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Glow Motors spraying fuel from front bearing

Old 01-28-2006, 05:18 AM
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kevmac48
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Default Glow Motors spraying fuel from front bearing

Have a few motors which spray fuel out the front bearing housing ,have tried neoprene seal bearings help for a little while but doesn,t fix the problem one is a enya 40 /TT46 pro and Os 28 heli motor any help on solving this would be much apreciated can email replys to [email protected] cheers Kevin
Old 01-28-2006, 09:15 AM
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w8ye
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Default RE: Glow Motors spraying fuel from front bearing

Most of the fuel spraying from the front of the engine problem has to do with the tolerance of the crankcase to crankshaft fit. Like you say, the seals don't completely stop the spray. If the seals to help any, it's a short term affair.

Enjoy,

Jim
Old 01-28-2006, 12:24 PM
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DarZeelon
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Default RE: Glow Motors spraying fuel from front bearing

Kevmac,


Like Jim wrote, the spray from the engine's front (runny nose...) is a tell-tale sign of an ill fit between the crankshaft front-rotor and the crankcase boss, within which it spins.

Unlike full size engines, a leak from there is not really a malfunction and at worse, it may make your engine difficult to shut-down with the carburettor (and will dirty your plane more).

Using a sealed bearing will not make the engine run any better.
Old 01-28-2006, 01:28 PM
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Spicoli
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Default RE: Glow Motors spraying fuel from front bearing

Here is a quote from my Jett 120L manual
"It is perfectly normal for the engine to blow a bit of fuel out the front bearing when running,especially at full throttle.This keeps the front bearing lubricated."
Old 01-28-2006, 02:43 PM
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djlyon
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Default RE: Glow Motors spraying fuel from front bearing

Actually his TT46 has passage from the casting face behind the front bearing to the intake venturie. Purpose is to try to route the fuel and oil comming to the front bearing back into the engine. It works to some extent with front bearings that have dust shields. Webra does the same thing. If you are standing behind the engine you only get a light shower instead of a heavy one. Remember the 2 stroke engine is a fuel pump. On the down stroke crank case pressure is pushing fuel air mixture into the combustion chamber. That same pressure is trying to push fuel and oil out the front bearing. Add to this blow-by pressure, especially from a worn engine and a loose fitting crank shaft and you can get a lot of spray. Live with it tell you start getting air sucked back into the case through the front bearing which will cause mixture control problems.

Personally I would worry if I didn't see something coming out the front bearing. I mean oil and fuel not parts.
Old 01-28-2006, 05:11 PM
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Default RE: Glow Motors spraying fuel from front bearing

The most oil I have ever seen from the front bearing was a drool of thick oil running down the side of the engine. Its much more common for engines to spit fuel out of the carburetor and coat the engine and plane with oil. This is caused by late closing of the rotor valve, which is good for high rpm. The old Supertigre G60s were some of the worst, the carbs made almost as much a mess as the exhaust.
Old 01-29-2006, 11:41 AM
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Default RE: Glow Motors spraying fuel from front bearing

ORIGINAL: djlyon

Remember the 2 stroke engine is a fuel pump. On the down stroke crank case pressure is pushing fuel air mixture into the combustion chamber. That same pressure is trying to push fuel and oil out the front bearing. Add to this blow-by pressure, especially from a worn engine and a loose fitting crank shaft and you can get a lot of spray. Live with it tell you start getting air sucked back into the case through the front bearing which will cause mixture control problems.
How is it possible that the pressure in the crankcase can only throw fuel out during the downstroke but not suck air back in when the motor is "carbureting", ie filling the crankcase?
Old 01-29-2006, 12:22 PM
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w8ye
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Default RE: Glow Motors spraying fuel from front bearing

You don't see the the air being sucked in. All you see is the fuel being blown out?

Enjoy,

Jim
Old 01-29-2006, 02:05 PM
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britbrat
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Default RE: Glow Motors spraying fuel from front bearing

The engine will run perfectly well & last as long as any other -- the goo is just an irritation, but not anything to get excited about if your engine-bay is adequately fuel-proofed. .
Old 01-29-2006, 10:12 PM
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Default RE: Glow Motors spraying fuel from front bearing

ORIGINAL: The_Pipefather
How is it possible that the pressure in the crankcase can only throw fuel out during the downstroke but not suck air back in when the motor is "carbureting", ie filling the crankcase?
When the piston is on the upstroke, the intake port is open so there's very little actual drop in pressure to draw air though any other small openings. On the downstroke all ports are closed so the mixture gets compressed until the transfer ports get uncovered. The average pressure at full throttle in the crankcase is about 1.5psi which is what you get if you tap into the backplate for a pump (called untimed pressure). Some engines have a boss directly under the carb for a pressure tapping and this opening is regulated by the crankshaft port so it's called timed pressure because it's only connected to the crankcase on the downstroke and gives around 6psi.

Old 01-30-2006, 12:48 AM
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The_Pipefather
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Default RE: Glow Motors spraying fuel from front bearing

Thanks for the explanation, downunder. The reason I asked is, on the full-size 2 strokes, its standard procedure to pressure-test the engine to about 6 psi before jetting the bike. This is done to ensure that carburetion is not upset by air being sucked in from any opening other than through the carb.
I have observed that even a blown magneto-side oil seal is sufficient to cause the engine to run very lousy, especially at part throttle openings. Are the glow engines less sensitive to air leaks because we tune the top-end during every run, thus compensating for any leaks? (having the same effect as swapping main jets on the full-size two strokes)
Old 01-30-2006, 07:34 AM
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Default RE: Glow Motors spraying fuel from front bearing

ORIGINAL: The_Pipefather

Are the glow engines less sensitive to air leaks because...
I suspect it's for 2 reasons, the first of course is what you said about needle twiddling and the second is probably because a satisfactory air/fuel ratio with glow fuel (especially when using nitro) is much wider than when using petrol.

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