Mainspar

My Feedback: (27)

Having an interesting, consistent problem with an inverted engine configuration. Engine runs well and predictably on the ground. Very stable idle, good transition to upper range and back to idle (despite a rich mid-range condition at about 1/4 throttle, tapering off to about 2/3 throttle - explanable by the carb description below), strong upper end. Can run in the rich mid-range for a good while and transition back to idle or full throttle just fine. The problem only occurs in flight - Within a few seconds after transition back to an idle or high idle, the engine dies. Does not appear (although difficult to gauge at times) to go rich and eventually burble to a stop. Seems to die within 2-3 seconds. It does not seem to matter whether the transition was directly from full throttle or part throttle, or whether the transition was gradual or rapid. Does not seem to matter how much fuel is in the tank. Except for the low speed flameout, the engine runs great in the air.

Alright here's the engine set-up (no laughing at my hybrid, now): K&B .45 Sportster, Magnum 40-46 XL Carb, Tower 40 muffler, 11x6 MA Scimitar Prop, 10% Nitro fuel. Had an OS #8 glow plug in there originally. After the first few flameouts, I swapped in a new Thunderbolt R/C Long. Seemed to remember reading somewhere that these were on the hot side, but then again, I probably bought the Thunderbolt plug about 10 or 12 years ago...might not be the same plug mentioned. The Thunderbolt plug ran slightly better on the ground, but nothing significant. Same flameout condition in the air. General weather conditions: 75-90 degrees F, moderate to low humidity.

I tend to mount my engines sideways (90 degrees) about 95% of the time and only have limited experience with inverted mounting, so do not know if the inverted mount is a factor or not. The fuel tank is positioned with center level with the spraybar. Any thoughts?

TedMo

My personal experience over last 60 years of running these engines has been incorrect low speed adjustments. The low speed needle is the main needle valve but seems to be ignored most. Retune.

Mainspar

My Feedback: (27)

Thanks. After the first couple of flameouts, I richened up the low end (from it's happy spot on the ground); same transition flameout in the air. Pushed it to the lean side; no improvement there either. Those two changes just made the engine run/idle/transition poorly on the ground. Magnum carbs are very sensitive on the low end needle. Only takes about a 1/4 turn to go from just right to too rich or too lean.

The old K&B carb had run reasonably well until I mounted the engine inverted, but it is old and could use a new set of O-rings; maybe a new idle disk too. Was pleasantly surprised when I experimented with the Magnum carb. Except for the rich mid-range, it runs great on the K&B .45. Just need to figure out the flameout problem.

Lnewqban

It seems to me that although keeping it on the ground, the engine internal conditions that sustain idle are marginal.
Those conditions worsen while in the air, moving from marginal to beyond the critical point that induces a flame out.
I would bet on combustion sub-cooling, which means, the flame cools down too much and extinguishes itself.

From high or middle to low rpm's transition, there is a temporary excess of fuel and oil respect to the amount of air, which is what we choke by closing the throttle.
That happens because the fuel is heavier and has more inertia that resists the flow reduction, and because is the flow of air what induces the flow of fuel.
Besides, there is less heat produced inside the engine (less mass of fuel is burned) while the cooling remains the same.

I would say some things are different between being on the ground and in the air:

1) Fuel level: If the plane is a tail dragger, the change in level and pressure over the spay bar is obvious (No change if it is a trike).

2) Cooling: The velocity of flying adds to the cooling of the internal combustion.

3) Aerodynamics: The position of the carb intake and exhaust stack relative to the air stream may change the pressure within either. Those possible changes may be negative for the proper mix (suction in the Venturi and pressure in the fuel tank).

In addition to trying hotter plugs, a plug with idle bar would help protect the glow element form fuel and oil splashes.

Best luck with that problem.