Engine and Transmission Tech

Quote:

All in the time it takes to enjoy a good scotchy scotch.

: )

Thanks, Fred Great info!

I'll save that with my notes from Dan Jones.

Me too, Carl. I wish Fred would move to the sunny south so I could just take him my car when it's almost done.

NCtim

Agree, that is excellent info. Fred, as always, thank you for taking the time to write it up. Excellent info.

Lookin a Freds' latest pose, he's heard how much "fun" West Hollywood is ? Party-down, Fred. Cheers, roverman.

Thanks guy's, always happy to help out where I can.
The picture is new years in Jamaica.
Apparently I had a good time.
Cheers
Fred

Time for an update... Looks like I had two problems... 1) a vacuum leak between carb & intake. Didn't show up spraying carb cleaner but I was doing that at idle. The shop I took it to did it while revving it some & it was evident then. When the carb was pulled you could see where the leak was on the gasket. 2) I was running lean. I'd been checking the plugs after the car had been idling a while. They checked them after higher rpm's an plugs showed white. It still ran lean after the gasket was replaced but odds are the vacuum leak was making it worse.

It got a little interesting at that point. The shop I took it to generally only works on Holley carbs. Called around a bit and got same message from the couple other places that cater to non-stock, non-FI modern cars. Shop said they'd do the Edelbrock just wanted me to know it might take an extra hour or two. My gut said it wouldn't be hard to tune but I didn't have the time this week & was out of runway. Given that and the fact that I'd be in a similar situation if I needed help again, I decided to go ahead and swap my carb. My rationale was the extra $100 (Holley minus what I expect my Edelbrock will sell for) would be more than offset by shop rates - if not in this shot, most likely by the next.

With the carb swap & vacuum leak fixed, the car is running much better. Has noticeably more power & not feeling any miss.

FWIW, the Edelbrock/Carter carb is about as simple to work on, as they get. Glad it worked out. roverman.

That's what I've always heard too Art. But I figured I can learn the theory on either one and make some adjustments, but when I need help it should be nice to have the one the shops around here seem to prefer.

"With the carb swap & vacuum leak fixed, the car is running much better. Has noticeably more power & not feeling any miss."

Rob,

Glad you're happy now. I know that feeling all too well. Sometimes you kick yourself spending that $100 but when it works you run in the house and yell "Honey! Ya wanna go fer a ride?!!!"

That's a feeling you'll remember for a loooong time and I sure hope you shared that with someone. This thread is going into my archives.

Cheers,
NCtim

Rob, luckily I remembered that you were having this problem. I started having almost exactly the same symptoms on my 351 Ford in my Jaguar. Except for my engine being a 351, I have the same ignition, manifold and carburetor that you have.
My engine was running fine and THAN the stumbling started. I was simply convinced that my problem was electrical, so I started throwing money at the problem and started making all of the changes that you did and also had the same results that you had, which were none.
Finally in desperation I read your final fix for your problem and even though I still knew that my problem had to be electrical, I threw on and old carburetor and presto problem resolved.
When I finally rebuilt my Edelbrock carburetor, I found a small sliver of fuel line hose that was blocking the fuel flow to the left side of my carburetor and after the rebuild all was fine.
If it wasn't for your description of your problem, I am sure that I would still have my head stuck in the engine compartment looking for my NON-Existent electrical problem.
My only saving grace is that my wife was driving the Jaguar when my problem started, so at least I can BLAME HER.
Of course if I would have been smart enough to read your final fix, than I would have saved a lot of time and money.

Spent a few hours with a guy who has a few drag cars and a performance shop/carb tuning business today. Smaller air bleeds, smaller jets (primary & secondary), larger squirters on the secondaries, and a couple of small holes in the primary throttle plates and it's like night and day. Noticeably more power. No more bog on transition. Smoother in cruise. Way better throttle response through the range. And, idles smooth @800 rpm (before i was about 1000 to get a consistent idle). Starts easier too. To be fair, I was there a few hours but he did my carb in probably 45 mins between other customers.

Most impressive to me was he made all of the changes based on the car specs (engine, cam specs, car weight, gear, some info about rpm at a few speeds) and was friendly and chatted while working.

My be my imagination, but the car even sounds better.

That's great, Rob. Nothing like having one dialed in right. The holes in the primary butterflies is an old hotrodder trick to help a car with a big cam.

I see that Rick has a successfully modified double pumper (mechanical secondary) carburetor on his car. That is quite an accomplishment as these are the most difficult to tune of any carburetor. Reasons follow:

Street carburetors come jetted from the factory with the assumption that there will be a certain range of air flows through the carburetor. The air flow (velocity) creates a pressure drop that sucks fuel into the air stream and the calibration settings assure that, under steady state conditions, the air/fuel ratio will be optimal over the expected range of air velocities. To achieve these expected air velocities, the carburetor must be sized correctly for the engine. A street tuned Ford 302 needs about 475 CFM of air at maximum output, while a Chevy 350 needs about 540 - 550 CFM. Since the Chevy 350 is by far the most popular engine, there are many carburetors around 600 CFM that are perfectly calibrated for the Chevy 350. By contrast, there are very few 500 CFM carburetors for the Ford 302. A reasonable compromise is to use a 600 CFM carburetor on a 302 that has a vacuum operated secondary system. This means that the carburetor will open no farther than necessary and so acts as a 500 CFM carburetor on a small displacement engine, thus delivering optimized steady state air/fuel ratios. But there is a problem under acceleration. Opening the throttle quickly causes air flow to increase more rapidly than fuel flow since air is lighter than fuel. This results in a lean condition, and is compensated for by the accelerator pump which squirts raw fuel into the manifold. With a big engine, the higher air flow restores the steady state fuel flow faster (remember, the primary throttle bore is fixed, only the secondaries respond to the vacuum signal) than in a small engine which leads to the strange result that the small engine often needs more fuel delivery from the accelerator pump than the bigger engine. So accelerator pump tuning is generally required when using a large, vacuum secondary carburetor on a small engine. This means a faster delivery pump cam or pump linkage setting, generally combined with larger pump discharge jets. Note that the accelerator pump fuel delivery is by spring pressure, and the operating linkage only frees up the spring to do its job. With larger jets, the fuel flow is higher and the spring delivers more fuel in a shorter period of time. About the only way to mess up a vacuum secondary carburetor, other than radical jetting changes, is to decrease the spring tension on the secondary side so that the secondaries open too far and too fast. By contrast, a higher spring tension may restrict top end power slightly, but the effects are minor.

But with a double pumper the situation is not nearly so forgiving. In this design the secondary throttles are forced open mechanically, and if the carburetor is too big for the engine (a virtual certainty for a 302 as there are few if any double pumpers in the 500 cfm range) the air velocities are below the design criteria over much of the operating range. This means that many modifications must be carried out to get acceptable operation, such as increasing jet sizes to deliver enough fuel at low air flow velocities, larger/faster accelerator pump deliveries for both the primary and secondary pumps, and often smaller air bleeds (smaller air bleeds allow fuel to start flowing through the jets at lower air velocities). So modifying an oversize double pumper for street use is well beyond the capabilities of an amateur tuner. Rick was fortunate to find a professional who knew what he was doing.

The question becomes as to what double pumpers are good for. The answer is racing, especially drag racing. Because the secondaries are opened mechanically, they will open a fraction of a second sooner than vacuum operated secondaries. This can reduce quarter mile times by a tenth of a second or two, which often makes the difference between winning and losing a race. But, for street use, the effect will not be noticed.

Good writeup Larry, very informative as usual, and helps clear the fog for many of us amatuer carb tuners. Most instructions are no where near so clear on what the air bleeds do and the logic of the acc pump shot. (And then there is the power piston...)

As for not noticing the difference on the street, probably but it sort of varies. The vacuum secondary is calibrated for a certain pressure drop across the throttle and will maintain that drop. At about 1-1/4" of mercury which is about what 4bbl carbs are flow rated at that amounts to around 5% of the vacuum the engine can produce. It isn't much of a stretch to say that a 5% increase in output would be noticed, and a double pumper can at least theoretically decrease the pressure drop to near zero (fuel metering gets to be a problem, so efi has a potential advantage here). How much of that 5% you can actually get I don't know, but that is probably most of what accounts for the better drag strip performance. Street driving tends to suffer of course so we generally just pay for driveability with a performance loss. Yet another reason for efi, but all fuel injection systems are not created equal. Many will not flow as well as a 4bbl carb and intake.

Jim

More carburetor information (if anyone is interested).

Conventional carburetors have (at least on the primary side) three different metering circuits: idle, transition, and main. At idle, there is vacuum and fuel delivery only on the idle circuit; as the throttle is opened, the idle circuit drops out and the transition circuit comes in, further opening of the throttle shuts down the transition circuit and the main jet system takes over. Interestingly enough, these circuits do not deliver raw fuel to the carburetor air stream, but rather an emulsified mixture of fuel and air. This is due to the emulsion tubes which are between the fuel source and the discharge. All carburetors have at least two emulsion tubes, one for the main system, and one serving both the idle and transition circuits. As an alternative, some carburetors have three emulsion tubes where the transition system has its own tube. The emulsion tubes are built into the metering block. At the bottom is a replaceable jet (main circuit) or a fixed orifice (usually) for the idle and transition circuits. At the top is an air bleed, and in the middle are a series of very small holes (usually 3 to 5) open to the fuel in the float chamber. About ¾ of the way up, there is a passage from the emulsion tube to the discharge system (main, transition, or idle). When vacuum is drawn on the emulsion tube it draws in both gasoline and air, forming an emulsion that is then delivered to the carburetor throat. A larger air bleed means there is more air in the emulsion and also that more vacuum has to be applied to the emulsion tube to get gasoline out of it. A smaller air bleed obviously causes opposite effects.

The most critical circuit for drivability and fuel economy on a mild engine is the transition circuit. With a light car and a big engine, most cruising actually uses the transition circuit more than the main circuit, so main jet changes have a surprisingly little effect on highway fuel economy. But transition circuits are generally calibrated on the rich side so as to avoid drivability problems in “universal” applications. The only practical way to tune the transition circuit is by changing the air bleeds. On most carburetors this means drilling the bleeds larger, but once they are enlarged there is no going back. The solution is to buy a carburetor with replaceable air bleeds (more on this later). It is relatively easy to determine if the transition air bleed is correct on a two circuit carburetor. The clue is the idle mixture screw setting. The screws should normally be set between about one and 2 turns open. If the setting is less than a turn, the emulsion is too rich and a larger air bleed is needed. If the setting is more than about two turns open, then a smaller air bleed is needed. If air bleeds can be easily changed, it is best to try for one where the idle screws are open as far as possible. With a three circuit emulsion system, transition circuit air bleeds can be enlarged until there is a stumble on light acceleration, or a surge during cruising at low throttle openings. In any case, before tuning is attempted, it is important to be sure that the idle speed screw is not set to the point where the throttles are opened into the transition circuit during idle. If more air is needed for idle, a trick is to drill small holes in the throttle plates to deliver the extra air (but some carburetors also have auxiliary idle air bleeds that can be set to avoid the need for drilling). One problem with drilling throttle plates is that this also leans out the transition mixture and smaller air bleeds are needed to correct this (very difficult to accomplish on a carburetor without replaceable air bleeds).

Keep in mind that for street use, a properly sized carburetor will have properly calibrated emulsion tubes and reasonably close air bleeds. But air bleed tuning is worthwhile, as well as accelerator pump tuning. Generally, only large, expensive carburetors have replaceable air bleeds and these have no place on small, street tuned V8s. But there are three reasonably cheap (under $300) small (around 600 CFM or less) carburetors with replaceable air bleeds. One is the Summit brand, which is patterned after the Autolite 4100 carburetor which is, even now, considered one of the best, if not the best carburetor for small Ford V8s. The other two carburetors are the HR and Slayer series made by Quick Fuel. the Quick Fuel carburetors are especially interesting because they have other premium features not otherwise found in this price range such as three emulsion tubes, replaceable idle feed restrictors, etc. They also have a novel control of the vacuum operated secondary throttles. Normally, this is done by changing springs in the vacuum opening unit. A stiffer spring delays the opening point, preventing too much air at low engine speeds. But the stiffer spring also limits the ultimate amount that the secondaries open. What Quick Fuel does is control secondary opening by a needle screw in the vacuum signal line. This unit can thus use a very light spring, and control opening by delaying the buildup of vacuum through the restrictor screw. These vacuum units can also be purchased separately (about $40) and be bolted on to any vacuum secondary Holley carburetor.

So, if one wants to experiment with carburetor settings, I recommend buying a Quick Fuel carburetor because any changes can be easily reversed. The difference between the Slayer and HR series is only in the secondary metering functions. The HR has a full secondary metering block with three emulsion tubes and idle circuits. The Slayer series secondary has only a main jet circuit. As long as the cam is not too radical, all the necessary idle air can be supplied by the primary side of the carburetor and the Slayer series will be fine (at a significant cost savings).

A final note on power valves. Power valves are almost universally confined to the primary side of the carburetor. With a power valve, the primary jets are sized to give a lean mixture, which is fine for cruising, but not good for power. The power valve function is to richen the mixture when power demands are high by delivering extra fuel to the main system (so the power valves open when the manifold vacuum decreases). Power valves can be obtained that open between about 3 inches of Hg to about 12 inches of Hg. The usual setting for a street engine is 6.5 inches of Hg. This is right for most engines, but if the cam is radical and idle manifold vacuum is low, a power valve that opens at a higher vacuum is necessary so that it does not open too soon. Note that in power valve equipped carburetors the primary jets are always smaller than the secondary jets. With the power valve open, the primary side then delivers fuel equivalent to the secondary side. It is thus important to maintain the difference between the primary and secondary sides when changing jets, otherwise the carburetor mixtures will not be balanced. So, if the primary jets are changed by 2 jet sizes or more, it is best to also change the secondary jets by the same amount.