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g51 quits in flight

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Old 09-22-2012, 07:58 AM
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copelab
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Default g51 quits in flight

I have a g51 which keeps dying on me in the air. I have replaced the tank and plumbing,the plug and the needle valve. It also seems to have an increased fuel consumption. I was getting near ten minutes with an eight oz tank,now it seems to suck it up in about five minutes. I use Omega 15% with extra castor oil added to each jug of fuel. It seems to have less compression after quitting an I am thinking it needs a rebuild. This is the Italian version;I also have a n.i.b. Chineese version. It has not been run but the compression feels almost the same when turning over by hand. Also,are the parts sold by Tower compatible for both versions of engines? Thanks
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Old 09-22-2012, 05:49 PM
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Default RE: g51 quits in flight

First of all ST engines shouldn't need 15% nitro. 5 - 10 % should do nicely. Why are you adding extra oil? Worst thing you can do for a 2 stroke. Could be running lean and getting hot. Try not squeezing the last RPM from it. All of my ST engines work fine, from the 40's up to the 4500. Almost never have to play with the high speed needle. Most problems come from wandering fingers.
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Old 09-22-2012, 06:01 PM
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Default RE: g51 quits in flight

Mike: I run all of my engines on 15% and always add a bit of extra oil . I normaly run my engines on the rich side as I do not have the need for speed. Usually I hardly ever touch the carb adjustments,maybe I should!
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Old 09-24-2012, 08:19 AM
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Bax
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Default RE: g51 quits in flight

A 15% nitro content is a bit high, but won't hurt your engine at all.

You mentioned that fuel consumption is drastically higher. Is the nose of the model wet with fuel? That could mean that the fit of the crankshaft to the crankcase is getting a bit loose between the carb and the front bearing. That's usually sealed by a thin film of fuel. If the clearance gets too wide, then the engine will start to spray fuel out through the bearing. Front bearings in model engines do not actually make the seal. What many engines have is a dirt shield and not an actual seal. If there was a seal, then there would be no way for oil to lubricate the front bearing.

To check, put the engine onto a test stand with a brand-new fuel system. Run the engine and see if you are getting fuel sprayed from the nose. You can hold your hand behind the propeller even with the cylinder. You'll quickly see if raw fuel is being thrown out. A piece of card stock will become wet with fuel if you hold one at the same location.

That's about all that will cause such an increase in fuel consumption. Otherwise, you're going to have a fuel leak somewhere and the fuel is going into the model or being tossed overboard. A very careful check is needed.

Finally, a worn-out engine may require such a rich setting of the high-speed needle that consumption has gone up because your throwing a lot of unburned fuel out of the cylinder. You need a lot of fuel so that the piston can make a decent compression seal against the cylinder liner.

Again, check, check, check.
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Old 09-24-2012, 11:24 AM
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Default RE: g51 quits in flight

Thanks Bax. I forgot to mention that there is a fair amount of varnish buildup on the piston, probably due to the extra castor I add.
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Old 09-25-2012, 09:31 AM
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Default RE: g51 quits in flight

A lot of varnish will cause the piston to have a tight fit into the cylinder liner. This will cause the engine to overheat due to friction. At some point the heat will cause the piston to start to seize in the liner and the engine will quit. By the time you've glided down to a landing the engine will have cooled off.

Use hot, soapy water and "0000" (four-ought) steel wool to clean the piston (disassemble the engine, first). Rinse thoroughly under water as hot as you can stand with the bare hands and let air dry. Oil and reassemble. Performance should be restored.
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