[Navy Horn 16]

I've been chasing my tail for about a year trying to figure out a problem with my fuel/timing. This issue only manifested itself at WOT, usually at the track or when running wide open like I was making a quarter mile pass.

I first noticed it at the big end of my better passes. I'd run 12.12 @112, and while that was pretty good, I knew it had more in it as I was breaking up at the top of 2nd gear and top of 3rd.

I had a "race prepped" Q-Jet that was done by Quadrijet Performance LLC here in Texas. When I put this Q-Jet on in place of my Jet Industries Stage III, I noticed immediate improved throttle response, and it was just an all around better carb.

The issue, as it turns out, is that the Pontiac Carb that I had done here was a 750, the Chevy version done by JET is an 850.

The problem manifested itself in 2 ways:
If I had vacuum advance hooked up, it would sound like a misfire right at the 2/3 shift and the top of 3rd.

If vacuum advance wasn't hooked up, it would stumble (but not sound like a misfire) at the top of 1st, and 2nd (I couldn't get to the top of 3rd on the roads around my house).

So, after changing plugs, timing, adding a center baffle to the carb, experimenting with race gas and different fuel, I finally put the Chevy carb back on just to eliminate the Pontiac carb as the culprit. Intermediate throttle response went back to not being as good, but the Wide Open throttle issues were gone.

So, my theory is that the 750 just wasn't enough carb for the motor. Even though I have a wideband on it and the AFR was right, the volume just wasn't enough. My best guess on the misfire is that at the top of the gears under load, it was pulling enough of a vacuum to draw advance, and pull timing well past the 36 total that I have dialed in.

The end result today was a lot of smiles, and the car hauling ass and running 11.61 at 116.07. I had 2 other passes at 11.62 and 11.64, so the car was remarkably consistent (maybe time to take up bracket racing). I improved my previous best by over half a second. (7.40@94 in the 8th was pretty good as well) This is in a car with all 4 seats that I drive to the racetrack and use on sunny days to go to HEB and get groceries.

I would have been happy with an 11.99... I'm ecstatic with this. The only problem is that I'm pretty sure that I've maxed out this setup and the only way to go faster is with nitrous or turbos, and I'm not doing that.

So, if any of you are fighting a similar gremlin, try a little more CFM with your carb and maybe you can kick it.

[Half-Inch Stud]

Fantastic. Great performance for the combo. Probably another 0.1 in the 60foot .time to update the rwHP in Sig.. So you posting the slow Q-JET for sale?




nobody knows what a HEB is.

[kyle_blake]

Did you say vaccum advance was adding timing during WOT? I don't think that can happen under ported or manifold source? I always thought it zero's out advance under WOT?

[242177P]

Originally Posted by kyle_blake

Did you say vaccum advance was adding timing during WOT? I don't think that can happen under ported or manifold source? I always thought it zero's out advance under WOT?

A restrictive carb creates a weak vacuum at WOT. I believe that's the reason they rate two barrels at 3.0 in-Hg instead of 1.5?

[steve25]

Great news to hear!

So good that you tufted it out and stuck with it!

[Cliff R]

Sorry to rain on the parade but CFM was NOT the issue. What you have determined is that there is some sort of problem with the smaller carb.

It will be something fundamental with the 750 carb not allowing it to stay full on hard runs, leaning out, venting issue, etc.

I've back to back tested 750cfm Pontiac Ram Air carbs, 1971 Pontiac 455 HO carbs, and my later 1977 (850cfm) Pontiac carb at the track (and on the dyno) many times over the years and the difference between the 750 and my 850 was about half a tenth and 1.5 MPH. The HO and 1977 carb ran within .02-.03" hundreds of each other and almost the same MPH on my own engine deep into the 11's.

On one private track rental we actually took 8 carbs with us and ran them back to back to back on two different vehicles all day long. Aside from the Thunder Series AVS running a little slower by about 2 tenths on both vehicles all the other carbs, Q-jets and Holley's ran about the same in ET and MPH on both vehicles. Included in the test carbs were several 750cfm Q-jets, a 4781-2 850 Holley, and a custom built big CFM Holley. It was interesting that my own 1977 Pontiac Q-jet ran the quickest ET on both vehicles (telling us how well it is tuned from many hundreds of dyno sessions and drag strip runs). The big Holley ran the highest MPH on the other car we took with us but just a touch slower in ET than my Q-jet.

Also consider we have those smaller "750" cfm Q-jets running into the 9's on Super Stockers making a LOT more power.

We also have the smaller ones prepared for Pure Stock applications running low 11's, and FAST Class cars dipping into the 9's.

As far as the smaller CFM adding enough vacuum to apply the advance, unlikely. Put a vacuum gauge on it, we saw right at 1" vacuum on the dyno with the 750cfm carb on a big CID Pontiac build around 500hp. The vacuum can should take at least 5-6" vacuum to start adding any timing at all.

Still good news you were able to find over 1/2 a second improvement, but if you figure out what's going on with the smaller carb it will run nearly as quick. This simply happens because most of the ET is made on the launch and the engine only spends a very small amount of time right up at the shift point where CFM would have a big impact on the end result.

To back up that statement, on a dyno a 750 carb would make equal power as the 850 to at least 4500rpm's and then only give up maybe 5-10hp clear up at the shift point IF the engine needed more CFM......FWIW.....Cliff

[Skip Fix]

So how is Alamo City since it has new owners? Heard they did some improvements? Been thinking of going out there. Longer but easier drive than going to Baytown form Katy. When I was there in the past test and Tunes had real variable track prep,one thing is fairly consistent at Baytown.

HEB is the big Texas grocery chain.

[ponjohn]

Navy - strong combo. I think there is more there, I believe you have some benefit in improving the 60 ft.


What head flow and cam?

[Cliff R]

Agreed and there is still more in it. 1.66 60' and 7.40 in the 1/8th should come up with another tenth in the 1/4 mile and I'll bet you'll find 1-2 more MPH on top end once it's all sorted out......IMHO......Cliff

[Navy Horn 16]

Originally Posted by Skip Fix

So how is Alamo City since it has new owners? Heard they did some improvements? Been thinking of going out there. Longer but easier drive than going to Baytown form Katy. When I was there in the past test and Tunes had real variable track prep,one thing is fairly consistent at Baytown.

HEB is the big Texas grocery chain.

It is definitely better than it was under the previous ownership. If you want to come out for a Test & Tune, look for their dates on Saturday and "Sunday Funday".. The lines are a lot shorter (no line at all yesterday) than on Fridays.. I've noticed that there are a lot more breakages on "Import Nights", which are also Fridays. Most Fridays the line can be an hour or longer once it gets past 8:00pm. I've never seen a line more than a couple cars deep on Saturday or Sunday.

I feel bad for people that don't know what HEB is. My friends from Memphis come down and are like "Dang, all we got is crappy ole Kroger. This place is AMAZING!"

[Navy Horn 16]

Originally Posted by Cliff R

Agreed and there is still more in it. 1.66 60' and 7.40 in the 1/8th should come up with another tenth in the 1/4 mile and I'll bet you'll find 1-2 more MPH on top end once it's all sorted out......IMHO......Cliff

I don't feel like making a rear-end change chasing tenths in the 8th mile. 3.42s are great on the highway, especially with the overdrive.

So, what do you think could cause a 750 carb to act up the way mine was. The AFR was dead nuts perfect all through the RPM range, even while it was breaking up.

[Cliff R]

Not sure but the first thing to check is the size of the fuel inlet seat. To run as quick as you are I'd use at least a .145" seat and 7 to 7.5 psi fuel pressure. No mention of what fuel delivery system you are using but it can be a big player with these things......Cliff

[steve25]

Here's something else to concider about Carb size and numbers in general that Cliff touched on.

All 4 bbl Carbs are rated at 1.5" Hg ( Mercury), if you have a vacuum gauge hooked up to your motor and you see that at your shift points the gauge is only reading 1" then your current size Carb is being underutilized to some extent.

1.5" of Hg is equal to a test pressure of 20.5 inches of water on a flow bench and the industry standard to test at these days is 28" ( although I have switched to using David Vizards floating pressure drop method) but lets go back to the 1.5"Hg / 20.5" relationship for a minute.
If a motor is pulling 1.5"Hg then each cylinder is only pulling at .187 Hg , or 2.56" of water.

This clearly shows that air is not really that much being sucked into a motor, but what is taking place is that the recedding piston on the Intake stroke creates a void that air at 14.7psi ( sea level standard pressure & temp ) rushes in to fill!

Now lets go back to the flow bench, if you had a head that was flowing a peak of let's say 200 cfm@ a 28" test pressure then that same head tested at a 2.56" test pressure would only flows some 65 cfm.

It's a generally accepted rule that if at peak rev's your motor is not pulling more then 1" Hg then your not cheating yourself out of much more then 5 hp if any at all!

["QUICK-SILVER"]

The with and without vacuum advance deal, points to secondary throttle blades not opening all the way. 90° needs checked on the bench and more important is WOT 90° once the carburetor is installed and opened up with the loud pedal. Besides vac at WOT it makes it harder on the front cylinders to inhale with "the big" fuel/air mix aimed to the rear.

Clay

[David Ray]

"Did you say vaccum advance was adding timing during WOT? I don't think that can happen under ported or manifold source? I always thought it zero's out advance under WOT?"

Well....that IS the main difference between full manifold, and ported vacuum sourcing.

Full manifold is placed below the throttle plates, and at WOT, has virtually NO vacuum to work with to operate a vacuum advance.

Ported vacuum occurs at the venturii area, far above the throttle plates, where the throttle bore reduces, which creates a vacuum suction area that will create vacuum at WOT, thereby operating a vacuum advance, adding timing at WOT.

Adding vacuum advance timing at WOT is redundant, and counter productive, UNLESS the engine is equipped with a fully functioning emissions device known as an EGR valve.

An engine does not require two separate acceleration timing curves, it should only need ONE curve, the mechanical curve for acceleration.

the vacuum advance should be called what Ford correctly identified it as, way back in the 1930's, a "Load Compensator" with their "Load-A-Matic" distributors. In essence, a vacuum advance should add timing when load is nil, to light, NOT at acceleration and should not be a second acceleration advance curve added.

If the non-EGR engine actually responds frm ported vacuum advance, FIX THE MECHANICAL ADVANCE, it isn't working as it should, and put the correctly adjusted vacuum advance back on full manifold vacuum sourcing.

[Kenth]

"Ported vacuum occurs at the venturii area, far above the throttle plates, where the throttle bore reduces, which creates a vacuum suction area that will create vacuum at WOT, thereby operating a vacuum advance, adding timing at WOT."
This is a joke, right?

Here is what GM wrote about vacuum advance (and i believe they got it right):

VACUUM SPARK ADVANCE

Along with the mechanical spark advance, another method is used to advance ignition spark under load. This is commonly called the vacuum operated spark advance unit. The vacuum advance unit supplements the mechanical advance, however, it operates differently as it varies spark timing in relation to engine load. Engine load can best be determined by manifold vacuum so consequently, the vacuum advance unit mounted on the distributor is connected directly to engine manifold vacuum.
Although spark timing is not a direct function of the carburetor, it is used on some applications to control the vacuum operated advance unit on the distributor during the idle and off-idle range. On these units a calibrated port is located in the throttle bore just above the throttle valves. It is connected by a vacuum line directly to the distributor vacuum advance unit.
In the curb idle position the throttle valve is below the spark port so consequently, no vacuum is applied to the advance unit and the spark advance remains in the retarded position. As the throttle valve is opened and engine speed increases the throttle valve moves past the spark port so that manifold vacuum below the throttle valve is applied directly to the vacuum advance unit. Timed spark advance is used on applications where engine design demands retarded spark for smooth idle and performance improvements at low speeds.
On other units the vacuum advance line may be connected directly to the manifold or carburetor bore below the throttle valve. In this case the engine will idle with full spark advance. The full advance spark at idle is used on engine applications where it is not detrimental to engine idle, and improves engine cooling. When setting ignition timing, the vacuum advance line should always be removed, especially on engines using full spark advance at idle as initial ignition timing will be set retarded if this is not done. The end result will be very poor fuel economy. To be sure, always disconnect the vacuum advance line to the manifold or carburetor when setting initial engine spark timing.

[Cliff R]

"Ported vacuum occurs at the venturii area, far above the throttle plates, where the throttle bore reduces, which creates a vacuum suction area that will create vacuum at WOT, thereby operating a vacuum advance, adding timing at WOT."

Incorrect on the carb in question here. I'm not even sure that actually occurs on any type of carburetor I've worked on as the correct source location to supply ported vacuum to the VA is typically just above the throttle plates relative position at idle speed.

Specifically for the Q-jet the ported source for the Vacuum Advance on his Q-jet is only a few thousands of an inch above the manifold source as Rochester very precisely located the supply to end up just above the throttle plates position at idle. They did this so FULL MANIFOLD VACUUM is supplied to the distributor right off idle. I've been on the dyno many times with the Q-jet that I use on my own engine as it's our "dyno mule" and believe me it shows the exact same reading (pretty close to zero) on a full pull as the manifold vacuum source does on the same carburetor.

I also drag race the car just like I drive it on the street with the VA hooked up to ported vacuum and it adds NOTHING on a full WOT run.......Cliff

[David Ray]

I was a GM ignition system design engineer, none of you were.

Now, in your most eloquent dissertations, outline just why a vacuum gauge shows vacuum below the throttle plates ehen they are closed, none when they are at WOT, and then, add to that why a PORTED port has significant vacuum to operate a vacuum advance, at the SAME WOT, and no vacuum at idle.

It had better be right on, because you guys aren't as it sits right now.

Also, someone please read the vacuum on both a full manifold vacuum port AND a ported port at both idle, and as high an rpm as you would consider safe (NOT WOT, please). Or, read it with the car in operation, so the engine doesn't expire.

Physics might work just fine for you guys when it is wrong, but it doesn't work at all when it is dead wrong, (that would be your way).

Also, please explain, in detail, why a carburetor has a venturii, and why one is required to actually pull fuel into the engine if there isn't any vacuum anywhere in the carb at WOT. Basic engineering and physics, please outline for everybody.

Key word here is: SUCTION (which is actually....ahem, VACUUM)

Also, please fully outline just how fuels in the float bowls are inducted into the venturii, if there is NO VACUUM in the carb at WOT.

Then explain why we don't need a venturii suction (vacuum) with electronic fuel injection.

Believe me, I DO KNOW THE CORRECT ANSWERS.

[GTOLou]

Originally Posted by Cliff R

Not sure but the first thing to check is the size of the fuel inlet seat. To run as quick as you are I'd use at least a .145" seat and 7 to 7.5 psi fuel pressure. No mention of what fuel delivery system you are using but it can be a big player with these things......Cliff

Fuel delivery...OP symptoms sound just like a lean misfire from inadequate fuel delivery.

[Kenth]

"Also, please explain, in detail, why a carburetor has a venturii, and why one is required to actually pull fuel into the engine if there isn't any vacuum anywhere in the carb at WOT. Basic engineering and physics, please outline for everybody.

Key word here is: SUCTION (which is actually....ahem, VACUUM)

Also, please fully outline just how fuels in the float bowls are inducted into the venturii, if there is NO VACUUM in the carb at WOT"

You need to learn about atmospheric pressure!
The engine is an air pump - it ingests air and compresses it. It´s often assumed the reason air enters the cylinders is because the piston sucks it in - it creates vacuum and pulls in the air. But, atmospheric pressure - about 15 psi - will push air (and fuel) into a void, provided its passage is non-restrictive.
This is the basics written once by Doug Roe, a real engineer.
His book Rochester Carburetors may open your eyes in more than one regard.
The correlation between carburetor and ignition to make efficient combustion for one...….