12a bridgeport timing and advancement
 

I know how many degrees they want the stock 12a's to be but are you bridged guys runing any more advancement or just stock timing? And is it more benaficial to lock the dizzy at fully advanced and call it a day or let the dizzy do its job? And anybody else still using the vac advance or is everybody plugging the lines? Should get some pretty interesting responses considering who I know has bridges on here. lol :)

My old SA bridgeport I was running 12* leading and 20* trailing without the vacuum advance. With the bridgeport the vacuum advance didn't really make any difference. By 3200 rpms or so the mechanical advance takes over anyway.

http://www.youtube.com/watch?v=09_ukjxh-zM

i'm running a P port, and i just have the timing locked @18btdc, with no split. i haven't done too much testing with timing, but what i have done shows that it does want an ignition curve.

it idles better between TDC and about 10BTDC, and it pings @1800-2000rpm @18btdc, so its telling me it wants a curve.....

for WOT, the book says 20, i will try 18-22 on the dyno and see what happens

good stuff guys.... thanks for the input. :) Oh and bad 83....... you ever make it up north? Carlisle maybe?

On the dyno my 12a J bridge made the most power with 29.5* total advance. BAcked it down to 28* for a safety margin.

Considering that the power band has moved up the rpm range, I would assume that you would want to "re-spring" the mechanical advance to take that into account.

Vacuum advance should still help a bit coming off idle, I would think. But wouldn't have much of a performance benefit since the rpm range where it is working would be well below your power band. I wonder if the action of the vacuum advance can be adjusted like the mech advance?

If you do run vacuum advance, with no emissions you will want the nipple farthest forward at the base of the carb. Good luck. :)


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Now when you guys are talking degrees advanced your talking about that at idle correct? May sound stupid but I just wanted to make sure. :) And I think I will try hooking up the vac advance.... I'm having some trouble starting when warmed up and this may help retard the timing when starting if I'm correct in that assumption.
And by "re-spring" you mean putting new springs in or just adjusting the internal springs?
You were also wondering about the vac advance being adjustable...... I seen in one of my manuals they showed how to adjust the trailing timing with the vac diaphram.... you can loosen the screws and slide it in and out...... if thats what you meant. :) if not just disregard that. lol
Thanks again for all the help guys.
And one last thing kentetsu.... I have a racing beat 600 carb.... I dont think I have a vac port in that area..... I dont thing the RB carbs really have much of any vac ports now that I think about the couple I've had. But I'll look again..... you mean facing away from the passenger towards the front of the car on the bottom?

On a bridgeport you want to set the timing at full mechanical advance, >6000rpm.
Here's RBs timing chart for your reference:
http://www.racingbeat.com/manuals/timingchart.pdf

Awesome! thanks man!

Wait...... isn't it saying that 24* leading and 5*trailing? for the bridge 12a first gens...... Thats about backwards to what all the others say!

Sorry for the late reply. This knowledge comes from about 15 years ago when I was restoring a '71 Lemans. Working with the distributor, I found they sold spring/weight sets to change the mechanical advance curve. Played around with it a bit, but I'm no expert.

It just strikes me as strange that you never read about anybody adjusting the advance curve on rotaries like this. It seems that it would be very beneficial with a port job, since you are changing the entire powerband.

Never found anyone selling different weights or springs for 1st gen distributor. Nor an aftermarket distributor. But I would welcome it!

I used a 10* split, never tried 15* but it's easy enough to try.

I know about them curving them but I just also heard of people locking the dizzys too...... but thats just turbos from what I understand. And I wasnt sure what degrees I was looking for in advancing it..... lots of different opinions on it. :)
I rebuilt the distributor over the weekend and noticed when you move the trailing vac diaphram advanced it moved the trailing pickup ALOT. I also broke the set screw loose that holds the leading pickups if i remember right and that sucker wouldn't budge! Is there something else I need to loosen up or what?

Yes, a good compromise for a turbo application is to remove the springs and lock the distributor. Boost requires less timing and hits about when the mechanical advance does, so it's not needed. Not perfect, but better than leaving the mechanical advance.

That is how you adjust the trailing timing. Be careful with the trailing split timing when you put the distributor back in. You may want to do a rough adjustment of it at idle before you adjust the leading timing at 6000RPM.

What have you found to be the best way to dial in the timing? Timing tape, adjustable timing light......?

I ran RB underdrive crank pulley on my bridgeport for waterpump speed correction. It has lots of timing marks on it. Adjustable timing light is a good tool to use regardless of which crank pulley you have.

Your running a track car pretty much tho right? I didnt think it was a good idea to put the underdrive pulley on something you intend to drive on the street?

K heres another question... lol What increments are the score marks on the trailing vac diaphram? Are they measured in 5* increments?