The Tillotson HR197A Carburetor-Air Fuel Mixture Setting

 The Tillotson HR197A Carburetor Air Fuel Mixture Setting CHT and EGT

In the previous post, it was noted that air fuel ratio needed to be adjusted. In most general aviation planes, the air fuel ratio is cockpit adjustable. The MZ201 engine, equipped with the Tillotson HR197A carburetor does not offer this feature without modifications.

Jack B. Hart's blog provides an excellent illustration of the HR197A. He did make such modification as seen in this link:

Carb Mods

The Tillotson carb has two "mixture" adjustments: Low Speed and High Speed. The Tillotson Manual suggests starting out with "one full turn out" on both. Other anecdotal suggestions from users of the carb on vintage snowmobiles is 3/4 turn out on Low Speed and 1 1/4 turn out on high speed. 

Setting the Low Speed is more or less the same process for setting idle mixture. Turn out--to richen until it stumbles and turn in slightly to smooth it out. 

Setting the High Speed is more complicated as it requires the engine to be run at full speed wide open throttle. One can restrain the ultralight and run on the ground for short periods, but with the lack of air flow from actual flight, there is a risk of overheating. 

Perhaps the most accurate way to determine the ideal setting is the use of the Tillotson Quick Jet tool. (See below) It allows precise and repeatable settings.

Each mark on dial is 1/30th revolution

Take careful notes

During flight, High Speed mixture exists while the plane is in a full power climb for 30 seconds; the CHT and EGT and EGT should be noted. The "ideal" setting would be to find the setting that produces what is generally referred to as 100-150 degrees ROP. (ROP=Rich of Peak EGT). This ROP setting procedure is similar to the system used for GA planes with cockpit adjustable mixture controls.

Four stroke engines can use a air fuel gauge that measures oxygen in the exhaust, but these systems do not work as well in a 2 stroke because of the oil mixed in with the fuel. 

Cylinder Head Temp or CHT should be kept below 380 F. (Fiate suggests a max of 500 F, but engine life at temps above 400 F is seriously reduced.) Anything that makes the air fuel mixture burn faster increases CHT. The fastest burn rate occurs with an air fuel mixture slightly more rich than 14.7 pounds of air per pound of fuel=approximately 50F ROP EGT. Leaner mixtures will have lower CHT and richer mixtures will have lower CHT.

Peak EGT will be at the 14.7:1 ratio referred to as "Stoichiometric". This would be ZERO ROP. Increasing the fuel (turning the High Speed adjuster out) will decrease EGT. Assuming the adjustment is reasonably "close" because the engine is running smoothly, if an increase in fuel causes a increase in EGT, you are on the lean side of peak. If an increase in fuel causes a drop in EGT, you are on the rich side of ZERO ROP. 

Once peak EGT is identified, increasing fuel to richen the mixture will reduce EGT. The ideal would be to be in the 100-150 F ROP or 100-150F below peak EGT. (100 ROP is theoretically maximum power; 150 ROP has a slightly lower CHT for maximum engine life--remembering that 2 stroke engines are cooled by fuel.) Keep in mind that these readings assume you are at maximum power and wide open throttle in climb (46 mph for the Aerolite 103) for long enough time to allow the CHT and EGT to "stabilize".  Having that cockpit adjustment mod like Jack B. Hart's would make this an easy process--without it, would require several repeated flights. 

Prior to actual flight, lean the high speed jet from the "initial reference" setting by one or two 30th marks on the Quick Jet tool and continue if EGT at full throttle run up increases--stop when it approaches max 1250F. (You will also note some "dull" acceleration during a takeoff roll at around 4000-4500 RPM.) Then richen the mixture five (5) 30th marks. Lean the low speed jet until the engine idle speed (set at 1650 rpm) drops, then back one 30th mark. Test the engine during take off roll to verify no "four stroking" in the 3000-4500 RPM range and no "dull" acceleration above 4500 RPM.

Note: Excessively rich LS jet will cause "four stroking" and even engine out during acceleration as engine travels past 3000-3500 RPM---this is because the LS jet has an idle port and an "intermediate" or secondary port that adds considerable fuel in this RPM range during which the HS jet is also "kicking in".  The danger from high EGT will occur during high rpm and high loads, and the engine is likely to have "dull" acceleration if too lean, hence the HS jet should be on the rich side. On the other hand, the LS jet should probably be set as lean as possible to avoid "four stroking" during take off roll. 

This "ideal" performance (prior to first flight during take off roll tests) occurred with settings of 1+6/30 on the HS and 1+3/30 on the LS with Density Altitude of 1200 feet. Best estimate is that engine is set at 150 ROP estimating that each 30th moved EGT down by about 25-30 degrees. (Re-adjustment may be required if Density Altitude changes substantially. ) A final check would be to look at plug condition after a take off roll. (Look for a light tan insulator and a dry electrode.)

Note that all of these adjustments depend on a propeller pitch setting that "governs" RPM during takeoff roll in the 4800-5200 RPM range. If the engine spins above that under load during take off roll, the engine RPM is being limited by the carburetor's maximum air flow and all mixture adjustments will be incorrect (probably too rich) due to excess turbulence in the carb's venturi.

Too much white on insulator is bad-too lean

Medium Setting for 1000-1500 Ft Density Alt

Rich Setting for <500 Ft Density Altitude 

Medium Setting "some four stroking" at 1500 DA; Rich Setting "some four stroking" at 500 DA. Appears 1/30 change is appropriate for High Speed Jet for 1000 Ft change in DA. Since EGT in 4500-4700 RPM "cruise" shows 1225+ EGT with the lower DA "Rich" setting, it may be prudent to run a bit on rich side.

Here is a link to articles about adjusting the mixture.

http://www.iwt.com.au/tillotson.htm

https://resources.savvyaviation.com/understanding-cht-and-egt-2/

In general aviation, running LOP increases fuel economy and mixture must be generally leaned at higher altitudes. Ultralights are generally not flying high enough to require in flight adjustment. Choosing maximum engine life over best fuel economy is a matter of pilot choice. To minimize risk associated with engine outs in an ultralight, I would always choose to maximize engine life. 

The Aerolite 103 nicknamed "BG" for its blue and green colors will be equipped with the Grand Rapids EIS showing CHT and EGT.