76C3forme

I hear ya'! I think the first few engines I built, I just cranked'em down!.

With the carb off, the inside of the intake looks like it's actually cleaner than brand new. Super shiny aluminum. Not a speck of carbon, oil residue etc. I didn't really peer in to see if it was oily any deeper in. This thing looks (on the inside) like it's just been taken out of the box.....

Jebbysan

And I have never used a torque wrench on intake bolts either....

Jebby

76C3forme

I know for sure this guy didn't !

I'm not really sure he may have used anything but his bare hands!

76C3forme

Well, I had a chance to take a peek at this corvette for a few hours again today. (I have a family function tonight so we only had a little bit of time today to work on his car. Plus.... I want to get some stuff done on mine!!)

Here's a quick update.

I re-torqued the intake manifold, re-installed the plugs and fired her up. Slightly better, but nothing fantastic / earth shattering. I also re-did a few tests to post some results:

Timing 12 degrees BTDC @ 1,000 rpm

Vacuum Readings
(after complete warm up and choke opening:

Fluctuating between 9.0 - 9.5hg @ 1000 rpm
Fluctuating between 12-13hg @ 1500 rpm
Almost steady / slight fluctuation 13.5hg @ 2000 rpm

At 2000 rpm:

"Quick Blip" of the throttle = vac quick drop to 0 then nice rebound to 17hg, immediately settling at 13.5HG @ 2000rpm

When Idling at 1000rpm:

"Quick Blip" of the throttle = vac quick drop to 0 (slight bog "muffle sound" from carb, but quick rebound from 0 - 13HG then settle at 9.5HG.

Covering up the primary throttle bore will make idle rough and stall out the car. The air sound / pulling from primary throttle bore sounds pretty normal.

So I still think there is something else going on. The cam just isn't large enough to produce this little vac at idle. I'm guessing I'm going to check the chain next.

Any other thought's just in case??

Funny, the engine, exhaust sounds good, (a little lumpy) but solid. Exhaust pipes look clean, no "eye tearing", no soot, just this slightly rough idle / low vacuum.

7T1vette

Early in this thread (just saw it today), you indicated that you took a 'baseline' vacuum reading. But, you didn't indicate that you removed ALL of the vacuum lines from intake, carb, etc and blocked them off. If you didn't do that, you do not have a 'baseline' reading. There would be lots of possible sources of vacuum leakage.

I.e.: bad diaphrage in vacuum advance can; defective vacuum modulator [if you have auto tranny]; bad check valve in main vacuum supply line; leak in vacuum reservoir; leaky brake booster diaphragm; etc; etc.....

{need I continue?}

I would bet that if you remove ALL of the possible outlets for vacuum from the carb/intake system the 'baseline' vacuum level would be higher than 12" Hg. {Probably around 15" Hg}

drwet

I think its time to put a degree wheel on it. It sounds to me like the valve timing is late.

76C3forme

Correct, that's what I meant by baseline.

I normally start with the booster, Vac Advance canister and modulator (This car is a manual though) and anything coming off of the carb, since they always seem to be the more common sources for leaks, then move on to removing / plugging "everything else".

As of today, Booster hose still removed and Plugged at carb base. Advance Canister is plugged at carb as well as the port of the timed vacuum. Vac port in back of intake with the check valves is also removed plugged in the intake. Vac at idle is 9.5hg max.

76C3forme

Yep, That's were I was going originally based on my initial readings, but the last few times I had a valve timing type of issue, my Vac reading was a bit more steady (and low) so I was kind of second guessing myself a little bit. (however the intake bolts were pretty loose so at least that's ll good now for the guy). And, it did "narrow" my fluctuation to about 1HG or so.


Originally the fluctuation was a bit "wider", (up to 3HG swing with a 8-8.5HG reading at idle), but after checking / torquing the intake, my fluctuation is only about 1hg, so I think my original thought about valve timing may have been correct. (Won't be the first time I've found a SBC chain "off"). I think he just has a few things going on all at once, that we just need to knock out 1 at a time. I think that's why he gave up after awhile. I can see his excitement even now, that we're narrowing things down.

I think he'll be in good shape once I see where the chain is.

7T1vette

You are probably on the right track. Valves are leaking [more than expected for that cam] for some reason.

76C3forme

I hear ya'. I think I got about 10-11" of Vac with the 292H cam I used in one of my 350's in the past so this is a little low.

(Unless they guy is mistaken and it's not a 270H ) !

spikebot 81

Any update?

76C3forme

So far, I havent been over there in the last few weeks. We both got a bit busy.

Last time I was over there (when I tightned up the intake manifold bolts), I noticed a bit of coolant that seemed to be "coming through" the rear intake bolt on the pass side. He loosened up all of the intake bolts (a little) just before I arrived (didn't drain coolant....). After the car was idling for a bit, there was no coolant to be seen BUT.... since the rear of the intake doesn't have water passages, I thought it was a bit odd. The rest of the intake was dry. Any coolant appearing at the threads of that bolt would have had to have come from "under" the gasket area.

I'm sure some coolant could have "seeped" down the gasket from further up (Mainly becasue the car is jacked up in the front and at a pretty good angle). The front of the car is 10" off the ground and he was trying to be helpful buy loosening each bolt a bit before I arrived so I could torque them down in sequence......, but I'd like to be sure nothing is going on there, (since we don't know how long these intake bolts were loose) and most likely replace the intake manifold gasket for him.

It's been idling / running off and on for quite some time and there is no coolant there. (I even loosened the bolt about 1/4" off the intake surface after everything else was torqued down just to make sure there was no coolant "coming up" through those threads". I'm aware that the SBC intake bolts don't pass through into the coolant like on a few bolts on the BBC's), so I thought that was kind of a weird spot to see coolant.....and wanted to be sure we didn't have some kind of "wacky cracked head thing" going on..

I'm going to replace the intake gasket for him, that way, I can take a good look in there / see if I can spot a possible gasket failure into the valley, make sure the intake gasket is the correct one etc / or something else to explain our low vac @ idle.. For the price of an intake gasket set, he agrees, and he'd like to take a look in there as well. Definatley no coolant in oil / or anwhere else when that happend....., coolant level is still perfect. I really feel that he never torqued down this intake "from the get go" when he installed it...

Vac is still 10-11hg @ 1000 rpm @ appx 16-18 BTDC. It should be higher. I still think valve timing is off and I'm thinking (hoping) that's what we'll find when I remove the timing cover.

firebirdTA

Any update to this issue? I have an 80 with an L48 that is running rich, low vac with needle fluctuating from around 12-13Hg and have been fighting this for a while.
-paul