Can't Get 17hg Vacuum

[colnago]

Okay, I've been making some behavioral changes, but I still can't get the vacuum to 17hg. Here's what I have:

- 352, fresh rebuild, cylinders bored 0.030 over
- Edelbrock 1405 carb (600CFM, 4BBL, manual choke, just finished complete tear-down and rebuild)
- Ford iron 4BBL manifold ("T" variant from mid-70s)
- Engine Pro 1776 RV cam
- Duraspark ignition (distro recurve with Mr Gasket 925D springs, 20-degree total mechanical advance, all in by 3500RPM)
- Altitude is 2200'
- Vacuum advance is currently on ported, but I keep going between the two, so I'm not married to either

If my initial advance is 16 degrees BTDC, I get 15hg. If my initial advance is 24 degrees , I get 16hg, and no pigging. I flogged it this afternoon, and could not make it ping, no matter what (but this is a 352, so the compression ratio is under 9:1). Gas is 87 octane.

What to do? It's driving fine. Even at altitude, shouldn't I get 17hg? Should I advance the cam, and back off on initial timing? Should I just drive it (no strip action, just my daily driver)? Should I back down on initial timing, and live with 15hg? Should I swap in a 351 (just kidding; this will be the only engine for this truck while I'm still married)?

I'm looking at sizing down on my carb, but that shouldn't affect my vacuum - should it?

Joseph

[tsherry]

By "fresh rebuild", how many hours /miles are on the engine? I've seen my own engines fluctuate a fair amount up to 500 miles of "average driving".

[colnago]

Well, I guess it's not so fresh anymore! It has just over 1,500 miles. I was paranoid to follow the break-in instructions, so Comp Cams break-in oil for 20 minutes, then a change, which I ran for 1,300 miles (I meant to change it at 1,000 miles, but life got in the way). My old engine maxed out at 14hg, but it would give the occasional backfire through tne carb (so it really needed a good scrub-down; now it looks like new). The rebuild isn't giving me any backfires. In comparison, she almost purrs, but the idle gives some rumble, so it's not glass-smooth.

I'm just not sure where to take things from here. She's running really nice; I just think there should be more vacuum. I guess my homework for tomorrow is to check for leaks.

Joseph

[Ranchero50]

If it idles butter smooth at 600 rpm you're fine. See what it does for economy and go from there.

[colnago]

If it idles butter smooth at 600 rpm you're fine.

In gear, or in Park? Right now, my idle in Park is 550, with a C6 tranny.

Joseph

[Ranchero50]

550 in park seems low but I don't remember the specs. Thought it was 600 - 700 in gear.

[Busboy]

My rebuild which ran perfect and had a stock cam only pulled 18 inches which is a little low but it still wasn't broken in yet. Advancing the timing would increase the vacuum but I kept the timing at specs. I would imagine it would increase after 1000 miles or so. I sold it so I don't know. You're situation can be at least partially on the aftermarket camshaft.

[cep62]

How accurate is your gauge?
I used to race where the "stock" class had to pull at least 15" of vacuum.

Different gauges would register different on the same engine.

[colnago]

Well, it's somewhere between Harbor Freight and Snap-on. I probably bought it at AutoZone, or similar, but I've never had reason to doubt it. After all, it's "only" a vacuum gauge. It has one job, ONE JOB! Sorry, I couldn't help myself. Back on topic, are there brands to avoid, or ways/places I can test/calibrate mine?

Joseph

[colnago]

... Advancing the timing would increase the vacuum ...

Advancing the cam timing, or ignition timing? Initial ignition is already at 24 degrees. If I advance it any more, I'm convinced the poor engine is gonna melt.

Joseph

[cep62]

Buy or borrow one or two , and see how close they are.

[Busboy]

That sentence was not a suggestion, it was an explanation of the effect that advancing the timing on my engine would produce.