the timing indicator just so you know which side of "0" is advanced. Use the "0" to set your timing with the dial back light.

 

[Don Jacks]

You're about 8 degrees too high if you're reading 36 on using the A.You want,on these engines,about 34-35 as measured from O at an idle.

 

[w ogden]

Thanks fellas for the input.
I thought that you should be using the O for the base but the guy helping me said that we would not be able to get the engine back to 12 advance from the O without stalling the engine! I am going to work at this tomorrow to see if I can improve on this thing a bit. I am sure I will be back on here with more questions.

 

[Phil Reed]

What brand of balancer do you have Walt?

 

[Don Jacks]

I'd suggest raising your initial up to 16-18 degrees,then working with your limiter bushings to max it out at 34-35 degrees at 3,000.With your max.ad vance as it now at 3,000 rpm,the springs/weights should be fine as is.Once you're done with that,hook up the vac.advance [manifold vacuum not ported] and see where it comes in.The vacuum advance on these MSD distributers is adjustable,and you might have to take some of that advance out of it.Since you already know that the engine is in fact on TDC at the zero position on the tab,the brand of balancer is immaterial.

 

[w ogden]

Phil, this a 348 tripower that I bought from a friend over ten years ago. He had it stored in his garage for a long time before that. It appears that it is stock OEM 348 not a F code. Was a 4 bbl engine/powerglide that he put the 3x2's on so any guess is good on the balancer. It is one of the round ones used by GM if that helps!

 

[w ogden]

Don, tomorrow I am going to set up a dial indicator for #1 and determine exactly where TDC is with regard to the marks on the balancer. Figure this will give me a base line to work from. I know from yesterday that we were very close but we did not have a gauge. Your point on setting the timing at 16-18 may help considerably. The MSD has a aftermarket adj vacuum can that I have not much experience with so I will have to bone up on it first and then see if it will do the job. I know we hooked up manifold vac. yesterday and it threw 10 degrees in right at idle so some tinkering is necessary for sure. appreciate the help as I do not work on these engines enough to consider myself proficient at all.