Old Faithful

Don't touch that jet!
A restriction at the base of the discharge nozzle serves as the main jet in MA series carburetors. In HA-6 (horizontal side-draft) series carburetors, the power jet in the base of the bowl performs the same function. Some well-meaning technical writers have instructed mechanics to hone these jets in an effort to cool cylinder head or exhaust gas temperatures. Apparently they believe this to be a "cure-all" for lean running engines. A mechanic would be ill-advised to follow such instructions since no process approval exists for opening these jets in the field. Furthermore, tampering with the jet size could mask other serious problems. An induction leak, an incorrectly sized economizer jet, an incorrect float setting, or perhaps even the wrong choice of carburetor could all contribute to a "lean" running engine. Another problem with this mode of attack is that the jets vary in design. Some jets are straight, while others are contoured or stepped. A mechanic who takes a drill bit, a ream, or sandpaper to the entrance of a stepped opening may soon regret that decision. Fuel flows could exponentially increase by merely breaking the stepped edge and thereby creating a venturi entrance. By contouring a stepped jet we've managed to decrease the pressure and increase the velocity of the fuel through the jet.
When you start boring out nozzles, several things can happen. As you begin to see appreciable results - the tendency is to take out more and more material. "If a little bit is good, a lot may be better." Wrong! Sure, your high-end fuel flows begin to look better and so do your CHTs and EGTs. But unfortunately, mid-range fuel flows may become exceedingly rich. This rich condition is especially apparent as you transition between idle and full power. An engine that bogs down rich may surprise a pilot forced to do a go-around when on short final. Actuating the throttle causes the pump plunger to spray an additional slug of fuel into the carb throat. Thoughtful engineers have carefully tailored the idle circuit, the accelerator pump, and main jet for optimum performance. A modified nozzle jet may cause the air/fuel ratio to be overly rich. There is a point of diminishing returns, where high-end fuel flows no longer increase but mid-range fuel/air ratios become too rich. And once this occurs, your wallet becomes increasingly lean as you shell out additional dollars for a replacement nozzle!

Choosing the correct
carburetor
A common misunderstanding through the years has been that an engine/carburetor combination that works well in one airplane will function as well in another style airframe. Additionally, it is believed that if the system performs well in a test cell, then it will perform equally well in flight. Unfortunately, to the chagrin of many a homebuilder, this is not always the case. Airframe performance, cowling configurations, baffling, and choice of air-box, all contribute dramatically to fuel/air ratios and to engine cooling.
When selecting a carburetor for your engine, always refer to both the engine and airframe type certification listings. Many have made the critical mistake of selecting a carburetor based solely on the part number(s) listed in the engine manufacturers parts and overhaul manuals. Some have selected a carburetor based on the part number currently installed in their airplane or on an airplane of the same model tied down on their airfield. Others have chosen a carb based on the OEM's application guide. All these methods will narrow down your search, but they will not with any certainty guarantee that you have selected the right unit for your particular airplane. Incomplete engine logs, the swapping of engines to airframes, and the uncertain history of some engines mandates a reliance on type certification data when deciding on the appropriate carburetor for your particular application.
To some, the theory and mechanical functions of carburetors may seem rather formidable. Yet, when each subsystem is viewed separately, it becomes readily apparent that these systems are easy to understand. In spite of all the intricacies associated with them, Marvel Schebler (Precision) carburetors remain the same efficient, reliable units our fathers and grandfathers flew behind with confidence for decades.