Vac. advance/Dist. problem

[MDR]

I have a miss above 3500 rpms, my set up is a Duraspark dist. and MSD 6AL box, inital timming 14Deg 34 total.If I disconect the vac. hose the miss goes away.I have backed the adj. screw in the advance all the way out and it didn't help.The Dist. is a rebuild from Autozone, could it have gotten the wrong Vac. Advance during rebuild, or does anyone else have a similar experiance or any advice to give?


Thanks Mike

[CobraEd]

Mike,

This is not that uncommon if you are talking about a slight breakup with no load on the engine. This is due to the high vacume when in neutral or cruising at a high rpm in a low gear with no load. I would guess that the car runs fine under load or hard acceleration if it is the typical scenario.

I have a carbed 302 with the Duraspark ignition and the MSD 6A box and have the exact same problem, but under normal driving, you don't experience it. I only see this problem if I am holding the car in gear way too long at part throttle. The car should pull really strong all the way to redline with no miss when you are on it and the vacume drops off.

By the way, what cam are you running, what lift?

Ed

[MDR]

Hi Ed thanks for the reply, you described exactly what is happening with my car. I am running the Ford B303 cam .480" lift.

Thanks, Mike

[Jack21]

Same problem.

351W, MSD dist, Accel adjustable vacuum advance. Misses under high vacuum conditions.

The brass tip on the rotor is about 1/4" wide. It sits right under the brass tip in the cap under static timing conditions (14 deg BTDC). The spark has to jump this gap to fire the plug.

With the engine running, the centrifugal advance moves the rotor tip away from the cap tip 20 deg crank, 10 deg dist. The vacuum advance moves the rotor tip an additional 16 deg crank, 8 deg dist away from the cap tip. Add them up, and if the rotor tip is too far away from the cap tip, the spark won't jump the gap and the plug misfires.

Still working on this, but here's what I'm doing.

Made a new rotor tip with some brass stock with a 1/2" wide foot on it. That solved some of it. Cranked the vacuum advance back some, and that helped, but defeats the purpose of the vacuum advance part throttle cruise economy. Next, I'll try rotating the distributor one gear tooth retarded so the center of the rotor tip advance motion ARC is centered on the cap tip. Will let you know how that works out.

So why don't OEM setups cause this problem on the family sedan, mid '70's - 80's vintage? The centrifugal advance isn't all in until around 4000RPM. At cruise, the advance is mostly vacuum. With your foot in it, the vacuum is gone, and the advance is mostly centrifugal. You don't have conditions where all the vacuum, and all the centrifugal is in at cruise RPM. And this one tooth off misphasing may have some merit also.

[nos347]

The problem that you are having is due to over advanced timing. To eliminate this you need to limit the amount of vacuum advance not the rate. I do this by drilling and tapping a hole in the plate and installing a screw to stop limit the amount of movement in the plate. This way you can properly adjust the rate of vacuum advance with the set screw in the can. On a duraspark dist the screw doesn't change the amount of vacuum just the rate. If you have any more questions, give me a call.

Marc Curtis
Jim Green's Performance Center.
425-774-3507

[MDR]

I took the car out for a ride today, the miss is pretty bad if I go through the gears slowly, or if I stay in any gear with RPM's above the 3.5K area. I really injoy the rumble of these cars and find myself holding the rpm's up higher than I would in any of my other vehicles.I think I will spend some time looking at the dist. and try to come up with a limiting screw like Marc sugested.(Marc can you be a bit more specific, what is the plate you are talking about?)What would I be giving up if I elimited the vac. advance all together?I wont be happy until I eliminate this problem.


Thanks Mike

[MDR]

I played with the car a bit today.Here is what I did to fix the problem I was having, I removed the vac. advance and after some careful marking and guessing I drilled a hole for a very small self taping screw in the arm that comes off the vac. adv. after cutting the threads in to the arm I had to remove the screw and grind it down since the length would pose a problem. Now when the vac. adv. kicks in there is about 1/2 the range of motion then the head of the screw acts as a stop against the dist. housing.I fired her up and took it up to 3500 and above with no miss, I'm a happy camper, hope this can help someone else.

Mike

[webebob]

How much advance are you running when the problem occurs?
(Including vac advance)
Bob