How to tell intake and exhaust valve with the intake manifold on?

OMG! Touchy are we....? And totally unable to understand an ironic comment it seems. That's ok - we all have our days. You may also want to look up the definition of "lol" in internet shorthand.

******** You may rest assured in the most certain knowledge that I've forgotten, yet still retain, more about Computer Systems and Sciences than you're ever going to have the slightest inkling about.

In any event, to everyone else, thank you for taking the time to answer my request for information and advice. It is most helpful and appreciated.

If it is a major rebuild, it is certainly beyond my capability and desire - I would be the first to admit it. If it does fall into that camp, on a 35 year old engine I note the comments from another poster and I could see it being a bottomless pit with a very hard to justify cost-benefit. Not sure what the heck I would do then...

The most I would have any desire or capability to address is some easy component replacement and/or adjustment. I am going to remove the valve covers again and re-examine and try to adjust everything armed from info from here and the service manual. I'll see where that gets me. I would hate to relegate old Betsy to the bone yard after having had her for 35+ years, so wish me luck.

I will LYK how I make out.

BSA

**** Here's a little more assistance that you can desperately use: Just leave the damned thing alone before you cause substantial bodily injury (or worse) to yourself or other unfortunate innocent victims in the vicinity.

.

"You may rest assured in the most certain knowledge that I've forgotten, yet still retain, more about Computer Systems and Sciences than you're ever going to have the slightest inkling about. "

A possibility most certainly, but highly unlikely. thanks for your comments.

In case your statement is genuine.

The mostly likely component that has failed on your engine is the vacuum advance on the distributor. If after 35 years the diaphragm is still intact it would be a minor miracle. The difference a working vacuum advance is rather subtle because it only functions when the engine is under light loads. But a defective vacuum advance will over time cause all sorts of engine damage including the type of problems you are having.

Get one at a parts store or on ebay:

-jim

A very astute observation Jim! Thanks - and it is the original as I really have had the car since day #1. It certainly should be replaced as a preventative measure. I'll look into sourcing a repladcement as part of all of this.

This thread just makes me roll my eyes, in so many different ways.

The vacuum advance diaphram is hardly 'the most likely thing to fail' on an old engine. Furthermore, if it fails, it will typically spring a leak. No vacuum advance will hardly make the cam go bad. If the centrifugical advance mechanism has seized up--and I have had this happen to me--then the engine might be running at full advance all the time, which is not good. However, again it won't do squat to the cam.

A '76 Corvette has value, even if the engine has some issues. The obvious choices are: 1) Fix the engine up properly; 2) Hope that driving and an oil change or two will more or less fix things; 3) Make sure all ancillary vacuum lines, plug wires, etc. are in good shape, pull the distributor cap to make sure everything looks good, and put on a timing light to ensure that timing is right, and vacuum and centrifugical advance are functioning properly; 4) Sell the car for some pretty good money to someone who wants a '76 Corvette.

Choice 4) is pretty final....if your car is in decent shape, and you still like driving it, then 1) would be a better choice.

As for finding a cam in a junkyard, this is a 350 Chevy flat-tappet hydraulic cam. The aftermarket is huge and cheap for 350 Chevies. We're talking a nice car, not some junker hillbilly pickup. Get a new cam.

I just want to make clear that this choice has nothing to do with fixing or not fixing a noisy valvetrain. It's a general check for a smog engine. The vacuum lines can crack; lubricant can dry up, etc.

Also, don't discount an exhaust gasket leak as the real culprit. Does it get less noisy as the engine warms up? Leaks tend to do that.

Who said it was the most likely thing to fail?

It is however extremely likely to have failed once in 35 years. That probability is getting pretty close to 100%. If he still has the original tires on this car it would be likely that they would have failed by now, also. And any other rubber component on this car is likely to fail after 35 years.

The failure of the vacuum advance will cause the engine to deteriorate. The vast majority worn out 350 engines made in the 70's wore out because the vacuum advance diaphragm started to leak. The reason is simple - mechanics never bothered to check the the vacuum advance and the effects of a non-functioning vacuum advance doesn't show up as an immediate loss of power or performance but instead take its insidious toll on an engine over 10k - 20K miles or more.

How do you know the cam has gone bad?

That is a very suspicious statement. How the heck can mechanical advance seize up in the full advance position? It would be a real challenge to get it to stick in the advanced position even if you wanted it to stick in that position Typically when the mechanical advance seizes it is due to rust. And it develops rust while sitting still. That means it always seizes in the full retard position.

You can hardly not know if your vacuum advance is bad on a 350. Same with all the Ford and GM V-8's I worked on with a manifold vacuum advance. You time it with the vacuum disconnected from the dist and plugged. When you reconnect it revs up. If you're curious you'll check the advance with your light before you set your idle. Goose it to see the mechanical advance. Whether you check it or not, if the advance is bad, I think you'll have noticeable performance problems. Although I never had a bad one, I did always check when I did a tune up. Hard to believe pro mechs don't do the same. Part of a tune-up.

--Vic

If it was manifold vacuum that would be an older engine.

That would not be for a '76. By then they had ported vacuum so the advance doesn't kick in until you stepped on the gas. So connecting the vacuum at idle should have no affect. On the older engines with points there was also the window in the cap for adjusting points - so you could see inside to see the distributor plate advance.

No there are no noticeable performance issues (at least not for the first 5k-10k miles). The advance works only under lighter loads. So if you accelerate hard it behaves the same whether the advance is functioning or not.

So how much advance were you looking for and how did you go about measuring it?

After electronic ignition - ignition tune-ups were mostly changing spark plugs. The base timing maybe was checked. And as you pointed out, the procedure for checking the timing is with the advance disconnected.

-jim

Forgot that ported vacuum. My 350's were a '76 and '78. Probably my '66 352 was the one with manifold vacuum. I think they all had the window in the cap for dwell adjustment. My '64 Bug didn't have a window, but I could use a continuity light and feeler gauge for timing and dwell.

Don't remember how many degrees. I always had specs for all this and followed the book. Best I can recall is goose to maybe 3 grand on the tach and see maybe 30-40 on the mark. It wasn't precise as I recall, since it jumped around. Just checking that the mech advance was in the ballpark. Lower and steady for the vacuum, maybe 1200 and 12-16 on the mark. That's just my recollection, probably bad. I don't think I ever ran a 350 more than 5 years before the car/van under it rusted out. But I tuned them every year. Plugs, wires, points, choke. They never failed me. Until the van 350 developed a rod knock. Not a fan of the 350, but the vehicles were bought cheaply and served me well enough.

I checked timing on my first electronic ignition engine, a 2.8, in '91 when I bought the car. Habit I suppose. My tach/dwellmeter and timing light haven't been used since. I'm just fine with that.

--Vic

Well:

On Apr 12, 5:15 pm, jim wrote: ....

It appears that you did. Why are you asking about what you wrote a day or two earlier?

The original vacuum advance canister, the heat riser vacuum motor, and the vacuum choke breaks on the carburetor of my '78 Pontiac all work fine. Apparently my car is the Miracle Shrine of Lourdes. Tell you what, I'll check the state of the vacuum advance canisters on the old Chevy and Buick HEIs I have lying around at home. They're original from the late '70s.

Please explain exactly what the effects of the failed vacuum advance are on 1) the engine's functioning; and 2) on the car's driveability. Then explain the causal mechanism by which a failed vacuum advance causes the engine to 'deteriorate'?

We don't. However, you jumped in by saying 1) "Get a new cam from the junkyard", followed by 2) "A bad vacuum advance is a horrible thing". I assume you were linking the bad advance to the possibly bad cam. Or are you just saying whatever comes in your mind, relevant or not?

1. Advance bushing starts to get sticky from age and deteriorated lube (NOT rust)
2. Centrifugical force at higher RPMs exerts enough force to activate the advance, even though it's not working freely
3. Springs are not strong enough to pull the advance back
4. Advance remains stuck at max

It doesn't take a whole lot of thinking to realize that the force of weights trying to advance the timing, especially at higher RPMs, is going to be much greater than the force of the return springs. If this wasn't the case, you would not get any centrifugical advance at all.

And of course the problem isn't rust, not down in the bushings, when the advance mechanism gets bad. It's loss of lube, or carbonized lube. The rust on the weights and springs doesn't make a whole lot of difference unless it's ridiculously extreme.

The electronic ignition on the 350 started in 1971, IIRC. It was an option for maybe 2 years but after that they all had it.

My point was there are no such factory made markings for that much advance. So if you had markings you would have created your own. And in the '70s under the first generation of emission standards it wasn't easy to find the specifications for timing. The base timing for any particular engine was contained on the under-the-hood sticker and you had to dig into the specifics of distributor model to find the rest of the info.

-jim

Well:

On Apr 12, 5:15 pm, jim wrote: ....

It appears that you did. Why are you asking about what you wrote a day or two earlier?

The original vacuum advance canister, the heat riser vacuum motor, and the vacuum choke breaks on the carburetor of my '78 Pontiac all work fine. Apparently my car is the Miracle Shrine of Lourdes. Tell you what, I'll check the state of the vacuum advance canisters on the old Chevy and Buick HEIs I have lying around at home. They're original from the late '70s.

Please explain exactly what the effects of the failed vacuum advance are on 1) the engine's functioning; and 2) on the car's driveability. Then explain the causal mechanism by which a failed vacuum advance causes the engine to 'deteriorate'?

We don't. However, you jumped in by saying 1) "Get a new cam from the junkyard", followed by 2) "A bad vacuum advance is a horrible thing". I assume you were linking the bad advance to the possibly bad cam. Or are you just saying whatever comes in your mind, relevant or not?

1. Advance bushing starts to get sticky from age and deteriorated lube (NOT rust)
2. Centrifugical force at higher RPMs exerts enough force to activate the advance, even though it's not working freely
3. Springs are not strong enough to pull the advance back
4. Advance remains stuck at max

It doesn't take a whole lot of thinking to realize that the force of weights trying to advance the timing, especially at higher RPMs, is going to be much greater than the force of the return springs. If this wasn't the case, you would not get any centrifugical advance at all.

And of course the problem isn't rust, not down in the bushings, when the advance mechanism gets bad. It's loss of lube, or carbonized lube. The rust on the weights and springs doesn't make a whole lot of difference unless it's ridiculously extreme.

**** I find it unlikely that he'll respond at all given the unworthiness and indefensibility of his risible remarks, which are unfortunately and in general more the rule than the exception here.

You misquoted what I said in order to give it a different meaning.

So you say.

Doesn't sound like you would be able to tell if they were not working as designed.

The effects are that the fuel in the cylinders is combusting much later in the power cycle when the engine is cruising under light loads. This will accelerate cylinder, ring and valve wear. And since the highest combustion chamber temperature is seen when lot more of the cylinder wall is exposed it will also coke the oil that is sitting on those exposed cylinder walls. This means the crankcase oil also deteriorates faster and that may lead indirectly to things like oil pump, lifter and cam wear.

You have trouble focusing on more than one issue at a time.

Lack of lube will cause a rod or main bearing to seize and that is because the lube is there to reduce friction - the bearings will melt without lube. But there is practically no friction involved in the distributor advance mechanism. Remove all the lube and it isn't going to stick by itself. It will function just fine without any lube (assuming it doesn't rust). It certainly will not seize due to friction involved. It will stick when the lack of lube leads to rust and that is because the rust displaces considerably more volume than the metal it comes from. If there is no rust the lack of lube will (in the worse case scenario) just make the mechanism become loose and sloppy over many miles of use. Rust is pretty much the only way the clearances can be used up and cause it to seize. It is very unlikely to develop that rust while the engine is running at high speed. Much more likely that rust will develop when its left to sit.

Not that any of this matters when the base timing is set where it is supposed to be. No matter what position it sticks in the result will be the same.

An absolute, unmitigated, steaming pile of shit.

Okay, I buy all that. My memory is worse than I thought. Not the first time my memory made things up, so I'm not shocked. But I'm sure I had no problem seeing both vacuum and mech advance on the mark on my V-8's for whatever good that did me. Also remember you could get a good estimate by watching it go off the mark onto the chain cover. So seeing the advance was working in some fashion wasn't hard to do. I don't think dist advance is as critical or failure prone as you do. Never knew anybody who went to the length of getting a dist tested on a dist machine, but I was never a hotrodder. All dists are probably off some degrees at certain ranges. Only dist I replaced was the one on the '64 Bug. Read the vacuum advance often failed and followed recommendations to put on a Bosch all mechanical. As far as specs I had Motor's manuals that covered quite a bit. I never lacked for specs when I did engine work. The used '76 Caprice 350 I bought had the cat pulled off and who knows what other emissions disabled but I never had a problem tuning it to run smooth at all speeds. Same with the '78 in the van. Anyway, the valve train noise the Vette guy hears has nothing to do with the dist. Different issue entirely.

--Vic

E II EE II E

HTH and good luck,

Ben

Just an update - I finally screwed up the courage to look more closely at the beast. I pulled the valve covers and intake manifold and put a micrometer on the push rods while hand rotating the engine. It turns out one exhaust lobe (at least) was worn down to a nubbin. The micrometer rotated 2 1/2 turns with other valves and barely once on this one. Several of the lifter seats were showing signs of wear and/ or scaling. I would not be surprised if other lobes were worn to some degree or another.

So - I ordered a cam kit (cam, springs, lifters, etc) from procams (a little more aggressive than the stock but not wild at all) and a complete gasket set for a rebuild. I have a chum going to lend me an engine puller and a motor stand. I figure maybe over the next few weeks I'll see about getting the motor out of the car for a looksee and cam replacement.

While I was at it, I also ordered a complete new HEI system on-line (by the time I replace the parts on the original it would be the same or more money wise), and this way I am eliminating the shaft play on a

35+ year old unit.

I have never done anything like this whatsoever before, but it does not appear to be rocket science, although there surely is a lot to learn in terms of tricks and tips! And for sure I'll be asking some questions along the way that experienced folks may roll their eyes at.....

We'll see how it goes - either "The thrill of victory, or the agony of defeat!" lol.

BSA

You must mean a Dial Indicator

This is a micrometer

-jim