Throttle up! Know your carburetor

Model Airplane News, Jul 2005 by Gierke, Dave

WE CONTINUE TO RECEIVE LETTERS AND EMAIL FROM readers who have questions concerning all phases of miniature-engine technology. This month, we'll investigate various problems associated with throttle carburetors.

FULL-THROTTLE FAILURE

Andrew Smith emails, "I recently purchased a new Magnum .61 engine and immediately noticed a problem while running it on the break-in stand. It won't run at full throttle; it starves for fuel and quits, no matter how rich the needle valve is set. In desperation, I pressurized the fuel tank with a squeeze-bulb, and this allowed it to operate properly. I have checked the fuel lines, filter and tank for leaks and obstructions but found none. I am using 10-percent nitro, 20-percent synthetic oil fuel from Wildcat, a 12x6 APC propeller and a K&B 1L glow plug. Can you explain why the engine will not run at full throttle?"

Andrew, I assume that the fuel tank is fitted with an 1/e-inch-o.d. brass tube and medium-size silicone fuel tubing with no splits or pinholes. If these items check out, then the problem might reside (TM)· within the carburetor itself. Sometimes, a metal burr left over from the manufacturing process becomes lodged within a passageway or orifice, restricting fuel flow; this might be what happened, since the engine ran normally when you pressurized the tank. You'll need to carefully disassemble the carburetor for further inspectiona relatively simple task, provided you have the proper tools. If you are hesitant about tackling this job, ask an experienced club member or hobby-shop employee to help. You can usually detect obstructions by observing and poking about the fuel passageways. If you find one, remove it with the tip of a hobby knife or a drill that matches the various orifice sizes.

THE IDLE RICH

David Lamonte of Atlanta, GA, writes, "My O.S. Max .25 LA engine had been running great in stock form using a Master Airscrew 9x6 propeller and Red Max 10-percent nitromethane fuel. The combination produced 10,100rpm on the ground, but the engine seemed to be running hot. I thought the muffler might be producing too much backpressure, so I drilled out the exhaust outlet tube to 7/32 inch. The engine didn't seem to be making any more noise, but it gained almost 1,000pm (11,000)! Now for the problem: for some reason, the idle and midrange mixture are too rich-even with the airbleed screw opened all the way. Any idea what's going on here? If so, how do I fix it?"

David, reducing the engine's backpressure by drilling out the muffler outlet allows more fuel/air mixture to pass through the engine at any given throttle setting; this provides enhanced torque and power but causes the carburetor to produce a rich idle mixture. Less backpressure will also allow the engine to run cooler, exaggerating the rich idle condition. Normally, this enriched mixture can be corrected by backing off on the air-bleed screw, allowing more air to enter at idle.

Because of your new situation, you'll need to enlarge the airbleed hole. You can do this by drilling incrementally larger holes (using number drills) and then running the engine to check the idle and midrange. Avoid making the air-bleed hole any larger than necessary to produce the desired results; bleeding more air into the engine at idle will diminish the fuel draw by narrowing the pressure differential between the tank (muffler pressure) and the carburetor (crankcase suction). Low fuel draw often produces erratic engine operation or an occasional flameout during throttled maneuvers. Your engine might not sound any louder to you, but a sound-level meter would probably indicate otherwise.

I hope that your flying-site neighbors are a forgiving lot!

MIDRANGE BLUES

Dan Paulson writes, "I have an almost-new GMS .61 ringed engine with a remote needle valve. The engine idles perfectly and runs great at full throttle; it's the midrange throttling that's giving me fits! No matter how I adjust the needle valves or how carefully I throttle up from idle, the engine spits raw fuel from the carburetor and muffler before it stops running. I've been using an APC 11x8 sport propeller, 10-percent-nitro fuel from Cooper and a Fox long-reach glow plug. Because I can't get the engine to run properly on the break-in bench, I haven't thought about trying it in an airplane. So far, the engine has been run for almost three hours. Help!"

Dan, ringed engines often require considerable break-in time before they will idle and transition to wide-open throttle (WOT) properly. In your case, three hours should be sufficient time to seat the piston ring. Several adjustments will help to solve the problem, but first, let's review why many 2-stroke engines have difficulty throttling through the midrange.

Unfortunately, the air/fuel mixture ratio must change between WOT and idle to maintain satisfactory combustion conditions within the engine's cylinder. This is a difficult task for a relatively simple fuel-metering type of carburetor. Here are the mixture ratio criteria:

* WOT. At WOT, the mixture ratio must be set quite rich to provide maximum engine power; the extra fuel lubricant also helps to carry away excess heat.