70 454, LS5

I'm seeing about 15" Hg when measuring manifold vacuum at idle. Seems about 3" or more low. So far I've ruled out vacuum leaks, removed the accessory hose, tested vacuum there. Doesn't seem to be a leak around the carb, spraying with starter fluid or passing propane around the area had no effect.

I once went so far as removing the carb, blocking off the opening, loosening all rocker arms over open intake valves, then did a vacuum test. I recall it held a vacuum well enough to rule out some manifold leaks.

Cranking compression test was fine, all cylinders around 205 - 210 PSI.

The only other items I can think of are ignition timing and valve timing. If the timing chain were off a tooth, wouldn't the vacuum be much worse? I paid a shop to reassemble the engine many years ago having faith it would be done right. No evidence so far to prove otherwise.
The local Corvette shop checked the timing, seemed OK. I don't have a light at this time so I can't retest.

I don't know what cam was installed, paperwork from 20 years ago is MIA. Anybody know if it's feasible to check valve timing using a timing wheel and dial gauge? Anybody done this?

The car idles pretty well, runs smooth but feels like it should have more kick.

I will retest the vacuum, pinch off the power brake booster hose and disconnect the accessory hose to be sure there are no external leaks.

I've heard of the outer ring on the balancer slipping with age, could give a false indication of timing. Seems far fetched...

Any thoughts?

 

Not enough kick? Are you sure your throttle cable is completely rotating the shaft on the carburetor? I'm not kidding...most if not all of these cars suffer from tired accelerator pedals...due to 40 years of mashing that sucker to the floor.

The local vette guru checks this first whenever a customer brings their car in...complaining about lack of get up and go. He finds that the cable is not, bends the arm on the pedal and tells them to come and get the car. They take it for a test drive, come back amazed and ask for the bill...which he says is no charge, because he didn't do anything.

Ahh...if everything was only this easy. I'm having on-going problems of my own which seem to defy solution, so I am nobody to offer expert advice. All I know is that when you find out what the problem is, you wonder why you didn't think of that earlier. Go figure.

 

15" Hg for engine vacuum at idle is decent, for an engine with some wear or if it has a more aggressive (than stock) cam in it. The 'base' 350 stock C3 engine generated 17-21" Hg when new. So, 15" isn't bad at all. If you remove all vacuum lines and plug them off, then plumb in your vacuum gauge and measure vacuum at idle, that will be about as good as it gets. You can adjust timing by the vacuum gauge reading to obtain an absolute maximum reading [for your engine], but that won't necessarily be the best timing set for engine performance.

But, if you obtain that maximum reading for your engine, then immediately put all the vacuum lines back in place, the difference in that second reading from the max reading will be a measure of "total vacuum leakage" for your vacuum system. If that difference is less than 2" Hg, you have a well sealed system.

 

Hi Faster,

Thanks for the tip. I did indeed find I wasn't getting full throttle, adjusted the cable at the carb for a temporary fix until I get under the dash again. I had to replace the throttle cable a while back, thought maybe the replacement was a bit long. Could the a problem with the pedal linkage.

The low vacuum is still a mystery...

 

The engine was completely overhauled, good compression, 205 - 210 lbs on all cylinders, very tight. I don't know what the cam specs are, can't find the paperwork from long ago when this was done. I don't think it's a very radical cam. I recall asking for something near stock, not wanting to deal with lumpy idles, etc.

I'll try testing vacuum again with ALL accessories disconnected or hoses pinched off. The PCV is one I haven't looked at though I've never seen a bad one unless the engine was pretty full of sludge.

Timing seems to be the most likely issue. Valve timing.... possible it may be a problem but tougher to diagnose. I'd think if the timing chain were off a tooth I'd see a more radical drop in vacuum.

Thanks for the help.

 

I've got an "old school" Comp Cams 268H in mine, yours is probably similar. 15" Hg vacuum at idle sounds about right for a non-OEM cam that is just a little bit more performance than the original.

Your timing chain and gears should have been replaced at rebuild. If not, the nylon coating on the gears would surely be missing now after 41 years and the chain loose as heck. Mine was, within a few hundred miles of ruining the engine. When I got my car 7 years ago, the water pump gave up the ghost. I was told to do a "while I'm at it" and pull the timing chain cover. The pan was full of nylon, as well as valve stem seals.

 

Hi Faster,

Thanks, this is exactly the kind of info I've been looking for.

From what I recall I have a double roller Cloyes timing chain. I think there were a few key slots in one of the sprockets which would allow you to set the timing a bit advanced, right on, or a bit retarded. I disassembled the motor twice in the time I've owned the car, had a shop reassemble given the price of parts. My assumption is the shop set it to neutral. If I get desperate, maybe try to figure out just what the timing really is.

I don't know what cam I have. It's been about 20 years since the work was done, still haven't found any paperwork. It could be a cam similar to what you describe. In that case it's reassuring to know 15" Hg isn't off the mark even though most articles on reading vacuum gauges say it's low. I recall the cam which the PO had installed was designed to solid lifters. The PO installed hydraulic lifters which may have been why the retainer clip popped out of at least one lifter and found its way into the oil pump. This was back in the mid 80's. I remember installing solid lifters for a while so the cam change must have happened the second time I had the motor out, early 90's.

The car idles fairly well, doesn't stumble (once I found a QJet which wasn't hacked), but doesn't have the pull I'd expect. Could be almost anything.

Are you running a QJet with 15" Hg? The QJet is vacuum controlled, makes me wonder if it needs to be set up differently for an engine with lower vacuum levels.

From all I've read, either it's normal or it must have a vacuum leak, late igntion timing, or late valve timing.

 

How sure are you that your vacuum gauge is accurate? When I bought my '73 I thought my vacuum was too low. Turned out my brand new vacuum gauge was bad.

DC

 

Good point, thanks.
I'll check my vacuum gauge against a hand held vacuum pump/gauge. Assume they should match or be very close.

Update: I checked the vacuum gauge against a hand held vacuum pump with gauge. They were within 1" of each other, sometimes dead on. I doubt the gauge is bad..

 

Had a low vacuum prob also.Was reading 8in, way too low! Engine sounded like it had a radical cam ,bubba tech haha.It was no fun driving around with this low vacuum causing the brakes to be crappy.Adjusted the timing for highest vacuum got it up 15in with 22 deg initial timing way to much,something has to be wrong.Decided to do a cam,intake swap.Took engine apart cam was retarded,cam sprocket was bolted up with 2 bolts and one lag screw!! Intake was leaking from under the lifter valley.Installed a new Crower 00320 solid cam with cool face face lifters advanced 4 deg.Now all is good I have 16in at 700rpm idle.I doubled the original vacuum reading and the engine is running nice! Love the response with those solid lifters! My next project is to install an aux vacuum pump to aid the power brakes should the engine stall.It will maintain a constant vacuum of 20in to the brake booster.Will post up results after the install...Alohas

 

It's looking like the lower than stock vacuum is probably due to the cam. So far I haven't been able to find a receipt for the cam, been nearly 20 years since the work was done.

Wondering what you're running for initial timing?
Factory spec is somewhere around 6 BTDC.

I am considering bringing this up to as much as 16 BTDC. If I do go that far I'll probably have to shorten the mechanical advance curve to keep the overall limit to 36.

 

That 6 BTDC was for emissions, smog system and warranty. It really hurts performance, either from my L36 or your LS5. I am running 15-16 degrees initial, 36 total. I really don't know why my mechanical appears to be only 20-21. I have what I believe to be the stock springs and weights in the distributor. My vacuum advance has a brass bushing to limit it. I really don't know what it is giving me, but doesn't appear to cause any detonation. Since I went to full manifold vacuum, the idle improved tremendously. Big blocks love a lot of timing.

What really freaks me out is revving the engine to check total advance. Even with my Equus advance timing light with digital tach, I still am not 100% certain that I am "all in" by 2800 or so. The mark on my balancer just seems to bounce around a little too much for me. Right now I'm playing with the concept of point bounce, 20W50 oil messing with the oil pump and gear...sometimes I think I just read too much stuff.

 

Have you adjusted the carb's idle mixture screws for highest vacuum?

 

I had about 15" on my LS5, fixed some vacuum leaks and tuned the carb and got to 21", stock cam. 15" with a performance cam doesn't sound low to me, but I admit to not being very knowledgable about cam specifics.

You can check top dead center on cylinder #1 to see if your balancer has moved. It's not hard, just pull the spark plug and slowly rotate the engine by hand until the piston reaches it's peak. Some people use a little tool to determine that, I used to use a toothpick and would get close enough. Let the piston push the toothpick between your fingers, when it stops pushing, turn the engine back a little until you can just feel the piston with the toothpick.

6 degrees BTDC is the initial spec, and isn't bad for that engine. In 1970, a lot of the "de-tuning" hadn't happened yet. What some people forget is that 6 BTDC is WITHOUT any vacuum advance at all. The shop manual says to pull the vacuum line from the distributor and plug it to test idle timing. Some people will say that you should use ported vacuum to the distributor, and therefore don't need to do that, but I generally run manifold vacuum. Even if I was on the ported outlet, I'd still pull and plug the line just to be sure.

 

Thanks, setting initial timing to 16 degrees BTDC was a suggestion I heard elsewhere as well. It appears I'm getting about 24 degrees of mechanical advance which I think is to spec. It can be shortened if necessary but would probably involve modifications to the distributor. No bushing on my vacuum advance. I'll need to reread Lars paper on vacuum advance and timing.

I agree, it's a bit nerve wracking to be cranking the engine up while watching for the end of the advance curve. I think mine went to around 4000 RPM. I'm not sure, as you witnessed with yours, the timing tends to jump around quite a bit.

I worked on it just a bit before getting side tracked, tracking down vacuum leaks on the accessory line. Found all three vacuum relays were leaking. When I pinched off the line, I don't think it made a significant change in vacuum read at the manifold. I'll recheck it when I get a chance, see if the idle mixture screws change the vacuum much.

Part of my dilemma is not knowing what I have for a cam. It was replaced many years ago. I haven't found any record of what was put in there. Assuming it's a mild performance cam, the lower vacuum may be expected. If this is the case, some other modifications to timing may be appropriate as well.

I wondered about the balancer at times. There's a tool which I could buy, fits in the spark plug hole, tells me when the piston is at TDC. Might be worth checking...

Thanks for all the input Faster, Jeff, and PK, I appreciate it.

 

One simple test for vacuum leaks: back out idle speed screw all the way. If engine does not idle down and die, you have a vacuum leak. Air must be getting in somewhere. This has worked well for me...

 

Let me share this which may be relevant. I have a 73 - 454 and it did run 18-19" with two different Q-Jets, a Holley and an Edelbrock. Yep got them all laying around to play and test with. Anyway, installed a modified Q-Jet and a stock Q-Jet and the vacuum was 15" if you fudge it. No adjustments or clamping lines changed the reading. Timing was right on.

Installed the Edelbrock with slight mixture tweaks and pow 18-19" again. What is going on? Well it seems I have two different Q-Jet base gaskets. One has a tab extending into the primary underside that covers the vacuum runner ports and the other gasket does not have the tab covering the vacuum runners. Look at your gasket and the old ones lying around or in the stores etc. Bet you have two also. Seems my guess is the one with the tab is necessary to cover the runner lines (look at the bottom of the carb between the primary blades).

There has to be a reason the vacuum on my engine and yours both 454's dropped from 18" to 15" when changes were made. I am going to try both gaskets this week and see which one actually may be affecting the vacuum readings. Just bet one of them is creating an internal vacuum drain that is undetectible with common troubleshooting like line pinching or carb spray around the base gasket. Let us know if you see the same thing?? Thanks and hope this may shed some light on your problems. JP