Xring01

Ok... I built a Roller Cam 302 to 1985 Mustang GT specs... It has Holley 600 cfm electric choke carb, and the MSD Pro Billet 83521 Dizzy (with steel gear).

I am wondering why the "Numbers" do not add up...

With the Vacuum Advance Disconnected and 12 BTDC 750RPM Idle... and the 18 Degree Cam in the MSD (they can be changed out to fit your needs), the engine should max out at total of 30 degress BTDC, however it doesnt,,,

When you rev the engine up it will go beyond 42 degrees BTDC... With the vacuum advance it will go past 50....????

So I have adjusted the Dizzy to max out at 36 BTDC at high RPM's, which gives me an idle setting of 6-7 BTDC... Running the car with No Vacuum Advance... It runs pretty good, But I think it would run better if I can set the idle at 12BTDC and max out at about 36BTDC...

To any of you Dizzy experts... WTF????

This makes no sense what so ever... I have called MSD, they are as confused as I am, offered to RMA it, however that could take 3-4 weeks... which sucks... I dont want to be without a running car that long. And I have to find the reciept that I paid for it...to submit on the RMA...

Any thoughts on this????

jojobanks

That sure is a lot of advance. Is the vac to the dizzy hooked up to a ported vac on the carb? or is it on full vac?

At idle, the engine needs additional spark advance in order to fire the lean (and exhaust-diluted) idle fuel/air mixture earlier in the cycle in order to develop maximum cylinder pressure at the proper point after TDC for efficiency, so the vacuum advance unit is activated by the high manifold vacuum, and adds another 15 degrees of spark advance on top of the fixed initial timing setting. For example, if your initial timing is set at 10 degrees, at idle it’s actually 25 degrees with the vacuum advance connected.

Xring01

I am 100% Sure I have the vacuum installed to the carb, per the Holley instructions... I have measured the vacuum at idle and get 0 at idle and up to 20" at higher rooms....

So when I connect the vacuum advance to the dizzy...the advance does not change at idle... It remains at were i set it with out the vacuum.

jojobanks

try hooking the vac advance to full vac and re-time your dizzy to 12*btc at idle.

Xring01

Lets assume I do that... This is my prediction....

If I were to set the Dizzy at 36 Degrees BTBC with the Vacuum Advance connected at high RPM's.. My intial setting would be retarded by 6-10 degrees...
and the engine wouldnt run...

That cant be good for an engine???

jojobanks

not necissarily. it really depends on how your dizzy is set up. From the readings it seems like it might be set up to run off of full vac. But then you may simply have to get a lower degree cam in the dizzy as well if it's messing with the centrifugal advance.

do you have the part # for your dizzy? I can probably check it out without you actually having to test it on your motor.

Xring01

The Dizzy is MSD 83521... Which is supposed to be correct for my application... I think their is something wrong with the Dizzy, or a mis match with this Dizzy, Cam combo...

Another thing I did notice... that when I backed the initial timing down to 6 BTDC at Idle, my Vacuum shot up 5", from 15" @12BTDC to 20@ 6BTDC.

jojobanks

sounds like you have a stock or very mild cam. 6btdc is probably a much better setting for you. Generally you want to tune your advance for the highest vac possible. Your dizzy has a hefty advance curve built into it, so lets do a few test runs and see what your advance looks like now. Hold the engine at 2k rpm and note the advance, then do the same for 3k, 4k, and do a WOT run as well. post your findings and we'll see if we can't dial it in a little better.