jim lockwood

I ran into a head scratcher today. I'm looking for thoughts and ideas.

The car is a '63 fuelie. The cam is alleged to be an 097 Duntov. The symptom that puzzles me is idle vacuum of only 6" Hg at an idle speed of 850 RPM.

Vacuum rises to around 13" Hg at 2000 RPM.

Initial timing is 10 degrees. 25 degrees observed at 3000 RPM

Idle is reasonably smooth, not inconsistent with an 097 cam.

Hiway air/fuel ratios are spot on.

Engine makes good power and fuel economy is ~17 MPG

Because idle vacuum is so low, the ratio lever floats between the economy and power stops, making it impossible to make meaningful adjustments to the enrichment diaphragm or to the idle mixture. If anything, the idle mixture tends to be rich, due in part to the improper ratio lever position.

The owner does not know what the valve lash settings are. My one and only hunch is they are too tight.

I want to know what could cause such a low vacuum reading. Any thoughts?

Jim

wmf62

i'd try the hunch first..
Bill

Dan Hampton

Just a hunch--late valve timing?

W.B. @ Bama

Great info at .... www.secondghancegarage.com/public/186.cfm

Nitrodamus

Cracked vacuum hose or one that has slipped off.

63split63

Verify that your gauge is accurate .

Bill

knockbill

+1,,, if they don't respond to re adjusting, maybe you need to verify the cam,,,

midyearvette

.....also aside from any leaks,late timing (slipped balancer) sloppy timing chain or weak valve springs, even at an idle....

65tripleblack

jim lockwood

That occurred to me, too, Dan. I tend to dismiss this possibility, though, because the vacuum level rises off idle and the engine makes great power.

We went all over the engine looking and listening for vacuum leaks. We sprayed brake cleaner at gasketed junctions. Nothing. Zip. Nada.

Too, a trait of FI engines is that a vacuum leak will cause a high idle speed and a happy idle mixture can be found by turning the idle fuel screw out a few extra turns. That wasn't the case. The engine idled happily in the 800 - 850 range and the idle fuel screw was almost completely closed.

I'm highly confident there are no vacuum leaks.

Ratio lever behavior confirmed that what the gauge told us was true.


Good stuff so far, guys. Keep it coming. I'm seeing a consensus starting to form around valve lash that is excessively tight and questions about what the cam really is. Anything else?

Jim

plaidside

Jim,
When I got my 61 FI, after sitting 25 years, the owner had no idea what cam was in the engine.
So before starting and after rebuilding the unit, I set the lash at .012-.018" and when it warmed up I had about the same vacuum as you.
Then I set the lash at .028 across and I had 15" of vacuum and it has been running this way for 3 years.
Joe

JohnZ

As I recall, '63 FI's had the distributor vacuum advance connected to a "ported" source (no vacuum to the distributor at idle); I know ZOT about FI, but if it behaves like a carbureted car, connecting the distributor to a full manifold vacuum source will give it more timing at idle, so you can close down the throttle plate at the same idle rpm, increasing the idle vacuum level. Chevy corrected this for '64 on FI cars, with a full manifold vacuum source to the distributor. If I'm off base on this, it just shows how little I know about F.I.

AZDoug

[QUOTE=jim lockwood;1580861069 I'm seeing a consensus starting to form around valve lash that is excessively tight

Jim[/QUOTE]

Too tight does effectively increase overlap and duration, and thus will give reduced vacuum.

Doug

jim lockwood

Very interesting, Joe. It seems safe to say the engine in your car does not have an 097 cam. Did you ever determine exactly what it does have?

In the case of this '63 fuelie, the owner believes the engine has an 097 cam, but he stopped short of 100% certainty.

From listening to it, I thought it probably had a hydraulic lifter cam. Maybe it does or maybe the solids are lashed too tight or <heaven forbid> it's got an 097 with hydraulics. Anyway you look at it, it's seems clear he's going to need to research what's in the engine.

Jim

MiguelsC2

Sounds like it is lashed too tight. My 097 will sound "hydraulic" when they are too tight.My vac is about 15-16hg @900rpm.

Just pop the covers and lash it to spec. If that solves the issue? There ya go.

jim lockwood

Right on the money, John.

I initially forgot this yesterday and hooked the vacuum gauge to the ported vacuum.

Got 4" Hg indicated.

After sanity prevailed, I smugly and confidently hooked the gauge to full vacuum.... and the reading rocketed up to 6" Hg.

Going forward, my recommendation to the owner will be:

1. Confirm the engine has a solid lifter cam
2. Measure actual valve lash.
3. If different (meaning tighter than) .012/.018, adjust to .012/.018
4. Recheck vacuum

FWIW, after we "hit the wall" with this vacuum issue, the owner drove the car home.... 186 miles away. Ran fine, he said.

Jim

jim lockwood

Any chance you measured manifold vacuum while the lash was too tight?

My own 097-cammed car makes 14 - 15 in. Hg at idle.

Jim

MiguelsC2

I feel they are a bit too snug now and I am still getting 15 hg. It still runs great. But the top end is a little weaker. If they were far off enough to cause a vac issue? I would think it would run like crap.

So there may be an issue elsewhere.

MiguelsC2

The lash specs I got here on the Forum were .09 and .18 when set to those specs it had great top and bottom end. With the classic mechanical tick.

I think I just went a bit tighter and lost a few revs. Mine has no mechanical sound now. But has excellent vac and bottom end.

jim lockwood

If the lash is too tight, as seems possible, the bottom end would suffer but the top end would improve a little.

This car pulls strong, cruises well, and gets 17 MPG.

It just doesn't make enough idle vacuum.