Focus on Competition
Technically Speaking ...
Greg Hahn Technical Director [email protected]
So far most of my tech talk has been concerned mainly with the electronic aspects of our hobby. This month I'm going to hit on a problem I have faced often, and I know it's also problematic to many others.
While test flying and working out the bugs in my new Scale entry (Beech 18), I ran into a situation that can catch you by surprise, and hopefully you'll figure it out before the airplane is lost to either an off-field landing or a spin in case of a twin. The problem occurs when the carburetor on a gas engine is side mounted, which is the normal position on a piston-port type engine.
The rotation of the carb in relation to the front or upwind side can often cause airflow problems over the intake side. When air flows perpendicular to the intake and its speed is greater than the intake speed, the result is a vacuum or low-pressure area that draws the fuel/air mixture out of the carb instead of into the cylinder.
If you've ever noticed raw fuel sprayed down the fuselage side of your airplane, this is what causes that. The end result of this siphon effect is your engine running lean soon after takeoff. It will often sound fat or rich, but it's actually going lean. Check the plug if you're in doubt, but normally it will be lean.
The easiest cure for this scenario is to install an air-intake stack or extension to the carb intake. They are available from several vendors and are very simple to install.
It's a quick, easy fix to what most of the time can be a baffling situation. Also, several engine manufacturers now often include them in the engine package, so check your box; you may already have one and not know it (Dave!).
Another issue while we're on the subject of carbs that often causes problems is the position of the diaphragm cover. If you look at the diaphragm side of a Walbro or Tillotson carb, you'll notice a 3/16-inch hole drilled directly over the center. This hole allows air to flow both directions from under the cover as the pumper operates the diaphragm to pump fuel.
Often, rotating the carb to make throttle linkages easier to connect puts this hole in direct line with airflow from the propeller. As the airflow increases with speed or throttle, the pressure flows into the cover hole and will eventually stop the diaphragm from operating properly, causing the engine to go lean again soon after takeoff.
There are numerous ways to combat this problem, but all that's really needed is to stop the air from directly hitting the diaphragm plate. My situation was solved by my buddy Jeremy Arvin while in the pits at Top Gun chewing the fat. I, of course, was thinking of the most difficult way to solve the problem, which included the use of a soldering iron and other scarce tools. He said to simply glue a piece of plastic in between two of the cylinders of my dummy engine to block the air, and that should do the trick. Sure enough, he was right. Problem solved in less than five minutes. So all that's needed is to stop the air—nothing major.
Lesson: If you have problems with engine performance changing while in flight, most of the time it's not mixture related; it's airflow related. Always go there first.
See you at briefing. MA
Transcribed from original scans by AI. Minor OCR errors may remain.
