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NEEDLE ORIFICE R E T A I N E R SCREW

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BODY A L L E N BOLTS

HOSE MOUNT BACK

By Rex Taylor, President (EAA 87893) MAPI Engines, Inc. Eloy Municipal Airport

Rt. l.Box 1000 Eloy, AZ 85231

J. HE POSA FUEL injector carburetor has become almost the standard carburetor on Volkswagen conversions in the past few years. Many Posa users swear by this simple little fuel metering device and have absolutely no problems with it. There are others who have had nothing but trouble from their Posas and if they haven't thrown them away, spend a great deal of time in a state of frustration trying to figure out how to make their engines start and run right. The root of the problem lies in the ancestry of Posa. It was never designed for aircraft use in the first place,

and has never had a set of installation instructions telling our poor frustrated builder how to use a carburetor designed for motorcycles on his flying machine. He takes his new Posa out of the box and opens the instructions to Step #1 which tells him to remove the gas tank. That makes sense on a motorcycle, but it blows the homebuilder's mind! The rest of the instructions are just about as ludicrous for aircraft use as Step #1.

HAPI Engines, Inc. has used Posas for over 4 years now on our engines with very good results. We learned their little tricks and idiosyncrasies in our early testing and now find them very easy to adjust and "fine tune", giving consistently easy starting, plus good mixtures in all throttle settings. We had become the largest user of Posas over the past 2 years since we placed the "SuperCarb" Mixture Control Carburetors (a much modified Posa) on the market. Recently, we were offered the patent, tooling, manufacturing rights and inventory of Posa and decided to purchase it in order to assure a carburetor supply for our own HAPI engines. We also knew that by owning Posa we could then add several design improvements to much better suit it to aircraft use. In the next six months at least 2 of these new features will be added to the standard "off the shelf Posa that will greatly improve the versatility and ease of installation. The major problem with Posas has been lack of aircraft installation information, so this article is a first SPORT AVIATION 59

Photo 1 — Posa carburetors and "Su-

perCarbs" are now being built in MAPI's new plant in Eloy, Arizona.

step to modify that situation that may help some of you who have Posas and maybe are not getting the kind

3. When you readjust the mixture needle anywhere

in the speed range, it affects the mixture at all speeds.

check would cost $500 or more, with a carburetor cost-

If you are thoroughly confused by now, you are about average where Posas are concerned, so let's start at the beginning and shed a little light of knowledge on this mysterious little device that is so simple, yet so complicated. The air entering the engine's induction system is regulated by the position of the slide in relationship to the hole through the main body. As the slide moves, the metering needle is also moved within the needle

numbers we homebuilders can relate to. 2. Posas have only 2 moving parts, the airslide and

the engine (in a low pressure area as the air passes over the needle) does the actual metering. Photo #3

3. Unlike many other carburetors, fuel metering

the needle and the orifice is critical and does wear with

of performance they are capable of delivering.

First, let's go over some of the assets of the Posa: 1. Posa will run in any position, updraft, downdraft or sidedraft (the latter being by far the most common), provided they are supplied with fuel either by gravity

feed or pumped and regulated to % lbs. PSI at the carburetor. This feature has been very appealing, you can replace a defective PSS that a rebuild and flow bench ing retail under $60 and get good results! Those are

orifice. A tapered flat on the side of the needle facing

the metering needle that moves with it.

shows the orifice which meters the fuel. The fit between

is infinitely variable and is automatically changed in direct relationship to air metering any time the slide

is moved. This means that once the right needle is

selected for the engine, and properly adjusted, fuel air ratio is kept at close to ideal proportions throughout the full speed range. 4. Posas are not as susceptible to carburetor ice as a venturi type carburetor, but given the proper atmospheric conditions we have found that ice Can and some-

times does build in the intake manifold behind the carburetor causing an engine shutdown just as surely as if

it had built up in the carburetor. The remedy is still the same, you must have carburetor heat available, even with a Posa, just for safety's sake. 5. Posas are available in the small frame carburetor in 17, 20, 23, 26 and 29mm throat sizes. The large body carburetors are supplied in 26, 29, 32, 35 and 37mm throat sizes.

Now let's talk about some of the liabilities we have

to learn to live with or overcome by designing a good fuel system that overcomes the carburetors deficiencies. 1. Posa does not have a float bowl or other metering devices to regulate head pressure of the fuel and stabilize fuel pressure. 2. There are no separate idling and "high speed" jets that can be adjusted separately. One needle does it all and if the proper needle isn't selected or the person doing the adjusting doesn't fully understand how it works, the fuel metering system will drive you nuts. Once understood though, it's very simple. 60 MARCH 1982

use so that over a long period the engine may tend to run progressively richer, particularly at low speeds, due to excessive clearance between the needle and orifice.

Photo 2 — Needles from #1 to #5 with the set gauges and wrenches for installing them come with each Posa. Needle selection and setting is explained in text.

We might as well learn the starting procedure at this point, since it seems to be another stumbling block in the use of Posas. It goes like this: 1. Turn on the main fuel valve with the throttle open until fuel runs from overboard drain. Then close the valve. 2. Close throttle with magneto "off" and pull prop through compression once for each cylinder, 4 times for a 4 cylinder engine to charge cylinders with fuel. 3. Crack throttle to approximately '/< open, turn

mags "on" and prop to start, opening the fuel valve as soon as the engine starts. The engine will run for a few seconds on the prime in the carburetor, so you don't have to be too quick when doing this. With the above procedure, you will find starting easy and never have excess fuel running on the ground, unless you have your Posa installed in the updraft position in which case it cannot retain the prime as it's supposed to and the fuel will indeed run onto the ground while starting. With electric start the procedure is to crack the Photo 3 — Close up shows the actual fuel metering orifice created by the tapered flat side on the round needle and the needle orifice.

Needle to orifice clearance should never be more than I1'-; thousandths clearance. Needle diameter is .125 and orifice should be .126 thousandths. The most common problem encountered in setting up a Posa in addition to the motorcycle instructions is

that the users try to set the idle speed first. This is

backwards from the correct procedure. Let's spend a little time and go through a typical carburetor adjustment session and discover how to

do it right the first time. The first thing we should do is to tie the aircraft down in a tail low position that approximates the angle of the climb of the aircraft. It is very important the fuel mixture be right, particularly when the engine is work-

ing at lOCK; power as in a take-off and climb situation. Your Posa probably came with a #3 needle installed so let's start with it. Screw the needle adjusting screw all the way in to the stop then back it out to 10 turns open with the fiat on the back side facing the engine side of the manifold.

throttle to '/•» open, turn on the fuel and mags, then count slowly to 4. or about 4 seconds, hit the starter and it w i l l usually start about the 5th or 6th blade over compression.

Now we have the engine running, let it warm up a bit at whatever speed it will run at and then we will adjust the high speed mixture. We want to do this procedure with about '.£ tank of fuel aboard, since Posas are affected by fuel head pressure. It is also desirable to set the mixture to be ideal at the lowest amount of fuel in the tank that you would logically take off with. With the engine warmed up we now open the throt-

tle wide open (aircraft is securely tied down, right?). Hold it open for 2 or 3 seconds for everything to stabilize, then turn off the fuel valve with the throttle wide open. Listen very carefully to what the engine does after the

valve is closed. If it gains rpm. it's telling you that the

mixture was too rich, so you screw the main jet in one turn and repeat the procedure. If it shuts down without

a gain in rpm it's wise to open the main jet one turn and repeat the test. Do this until a definite gain in rpm occurs, then start leaning the mixture one turn at a time and repeating the test until the engine will die after fuel shutoff without any rpm gain. This is the best mixture setting.

Photo 4 — Setting the needle length with the gauge matches the fuel delivery rate to air flow requirements of the engine.

SPORT AVIATION 61

Photo 5 — The large body Posa is available in 26-29-32-35-37mm sizes. Small body Posas come in 17-20-23-26-29mm

sizes.

I like to open it one turn to the rich side after finding the best mixture setting; just be sure that I'm always on the rich side of the maximum power setting and not cooking the pistons and valves. Now you have the high speed mixture set and can do nothing more by adjusting the main jet. It is only designed to set the high speed mixture, not to adjust the carburetor in any other speed range. To gain midrange or low speed control we must select the needle that best suits the engine's fuel demand requirements. Let's assume that the top end is O.K. now, but the idle is very rich and the exhausts are belching black smoke, with the engine running roughly. In this case a higher number needle is indicated. The higher the

needle number, the greater the taper angle; conse-

Photo 6 — MAPI "SuperCarb", a Posa modified to include cockpit adjustable mixture control and separate idle adjustments.

62 MARCH 1982

quently, the top end gets more fuel and the low end

gets less. We remove the needle adjusting screw from the carburetor and install the higher number needle, carefully setting its length by the gauge supplied with the carburetor (see photo #4), then go through the valve shutoff procedure to again get the high speed mixture right. After the high speed end is set, you will note that idle mixture is now leaner, possibly O.K. now. If it's still too rich at idle you have to go to the next higher needle yet. You may get to a point of beautiful idle, but find that the engine doesn't want to take a throttle advance, tending to run rough or cough and spit when throttle is applied. This indicates too lean an idle setting. Since the Posa has no acceleration pump, idle mixture must be set slightly richer than normal so that it will take rapid throttle advancement.

It is not unusual for builders to fight this hesitation problem for weeks thinking that it is a mid-range carburetion malfunction, when it can be cured immediately by a slightly rich mixture at idle. If by some chance you do come up with a lean idle problem, you can cure it by installing a needle with a smaller number than the one currently installed. In our HAPI designed "SuperCarb modification" to

the Posa, we have incorporated a separate low speed idle air circuit that does give us considerable control over the low speed mixture, without altering the metering system except at low idle. Our low speed air jet is

entirely independent of the needle, adjusting air metering rather than fuel. We also incorporated in the SuperCarb a cockpit

adjustable mixture control on the main jet that allows mixture to be adjusted for best economy according to

an Exhaust Gas Temperature Gauge, and, of course, allows mixture to be set at the ideal for your cruising

altitude. One great advantage of this is that it allows us to set the full throttle setting at take-off slightly rich, giving extra engine cooling when maximum power is being produced, and then adjusting to a lean fuel conserving mixture after cruise is reached. HAPI Engines, Inc. is now producing the Posa Carburetor and the SupeK'arb in our Kloy. Arizona facility. For those of you who may have Posas but don't quite understand them, the new installation instructions are available for a nominal fee. Parts and rebuilding services are also available for older carburetors. Many of our customers send in their "stock" Posas to be converted to SuperCarb.

Photo 7 — Typical Posa Installation, seen on Butch Gratton's KR-1. faces carburetor into ram air.

AVIATION 63