Small Backfires only with vac lines connected

[KoenigsTiger]

Hello Everyone!!!

I finally made a few laps around the neighbourhood!!! It's a rocket ship after 4k , before that there is much to be desired! It's a 3.2SS with 964 cams, stock exhaust for now.

OK, here is the question:

I was setting the timing, I think I did a good job of it. It was 8:30 PM and I figured 5800rpm was enough of a test on the neighbours.

When I quickly open the throttle to 2-3k, the response is fine, but there are small backfire sounds when I release the throttle quickly with the vacuum lines hooked up to the distributor. I have to double check but pretty sure from the air cleaner, and sounds from the exhaust.

With the vacuum lines disconnected there are no backfires...

The distributor was supposed to be rebuilt, and when I disconnect the retard line the rpm does increase. At 5800RPM the advance is 21*BTDC +/- 2*

Thank you very much,
Gene

[kach22i]

I don't know the year of your car or your engine, but most of the engines at 6,000 rpm are supposed to be between 32-38 with 35 being the best.

From what I've read and experienced backfires are from a lean condition. You need to enrichen your fuel to air mixture a bit.

Perhaps the vac lines are affecting the fuel to air mixture somehow.

[Flat6pac]

The retard is only activated to pull down the idle when the butterfly on the throttle body is closed so the hose has to be connected below the butterfly to vacuum retard.
The advance side pulls the distributor forward with high rpms so the hose is connected above the butterfly in the venturi effect.
The 3.0 dist. is set at 5 degrees btdc without the retard hooked up. The dist has for example 20 degrees of centrufigal advance. That gets the spark to 25 btdc and the vacuum advance kicks in and gets you another 5 to 7 degrees advance giving you 30+ degrees advance on the big cubes of 3.2.
Bruce

[KoenigsTiger]

kach22i,

I dont know, 32-38* sounds a bit high to me. Thank you for trying to help!

Bruce,

"The retard is only activated to pull down the idle when the butterfly on the throttle body is closed so the hose has to be connected below the butterfly to vacuum retard."

I was under the impression that the retard line, retards the spark when the throttle is let go all the way down. So when the pistons are still going up and down(side to side)fast don't detonate because the fuel has been reduced.

Thank you very much for all the help!!!
Gene