April 2001

I have decided to abandon the Revflow (POSA) carburetor for the following reasons,

1) Even when full throttle mixture is within specification "stock" needles seem to cause the engine to run lean in the 2700 - 3000 RPM range. CHTs climb rapidly in this RPM range suggesting detonation.

2) When full throttle mixture is adjusted per instructions idle mixture is excessively rich. This has caused the engine to stall on a number of occasions (all on the ground so far) during acceleration.

3) Friction on the slide requires higher operating force especially at low throttle settings. High temperatures increase this to uncomfortable levels.

4) Mixture is sensitive to relatively small changes in fuel pressure. Vent position, fuel level in tank, atmospheric changes and flight maneuvering all affect engine performance significantly.

5) The absence of an accelerator pump causes the engine to stumble slightly on rapid acceleration such as during a balked approach.

6) From my reading it seems that most of the actively flying KRs have either changed away from the POSA or never used one in the first place.

After nearly 9 months of testing I simply have too many issues with its operation.

Selecting a replacement:

There are a number of possible replacements including the Ellison Throttle Body Fuel Injector, Great Plains (3 models), AeroConversions (POSA) and a host of others such as the Zenith (Ford Model A) and Tillotson types to name a few.

Requirements:

1) The carburetor must be able to flow enough fuel and air to meet the maximum demand of the engine.

2) The carb must provide for smooth operation of the engine throughout the full RPM range.

3) The idle must be consistent and smooth with not tendency to stall after prolonged periods of idling.

4) EGTs must be within acceptable limits throughout the RPM range.

5) An in-flight adjustable mixture is desirable although not essential.

6) The replacement should require a minimum number of modifications to the engine/cowl/controls.

Air:

The 2074 cc Revmaster engine has a rather modest air flow requirement of around 100cfm. This is primarily a factor of our low red line (3200 in this case).

In case you are interested in the math here it is:

CFM = Displacement (in cubic feet) x RPM (red line)/2

Since 1 cubic foot = 1728 cubic inches and the displacement of the 2074cc Revmaster expressed in cubic inches is 128.4 the formula looks like

CFM = 128.4/1728 x 3200/2

CFM = 118.8 at 100% volumetric efficiency.

Since the VE of the Revmaster 2100 is more likely on the order of 85% the actual CFM requirement becomes

CFM = 118.8 x .85

CFM = 100.9

In a normally aspirated engine there is no advantage to over-carburating. If the engine can only flow 150cfm then replacing your existing 150 cfm single barrel unit with a 650cfm Holley Double-Pumper will not increase engine performance. Turbocharging or supercharging of course change the equation.

So the next question is how do you ascertain the cfm rating of a carb?

If you are buying a new unit the manufacturer should be able to provide a rating. Usually this is expressed in a range for example the Ellison EFS-2 (often used in VW conversions) is rated between 150-200cfm.

My situation was slightly different as I was planning to use a carb for which this information was not available. The sizing of the carb would have to be estimated.

The air flow rating of a carb is determined by its' venturi size. The existing POSA (Revflow) carb had an air passage size (this carb has no venturi as such) of 34 mm. Since the engine reached full RPM well prior to the throttle reaching the full open position the 34mm Revflow is likely capable of flowing more than enough air to meet the requirements of the 2074cc Revmaster.

Based on this it seemed reasonable to assume a similarly sized conventional carb should be capable of flowing enough air to develop full power.

The Tillotson Carb:

These conventional float bowl carbs were widely used by outboard motor manufacturers like Scott-Atwater and Chrysler. I have heard they were also used on Harleys but I don't know if they were standard equipment or aftermarket. . They are side draft units and the older models had a high end mixture adjust which made them candidates for aircraft use.

HAPI engines sold these a number of years ago and through the internet I contacted another KR builder who has been successfully using one on an 1835cc engine. He also began with a POSA but quickly changed to the Tillotson.

Unfortunately HAPI engines is no longer around but the builder provided me with the model number and an installation instruction page provided by HAPI.

To find a similar unit was a challenge. All I knew was the Tillotson model number and that it had likely originally been used in an old outboard motor. The model number OM41A 2 was little help since most salvage shops wanted to know the make and model of the motor, not the carb. I was able to find a shop with a good selection of used Tillotson OM series carbs in a box. The largest I could find was an OM 4, which the shop sold to me with a rebuild kit for $50US.

I am told the OM4 was used in 40hp Scott-Atwater motors in 1958.

Upon closer investigation of the new carb several interesting similarities (coincidences) came to light.

When I disassembled the carb and measured the venturi I was surprised to find that it was very close to 34 mm (same as the Revflow). The actual size seems to be 1 5/16" (33.4mm) a number I found cast into the body of the unit.

The next hurdle to overcome would be mounting the carb to the existing Revmaster intake system. The Tillotson is a flange mount carb vs. the POSA spigot mount. The ID of the intake pipe (to which the hose from the POSA slipped over) was almost exactly the same as the ID of the carb air passage aft of the venturi.

A standard VW exhaust flange fit the Tillotson perfectly. I had a spare exhaust flange (from an old exhaust system) which slipped right onto the intake pipe and was easily welded in place.

Dimensionally the Tillotson is compact. It fits easily in the space occupied by the Revflow but is slightly closer to the bottom of the cowl (about 1/4").

Initial tests:

The first test was encouraging. The engine started easily but idled at 2000 rpm. This would make landings interesting.

A new butterfly was fabricated and the idle dropped to 800rpm. With the low speed idle adjust the mixture could be set to give smooth performance at the low end.

High RPM showed a need to increase the size of the main fuel jet. The engine was running far too lean. The original was a #37 drill size. Another trip to the outboard shop produced a jet with a size of #31 (0.122") from an OM2 model. You can drill or ream the jet to size but must be very careful to keep the hole centered in the jet body. It's very easy to go through the side of the jet (guess how I know that?).

With the air filter installed the engine could now be run at full throttle and pull 3150 rpm static, which is just about what the Revflow was giving. Full RPM is achieved just prior to full throttle open which suggests that the 33.4mm venturi is flowing well enough for this engine. The EGT is around 1300 deg F which would indicate the mixture is not too far off. This can be further checked later by use of the mixture control.

The final test before proceeding with a permanent installation was to check for fuel flow to the float bowl.

Previous tests made me confident of a minimum 15 GPH at the end of a 1/4" hose at the carb inlet. However this did not necessarily ensure an adequate supply of fuel into the float bowl since the effect of the needle valve and seat assembly must be considered.

To check this the float bowl was removed and the time to drain 16 oz of fuel from a very low tank (<2 gals) and nose high attitude was measured. The time of 1 min 51 sec shows a fuel flow in excess of 8GPH into the float bowl. This should be adequate to meet the full power demand of 5GPH. A full power run up test will be done with the a/c secured in a tail low position to confirm this finding.

Installation:

The throttle requires a bellcrank in order to get the correct throw and geometry. It looks simple but took me a while to figure out (I'm told that this is a bad time for me intellectually according to my bio rhythm chart).

The mixture will be another challenge. The control is stiff due to the shaft seal and may exceed the capabilities of the control cable.