engine that wouldn't quit-and other questions, The

Model Airplane News, Jun 2005 by Gierke, Dave

OUR READERS CONTINUE TO SUBMIT QUESTIONS concerning all phases of miniature-engine technology. This month's topics for discussion are engines that won't stop running, engine mounts, setting the needle valve before startup and nitro content of fuel for 7frac12;A engines.

Q THE LITTLE ENGINE THAT WOULDN'T DIE

Brad Delovese emails, "I have a Magnum .40 engine (a 2-stroke) that refuses to quit running when I close the throttle. It simply sits there and idles very slowly. I have to stop it by throwing a rag into the propeller! Since the throttle barrel is closed tightly, where is the engine getting the air to continue running? I've checked the carburetor-to-crankcase seal by squirting a bit of fuel around the interface and looking for air bubbles, but there aren't any."

A Brad, you aren't the first to observe this phenomenon! Many modelers would like to have your problem, however. Most idle-related letters that I receive ask how to obtain and maintain a lower idle. There are several locations where air can bleed into the engine's crankcase, including:

* Secondary needle valve

* Primary needle valve

* Carburetor attachment to crankcase

* Throttle barrel/throttle-barrel housing interface

* Backplate

* Crankshaft seal and fuel-return passageway

Most of these potential air-leak sources can be detected using your method: apply a bit of fuel or low-viscosity oil to the various interfaces while the engine is idling, and watch for bubbles.

A source of air infiltration that is seldom considered is the fuelreturn passageway in the front crankcase housing. This tiny passageway commutes from behind the front crankshaft-support ball bearing to the air-induction hole below the carburetor's neck and immediately above the crankshaft. Easily overlooked, the passageway can be a drilled hole or a machined groove. Fuel that has leaked past the shaft-crankcase seal is "sucked" back into the engine before it exits behind the drive washer, eliminating the possibility of an engine compartment mess. The system works well, provided there is a shaft-seal fuel leak in the first place! If the seal doesn't leak, the fuel-return passageway can bleed a bit of air back into the engine. This may be what's happening with your engine.

If this engine belonged to me, I'd enjoy the idle "problem"! Bet your friends that your engine will idle longer than theirs-or any other engine on the field-and have a little fun! When your engine wears a bit, the annoying idling situation will become a thing of the past. By the way, throwing a rag into the propeller is potentially an unsafe practice. In future, try pinching the fuel line!

Q ENGINE CONNECTION

Mr. B. Samuelson emails, "Modelers spend hundreds of dollars on an engine and then mount it on a cheap, glass-filled mount. I've always thought that the bottom of the engine lugs should be machined flat and parallel to each other, so they can be screwed down to a sturdy, one-piece aluminum firewall mount. Will you please comment on this?"

A Mr. Samuelson, my longtime friend Luke Roy-holder of many AMA national control-line speed records-insists that rpm and horsepower gains reside in the engine-mount interface. The problem, as you suggested, is composed of two parts:

*Bottom of the engine-mounting lugs. These must be machined flat and parallel to one another to prevent the crankcase from becoming distorted when the engine is screwed down to an acceptable engine mount. Crankcase distortion causes the engine parts to become misaligned, thereby increasing wear, reducing performance and shortening engine life. The good news is that, unlike common engine-manufacturing practices of the '50s and '60s, modern CNC (computer numerical control) production includes this important lug-squaring step.

*Engine mounts. Old-timers will tell you that there is no substitute for a sturdy aluminum engine mount that has been carefully

I machined to the same flat and parallel specifications as the engine lugs. These mounts are one-piece affairs that can be securely fastened to the model's firewall or shock-mounted (e.g., with rubber grommets) to reduce engine noise.

To reiterate, the interface between engine and mount must not produce crankcase distortion. Two-piece aluminum mounts, glassfilled plastic mounts and wooden mounting beams are all considered inferior to the one-piece aluminum mounts. This brings to mind the time I used my strobe (an adjustable, rapidly flashing light) to watch a pylon-racing .40 perform on my test stand that was equipped with my first (and last) fiberglass-filled plastic mount. I was shocked to observe the engine "walking around," as disclosed by the stop-action effect of the synchronized flashing light. Of course, this technology doesn't come cheap: whether they're cast or bar stock, aluminum mounts cost considerably more than inferior types.

Q GLOW-PLUG BURNOUT

Jim Shannon writes, "I have a one-year-old O.S. LA .40 with more than 100 flights. The only trouble that I have experienced is with glow-plug burnout-at least one for each flying session. Although the engine runs well, I've noticed that the waste oil on the wing and tail looks black. I use 5-percent nitro fuel with 18-percent synthetic oil, an APC 11x6 prop and O.S. no. 3 plugs. Could the engine be wearing out already?"