Posa Carb Notes:

General:

The POSA carb is an ancient device. One was recently found in an excavation near Stonehenge. It is a simple fuel/air metering unit consisting of an orfice/needle combination for fuel regulation and a slide (which carries the needle) for air metering. In it's most common form there is no float bowl, fuel is simply supplied directly to the orfice.

I was amazed at the amount of bad press surrounding the "POSA". This led to serious consideration of replacing it before the engine had even been run. I decided to try it anyway but had low expectations of success. I was so skeptical in fact I remember being shocked by how well the engine started and ran the first time I tried it. With this early success I decided to press on with the Revflow carb and intake system unchanged.

Without a float bowl, the POSA is sensitive to fuel pressure variations. It was designed to be used in gravity flow systems. Gravity flow systems depend on head pressure to flow fuel. Head is the distance between the level of fuel in the tank and the carburettor fuel fitting. The larger this distance the greater the pressure (and consequently flow) to the carb. High wing aircraft with wing mounted fuel tanks are especially well suited to this system with heads in the 2 - 3 foot range. Low wing aircraft present more of a challenge to gravity flow fuel systems simply because we typically have less head available.

In the KR series of aircraft head can be very small at low fuel levels and high climb angles. This can be ameloriated somewhat by using large diameter fuel tube (3/8"), keeping runs short and straight as possible and avoiding the use of restrictive filters and shut off valves.

Revmaster recommends adjusting the needle of the RevFlow carb to produce a full power full rich mixture EGT reading at least 100 deg F below peak EGT. You should be able to lean to peak and then reduce EGT back to this 100 deg below using the mixture control. This is all the adjustment available on this carb other than the idle speed stop.

Rex Taylor produced a video some years ago which should give you an idea of how to proceed adjusting a Posa type carb. When you are adjusting yours do the final adjustments with the induction system and all filters in place however, not as shown in the video with just the carb.

Watch EGT closely as throttle is advanced slowly. The EGT should rise continously as throttle is increased. If not you are possibly too lean. Adjust the needle out by a turn and try again. If after adjusting out 2 turns the problem remains or worsens try using the mixture control to lean at full power. If the mixture control can be used at full power to increase EGT more than 200 degrees F the needle should be turned in (leaned) or replaced with a leaner profile.

Be careful not to adjust the needle out so far that the adjusting screw is in danger of working its way out completely. Count the number of turns from installation to fully seated and try not to turn the needle out more than 3/4.

Do not tighten the needle valve adjusting screw too much when seating the needle (to count # of turns out). The needle could stick in the full lean position. You should be able to pull the needle out against the spring with your thumb and forefinger when correctly installed.

On one occasion after a number of high power tests the throttle was found to be binding on advance. No external impedements could be found and the binding was no longer noted after the engine had cooled. It is possible the slide is expanding at high carb temperatures causing binding.

Upon inspection a significant amount of scoring was observed on the back face of the slide and the corresponding face of the carb body. This was evident in smaller amounts when the carb was cleaned and reassembled last winter. The scoring on the slide and carb body were both polished out using 400 grit wet paper and then lapped together using valve grinding compound. The slide actuating spring was removed and straightened slightly to improve its geometry. The slide now operates smoothly. This will be monitored closely.

An aluminum cover over the gascolator helps reduce the likelyhood of vapour lock (recommended by Revmaster) and for the Cadillac solution a 1" blast tube (from the back of the engine baffle) can be added easily for almost no weight.

The needles themselves can be ground somewhat to fine tune performance. The amount of adjustment available is limited. If a great deal of adjustment is needed perhaps there are other problems, which should be addressed first. Changes to the needle profile should be carefully considered and accurately made. My experience with modified needles has been very poor. (although I am hoping a new needle modified by Revmaster may be more positive).

Needle tip thickness, overall needle length (under the head) and length of taper determine needle charateristics. A tip of 0.055" is considered rich while a 0.065" would be a lean needle.

Keep records of engine performance including RPM, OAT/Altimeter setting (or P ALT) fuel level EGT, CHT, Oil temp and pressure, fuel and oil consumption.

Etremes of temperature are likely to affect the needle valve setting to some extent. Use the adjustment worksheet to adjust the needle valve regularly and keep the record for monitoring.

The carb adjustment seems very sensitive to changes in the intake system. Adding a filter has a large effect on the top end performance.

Sep 9 - 10, 2000:

While adjusting The Gadget's carb per the Rex Taylor video it was not possible to get the mixture rich enough at full power. The lowest Full power - full rich EGT readings I could obtain were about 100deg F under peak. This could only be accomplished with the needle adjusted to it's furthest out (rich) position.

I adjusted the mixture cable to increase mixture control throw and began to analyze the fuel system.

I began to suspect it may be necessary to modify the carb body to accept a larger fuel inlet fitting. The 1/4" flared fitting supplied on the Rev-Flow unit has a flow diameter of approx 5/32" and requires a reducer be used between a standard gascolator and the carb. This means you are likely to use a 1/4" hose here as I did. At low head pressures (common in low wing aircraft like the KR) fuel flow could be borderline. Symptoms of this would include difficulty adjusting carb at high power settings and an increase in RPM while in tight turns. Increasing to a 3/8" hose between the gascolator and carb not only increases fuel flow but it will reduce the possibility of vapour lock.

Fuel flow tests conducted on C-GGFW show a marked increase when the 1/4" hose and fittings are replaced with 3/8". Canadian inspectors strongly encourage the use of 3/8" tube (min) throught the fuel system (at least up to the gascolator). GFW was constructed with 3/8" tube to the gascolator but 1/4" hose from there to the carb. Worst case fuel flow (near empty tank & 15 degree nose up) was 12 GPH. This seems reasonable to supply the needs of my Revmaster 2100 but I was having difficulty getting the mixture rich enough at full throttle settings. Replacing the 1/4" hose and fittings from gascolator to carb with 3/8" produces an increase of 9GPH in fuel flow to 21GPH! This is well in excess of the most stringent FAR requirements of 15GPH for a 75HP engine.

Sep 17 2000:

I also relieved the inside of the 1/4' 90 deg flare fitting necessary to connect to the carb without interferance from the bottom of the cowl. After this worst case fuel flow went up to 13 GPH. Tests with a regular 1/4" flared tube bent on a standard tube bender give 15 GPH but the radius is too large for good clearance on the cowl bottom.

Once the cowl and air filter was installed the mixture seemed to become very rich requiring re-adjustment of the needle. This was expected but the effect of the air filter on mixture was larger than anticipated. Compare static RPM between the two filtered air sources and unfiltered for indications the cowl filter may be too restrictive. I am hoping the two filtered air sources may be better matched than 1 filtered and 1 unfiltered resulting in a smaller mixture change when selecting alternate air. This may mean more consistent engine performance between the two air sources.

Sep 18 2000:

Still unable to achieve good high end performance with the intermediate needle. As power is advanced EGT rises to a maximum of 1200 at approx 80% throttle. Further throttle advance causes engine to begin stumbling (popping). It seemed the mixture might be too lean at this setting however the engine smooths out when the mixture is pulled back and EGT continues to rise to a maximum of about 1350. Turning in the needle seems to delay the onset of the problem somewhat.

Sept 20 - 25, 2000:

A fitting was fabricated allowing the use of 3/8" hose to the RevFlow carb body. Unfortunately this did not appear to have a significant effect on the full power mixture however. I will keep the larger hose and fitting since it is less likely to become vapour locked or obstructed by debris. Worst case fuel flow to the carb is now over 20 GPH.

The fuel tank vent position is important since if it is in an area of disturbed air flow the tank may flow fine in a static test but poorly under power. GFW's vent was in the stock position about 1" above the forward deck line and with the cowl off (or at a high climb angle)may have been in a disturbed air flow. In order to eliminate this item from consideration an extention was fabricated placing the vent 4 inches above the deck line. Yes, it looks like hell but we're going to do this thing by the book, one step at a time until I'm happy with the way this motor runs.

A call was made to Revmaster and Joe Horvath was (as usual) willing to help. After checking the usual items (mixture control position, fuel flow, gascolator shield) he requested the dimensions from my needle. Specifically:

Overall length - 2 15/64"

Length of taper - 1 7/8"

Tip thickness- 7/128" (0.055")

After some consideration Joe suggested increasing the length of taper (not decreasing tip thickness as I would have thought) may richen the mixture at high power settings and allow the needle to be adjusted in to about 3 turns.

He quickly ground a new unit and sent it to me.

In the mean time I discovered that while the engine ran somewhat lean at the top end without the intake assembly and filter, adding these items seemed to richen the mixture. So much so that the stock (lean) needle could be installed at about 4 turns out and give acceptable full power EGT readings. This seems self-evident now however until I moved the tank vent up out of the disturbed airfolow behind the engine baffles I was having trouble getting consistent EGT readings (espesially with the cowl off).

One interesting item to note is that the EGT rises steadily as power is increased but then seems to settle slightly a few seconds after the engine reaches full power. It then remains steady until the mixture is adjusted. Joe Horvath indicated he had not noted this previously but would watch for it in a future "dyno run". I would suggest that this phenomenon was related to the gauge/probe in some way but for the fact that it is not apparant in engine tests without the intake/filter combination. It is possible this induction system does not flow well.

Summary:

At 5 hours the engine is running well with the Revflow carb, but still slightly rich at full power and idle. Further leaning is possible.

Abrupt power changes will cause the engine to stumble as there is no acceleration pump. Accept this as normal.

Power changes in cruise will have a significant effect on mixture (EGT). Prepare to re-adjust mixture with power setting changes.

The engine will "load up" with fuel after prolonged periods of idling. The engine should be "cleared" periodically during descents to ensure power is readily available. Those of us who trained in C-150s were likely taught to do this every 500' during descents and this seems to work quite well with the Revmaster as well.

Ts and Ps

Initial climb

CHT - Max 450 deg F (at base of plug)

EGT - 1200 deg F

Oil Temp - Max 180 deg F

Oil Press - 40 PSI steady

Cruise 2700 RPM

CHT - 400 deg F

EGT - Lean to 1120 deg F

Oil Temp - 175-180 deg F

Oil Press - 38 - 40 PSI steady