Concerning carbs, Vacuum or Airspeed...?

...I've come up with 2 scenarios:

1) I am using a square-bore carb on a spread-bore (Q-Jet) intake via an adapter. The adapter is technically the "wrong" type, because it's for an open, single-plane manifold. I should be using the dual-plane style adapter, which has 4 individual holes for the barrels/throttle plates.

This could somehow be causing a reduction of airspeed at the primaries, leading to a lean condition at lower engine speeds during part-throttle. At higher speeds, even with the same vacuum pressure, the airspeed increases to an acceptable level, as to allow the right amount of fuel to be drawn into the airstream, and making the engine run better.

Replacing the adapter with the proper type *may* fix the problem...

2) Nothing I do will compensate for the lowered airspeed (partially a function of the incorrect cam, and lower vacuum) at lower engine speeds, and I will need to switch to a real spread-bore carb (Q-Jet or otherwise) in order to increase airspeed at the primaries and allow the proper metering of fuel into the airstream.

.........

~jp

Jon,

At at sustained 70 you should have been showing a steady 9 - 13 inches or more of vacuum. You should have been solidly in cruise mode, but you were just barely there.

At cruise mode, the vacuum pulls the metering rod down so that the larger diameter rod is in the jet, at power setting the rod is up with lesser diameter rod in the jet. Cruise is lean, power is rich.

The carb size isn't the problem. I have run a 600 to 650 CFM carb on that size motor with no problems whatsoever, street or strip. Running at such low vacuum tells me that you are working your engine hard to maintain speed, whatever the cam. The spread bore to square bore adaptor shouldn't effect it that much either.

I have followed your troubles here on the group and I am baffled as to what your problems are, but the carb size is not it, of that I am sure. Possibly a defective carb, maybe, or the cam being indexed a tooth or so off, or some other such obscure problem. But not the carb size.

The position of the metering rod in the jet is merely a function of intake manifold vacuum, it has nothing to do with air flow / velocity.

Best of luck to you.

George

All understood and agreed. But that does not explain why the same EXACT level of vacuum is incredibly lean (stuttering, stumbling) at lower engine speeds, but just fine at 70mph.

I'm not debating carb size... The stock carb for the truck was a (bigger) 795cfm Quadrajet on a (smaller) 305ci engine.

I'm talking primary size...not overall CFM's.

Airspeed (not vacuum) does and will affect how well gas is pulled into the airstream. The way I understand it, gas is not pushed out of the open jets, rather it is pulled out by air movement. The fuel pump won't push it out, as the pump effectively will be out of the equation if the bowls are filled and the floats are in their full, upright positions (choking off fuel supply).

Is that not correct???

Vacuum WILL affect the position of the metering rods, but that will only serve to regulate how much fuel the airstream can PULL into itself.

If the volume of air and the time in which it is passed into the engine remain constant, then airspeed will be affected by the size of the primaries. Small primaries, faster airstream.

I have been researching this all morning, and I keep seeing mention of spread-bore (smaller primaries) being better for the street, not just for economy.

Not saying that a square-bore can't work well, not saying that at all...just that a spread-bore may be better in my situation. PARTICULARLY at lower, city speeds.

Again...not talking carb size...just primary size. Smaller primaries = faster airstream, which could indeed lead to better emulsion of fuel and air.

This could explain why the engine ran so much better on the interstate. The airstream was moving fast enough to draw the fuel out of the jets.

Never claimed otherwise...

Thanks :-)

~jp

Running at

Gas IS pushed out into the airstream by the difference in pressure from the venturi to the float bowl. It is NOT drawn out by vacuum. In a venturi carb airspeed and vacuum are directly related. It is air flowing through the venturi that creates the pressure drop, more air or higher airspeed creates a higher pressure drop. What are you using for an air cleaner? It may be causing a problem if it is disturbing the airflow across the carb vent or possibly the vent itself is blocked. The adapter shouldn't make much of a difference unless the bores are way off.

interstate.

velocity.

The air cleaner is this one:

I'm having a difficult time explaining this... I never studied physics :-(

I think we're saying the same thing, but I'm not communicating it well. Sure, the difference in pressure that causes the fuel to be pushed or drawn (doesn't matter which, point is the gas moves from point 'a' to 'b') is caused by the the airstream, not vacuum.

That's what I've been trying to say. NEVER said it was pulled out by vacuum.

I said "Airspeed (not vacuum) does and will affect how well gas is pulled into the airstream."

I did however confuse my statement by following it up with "The way I understand it, gas is not pushed out of the open jets, rather it is pulled out by air movement."

Air movement is still the cause, but I meant to say "gas is not pushed out of the open jets UNDER PRESSURE"....pressure meaning from the fuel pump as in fuel injection...sorry.

Regardless, the high velocity airstream is what causes the difference in pressure and hence draws the gas out of the float bowls, and through the jets..

Vacuum without air movement is obviously possible (vacuum chamber for instance) but without the movement of the air, the gas is going nowhere.

Again... I'll state what I did at the beginning of this thread:

At 6" Hg the truck runs like crap below interstate (55mph) speeds. The MANIFOLD vacuum reading is ONLY relevant when discussing the "mode" that the carb is in--in this case, "cruise mode". We're not measuring how much air the engine is actually pumping, just MANIFOLD VACUUM.

At 70mph+, while maintaining the same MANIFOLD vacuum the engine suddenly ran good. The carb was still in "cruise mode", just moving a whole lot more air through it. It felt as if it had power to spare.

The only differences I can imagine here are increased airspeed, directly related to engine speed. (engine turns faster, pumps more air)... and that possibly the increase in airspeed is allow more fuel to be drawn into the engine, richening it to the point of running normally.

I don't think I'm way off base here...I think I am actually close to something. But unfortunately I'm having a hard time explaning myself today...

Does that make more sense?

~jp

Steve W. wrote:

Hi Jon,

...

You are doing OK.

The vacuum is caused by airflow through a restriction, in this case a venturi. But I think you are well aware of that. As you state, it is the airflow that caused the vacuum, but only when we talk about "venturi vacuum" not manifold vacuum. Venturi vacuum and manifold vacuum are direct opposites. When intake vacuum is high, venturi vacuum is low, and vice versa. It is a matter of flow, versus static pressure.

Yes, in that airspeed (velocity of air through the venturi) is the basis for venturi vacuum. You are absolutely right so long as you are speaking of venturi vacuum. Manifold vacuum is determined mainly by engine rpm, and throttle plate position. This also determines venturi vacuum, but they are inversely proportional.

This argument has been going on since mankind was able to walk, does atmospheric pressure push when pressure on the back side of a restriction is reduced (vacuum)?, actually I think it sucks!:)

Again, you are absolutely right.

It has been too many years since I had my hands on one of these carbs, but I think there is a tap for venturi vacuum. It would be very enlightening to see what is happening to venturi vacuum at highway speeds. Venturi vacuum would be a better indicator of relative airspeed through the carb.

You are doing fine by me. Somehow I think when you finally get to the bottom of this, you will find the problem is relatively simple, bad carb, misindexed cam, or some such thing.

The smaller primaries used in the quadrajet were for around town throttle response as much as economy, if not more so. The Quadrajet originally was only used on GM big bore engines. When it was more refined it was later used on a broad platform of engines, even the Pontiac overhead cam 6. However I once used a 500 CFM two barrel Holley on a street machine. Those were really big bores and the off idle response was really good.

I don't think you are off base on carb size, even primary bore size, for a street machine. I think a combination of things could be contributing to this as well, as I have already stated.

Could you borrow a similar sized carb from a friend (Edelbrock, Holley, or anything else that will bolt to your manifold and give it a try, to verify if the engine itself is sound? That would rule out or confirm cam problems, distributor problems, or anything else. Just a thought.

The biggest factor you have stated is the low vacuum, and when you have cleared that up I think the other problems will disappear.

Don't give up, you are getting a good education through all of this.

George

Thanks!

Gotcha... I wonder how close to ported vacuum the venturi vacuum is...? Wouldn't be a problem to hook up the vacuum gauge to a ported source via a "Y" in the line.

Glad I'm on the right page.

Ha! Right now the whole thing just sucks!

I'll look closely through the cutaway diagrams in the manual. I'd be very interested to see this myself.

Probably so...

Well, since the manifold vacuum and venturi vacuum are inversely proportional, would it not make sense that low manifold vacuum at idle would translate to low venturi vacuum at cruise speeds?

They are inversely proportional, however they're both affected by the same thing--the engine's ability to suck (although as stated earlier, right now my engine *really* sucks!).

Agreed.

I have 2 Quadrajets. One is the original carb to the truck, which came on the 305. It didn't run great on the 350 though. The mechanic said it needed to be rebuilt.

Then I have one someone donated to me. It's of unknown origin. It has a hot-air choke, as opposed to an electric choke. I don't know the story on that one.

I agree... I just wonder how much of it goes back to the cam.

Believe me, I won't. I am rather enjoying the education I'm getting. I've learned a LOT in the last several months. But as much fun as the learning part is, right now I'm ready for the fruits of my labor.

Hopefully soon...

Thanks,

~jp

Jon,

...much stuff...

Actually, low manifold vacuum at idle speed relates to such as, idle speed, timing (ignition), timing (camshaft), camshaft lobe overlap, and throttle plate opening (angle) necessary to achieve idle speed. Venturi vacuum at cruise speed is related directly to the amount of throttle plate opening (angle) required to maintain speed under load, and most else is quite minor.

The more you open the throttle plates, the lower the manifold vacuum will be (less restriction to the airflow), but this allows more airflow through the venturi which translates to higher venturi vacuum. Bournelli is your friend here.

Right on that one, though that is what we would like to see fixed, right??

...

An hot air choke carb could be used if you put it on a warm engine, or if you are willing to putz with it until you build some head heat, the with the choke circuit out of the system (rotate the cover till the choke plate is wide open and tighten the cover screws there) it should run quite well, assuming it is a good carb. A carb shouldn't be as touchy as what you have been experiencing, so don't be afraid to experiment a bit, just keep the fire extinguisher handy :)

One other factor, just remember that patience is a virtue, though sometime it is difficult to come by.

And yet another factor, at some time you may want to refer to some of my earlier posts and check the degreeing of your camshaft. That would get one more variable out of the equation but it requires minor disassembly of the engine to gain access to the front of the crank and the lifters of one cylinder, something that shouldn't be done frivilously. But having said that, I have come to the point that I never trust degree marks, alignment marks, and the such on cams, and sprockets. Been fooled too many times. And I like to know exactly what is going on. It helps me sleep better at night.

Wrenching can be fun, even enlightening, provided you don't have to do it to excess. And the occasional reward of actual performance helps the old attitude too.

Hope you have a good weekend Jon, I am going to sign off here till some time late Sunday afternoon, all the best to you.

George Vigneron

I'm no cam expert but I wonder if it is just your cam behaving the way its supposed to. You posted the engine before and it did have some kind of high rpm cam right? And for the cam experts that might be reading this....Is there a better cam that stock that is good for LOW rpm like around town driving, cruising down the highway?

Gotcha...

Well...I may sell off both Quads and buy a good new one with the electric choke (my preference).

I will at some point. I need to read up on the process first. I also want to know what all's involving in swapping a cam in an otherwise fully assembled (and installed) engine. I may as well, because it is in the cards at some point. But I'm going to get quotes around town first.

Yeah... I made some adjustments Friday night. Installed the new compatible temp gauge sender which was a welcome relief. Past experience has me spooked enough to where I NEVER drive without a temperature gauge. I did it while the engine was warm, but not hot. I had to...I had places to be and people to see on Friday night. Cleaning up the antifreeze was less than fun though ;-)

Before heading out on Friday, I brought the idle mix up to factory recommended specs and reset time to about 15-16=B0 BTDC at idle. I may back the idle mix off a little.

With those changes, you can once again smell crazy gas fumes at idle and off-idle. The advanced timing helps overall. Cruise mode was more consistent, but still needs to be richened up a bit. I can maintain cruise speed *fairly* well while keeping manifold vacuum above 5" Hg, and thus keeping the carb in Cruise mode and out of Power mode. On flat grades, I can get vacuum up in the 12" range while cruising and it won't stumble.

The only reason the manifold vacuum is relevant here is 1) it lets me know what "mode" of operation the carb is in, and 2) it's a good indicator of how much "foot" I'm having to use in order to maintain speed.

So right now, it's less foot, and that's a good thing.

I'm still getting strong gas fumes, but mainly on slower, back-road driving.

I gave myself a much needed break from wrenching and reality after Friday evening. Saw a friend's band on Friday night, knocked off 200 rounds of .357 Magnum through my new Ruger on Saturday, and did work around the house on Sunday. That was the first relatively calm weekend I've had since moving and to say that it did me good, would be a huge understatement. Hopefully this week I can get back to tweaking the truck with a newly clear head.

I'll keep you posted.

~jp

Yeah, agreed... I needed a weekend off from screwing with it. I got it as close as I could, then called it "good" and proceeded to decompress over the weekend.

In order to check all that, won't I have to pull off the water pump and all the accessory belts and timing cover?

Can the cam be swapped without removing the heads? I know the lifters, pushrods, and rockers have to come out and be replaced and adjusted, but I'm hoping that if I must do it, I can avoid pulling the heads completely. I'm still gonna call around and get quotes.

~jp

Shep wrote:

I'm betting that it is. The dude that installed the engine was an old-timer that personally owns and operates his shop. I trust him and he came recommended from others.

I think he probably would've noticed something like that.

The engine developes peak HP at 5100rpm, and peak torque at 3750rpm. I don't know if that's what you'd consider a high-rpm cam, but I'd say that it is, simply based on the vehicle that it's in. 2000rpm is where I keep it while doing 75mph down the interstate in OD, and hitting

3000rpm is something that almost never happens.

I think the stock cam for the 260hp motor would be a better choice. The two engines are otherwise identical. The 260hp's peak HP comes in a little lower and the cam has less lift and duration.

When the time comes to actually replace the cam, I'll be hitting up one of the "majors", like Comp or Crane, and hopefully speaking to a rep over the phone to get something that'll give good driveability and is matched for my engine, truck, axle ratio, and driving habits.

Like I mentioned previously, I'm not looking to build a stump-puller or off-road monster. Just a good street truck with good low-end power in reserve to pull a trailer every now and then for camping.

~jp

ShoeSalesman wrote:

Update...

I don't know why I didn't think of this before. I Googled the entire engine...specifically "12499529", which is the GM Part #. There's threads galore on forums about low vacuum and crappy low-end performance, especially in trucks and general off-road 4x4 applications.

I did gather more info on the cam. It's the old Corvette L-46/L-82 cam, GM part # 3896962.

Per gmpartsdirect.com:

"This hydraulic flat tappet is used on the 68-81 L-46 and L-82 Corvette. The duration at lash point in degrees (intake/exhaust) is

312/312; duration at .050" tappet lift (intake/exhaust) is 222/222 and maximum lift with 1.5:1 rocker ratio (intake/exhaust) is 450/460. Valve lash is zero/zero and lobe centerline is 114 degrees."

Doesn't say anything about overlap, but I assume that can be determined by the above info (???)....well, I found something that indicates overlap is around 61=B0, but that's not a concrete source.

Per another forum: "This is essentially a L-79 cam with three degrees more duration and retarded 4 degrees. The "151" will make a little better low end torque, and the the "962" will make a little more top end power and is a good choice for a 350. If you want a little more low end torque, you can advance it four degrees to the same centerlines as the 151 and still get the benefit of the slightly more duration."

My cam is the one being referred to as the "962".

Would advancing it 4=B0 be worth trying before throwing money down on a new cam? It seems like a lot less work than replacing the whole cam and might just yield good results.

Opinions?

~jp

....

Jon,

My experience is that a cam should be advanced or retarded 8 degrees or more to show a measureable difference in performance (been there, done that). Having said that, if I were you, at this point in time, and knowing this present history of the engine, I would advance it about 12 degrees, then check for piston / valve interference, and if it shows about .110 clearance on the intake valve and at least .080 on the exhaust I would go for it.

It is easy to change a cam, but it entails removing the distributor, the valve covers, intake manifold, all push rods and lifters, the water pump, and the front cover. Also doing it in the car / truck you must have enough room to get the cam out of the block, which on some application will require removing the radiator as well, it varies by the application.

Mr. Gasket (for one) makes offset bushings for the cam pin, and also offset crank keys (think Mr. Gasket does, maybe not, but others do). Combining the two you can easily change the cam degreeing as much as you want to, within reason.

More later,

George Vigneron

Ok...quick question...What's the best way to measure the piston/valve clearance?

That's pretty much what I figured when I envisioned the process. I know removing the radiator, grill, and fan will probably be necessary if I decide to change the cam.

I've seen these offset keys in a few performance catalogs. I knew what they were for, but didn't consider that I may need one until now.

As always, thanks for the good info George.

~jp

Jon,

......

Well, if I have the heads off, I tape a fat piece of solder on the top of the piston where the valve will contact it and then replace the head, without gasket but with valves and valve train intact. Then I turn the engine over by hand through one full cycle, two revolutions. Then remove the head and measure the "squeeze" of the solder with a vernier calipers. Modeling clay can be used as well, just messier to clean off the piston.

However, if you have the engine assembled, as you do, you can merely add a spacer under the rocker arm, above the valve stem, thick enough to remove the valve lash, plus the needed clearance. If you have hydraulic lifters, you should change to a solid lifter just long enough to make the measurements. You don't want the hydraulic lifter to pump up and change effective length once running, and cause some unnecessary "noise" or "contact." Then turn the engine over by hand one cycle, two revolutions. If no contact takes place, you are OK to go. If contact takes place you will feel it and of course you will stop and not force the piston into the valve, or vice versa. That way, the clearance is established, and the valve lash is an added bonus of clearance.

Do this for both intake and exhaust. Note, greater clearance for the intake valve because the piston is approaching the valve as the valve opens, less clearance is needed for the exhaust valve because the piston is moving away from the valve when the valve is opening.

There are probably neater, easier, cleaner ways of doing it, but this way is very simple, effective, and never fails. My kind of testing.

.......

Actually, removing the grill makes it much easier. Good point, I should have mentioned it.

If you have any machinist experience, and have a well equipped home shop (drill press, hack saw, file, and micrometer or vernier caliper as minimum requirements), you can make your own offset bushings, to your exact specifications. And it doesn't take all that much equipment to do it. More equipment means easier, not necessarily better.

My pleasure, it is making me shake some cobwebs out, it has been some years since I built a race engine. And I built mostly Fords anyway, just because, so don't ask.

I play with my GM diesel nowadays, a pleasant change, and a new learning curve.

George Vigneron