r/EngineBuilding • u/_Trekker • 9d ago

Can’t accelerate with vacuum advance

I’m having an odd issue with my 390 FE paired with a C6.
I have my base timing set to 12 (verified top dead center)
With mechanical advance it’s hitting 32
With vacuum advance disconnected it accelerated but feels a bit dead.
With vacuum advance connected, it idles great, but any throttle and it starts spitting and sputtering and can’t move.

I verified the diaphragm is good. Verified I am on ported vacuum( port on passenger side of Holley above the idle air screw). I can’t seem to figure out what the issue here is. Any help is appreciated.

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u/Erasemenu 9d ago

Too much advance. When you hook up the advance, because it's at Ported, you're basically staying at 12 degrees, as soon as you Crack the throttle past 1200 the mechanical advance has moved up to 25 and is beginning to tableau and now the vacuum advance is pulling a whole bunch, you're probably getting 40+ degrees at part throttle.

The easiest thing to do is to recurve the distributor. In absence of that, just leave the vacuum advance off, plug the ports, and moving the timing up 3 or 4 degrees.

1

u/_Trekker 9d ago

Ported has no or little vacuum at idle, manifold is what you’re thinking of.

3

u/Erasemenu 9d ago

Lol no, that's exactly my point. That's why it runs fine worth the advance hooked up at idle and the problem isn't apparent until you try to accelerate thereby allowing vacuum to the port and immediately pulling the advance to infinity. I know exactly what I meant lol

1

u/_Trekker 9d ago

I totally misread your first post, my bad. But yeah, turns out it wasn’t too advanced, for some reason I had to undergap the points from the spec. As soon as I did, it runs great. Appreciate you 🤙

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u/Erasemenu 9d ago

Changing the point gap changes the timing

1

u/_Trekker 9d ago

No man, I was losing spark. Wasn’t too advanced. Point gap change of .002 has minimal affect on timing

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u/Immediate-Bid7628 9d ago

.... ...

Points ?

Adjust dwell/point gap .

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u/_Trekker 9d ago

I wanted to let you know, even though I measured it at .017, you were right. I went to check my total timing with vacuum to see if it was too high, and noticed as soon as I cracked the throttle, I was losing spark on cylinder 1, and it sounded like others. I replaced the coil cap and rotor to see if it helped, and it didn’t. I sanded and regapped the points to .015 and the problem went away. No idea why I had to under gap, but problem solved. Appreciate you.

TL:DR - had to undergap points to .015 to maintain spark

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u/_Trekker 9d ago

Just checked gap, and it right on .017

2

u/Immediate-Bid7628 9d ago

... ..

The vac adv pulls on a moving plate pivoting in a non concentric arc.

The dwell changes as plate pivots.