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must prime carb to start engine

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Old 05-18-2014, 01:18 PM
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buckinabush
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Default must prime carb to start engine

I am not sure why this is happening, but in order to start my tower hobbies trainer I have to put a couple of drops of fuel directly into the carb for the engine to start. Even after running the engine for a couple of minutes and shutting it off, I must re prime the carb in order to start the engine. can someone please let me know as to what may be happening and how to resolve this issue. Thanks in advance.
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Old 05-18-2014, 01:26 PM
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Most trainers have the tank mounted too low. Got to make it look like a Cessna with a windshield, so with a vertically mounted engine this can happen. Also you may have too much fuel line for ease of use, but shorter is always better.

You can open the throttle and choke it by pulling the prop through while holding your thumb over the carb. Some are easier to start if you just put your finger over the exhaust when you first spin the prop with a starter. Lots of techniques,
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Old 05-19-2014, 05:36 AM
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Have you tried priming it the way High Plains described? An alternate method is to block the muffler and flip the prop.
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Old 05-20-2014, 01:36 AM
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buckinabush
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I actually tried a method of both of your answers. I put my finger over the exhaust opening and the carb at the same time and spin the prop to bring fuel into the carb. What I noticed when I do this is after I remove my fingers, the fuel sucks back out of the carb.
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Old 05-20-2014, 04:13 AM
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The real question here is, does the engine keep running and pull the plane around okay?

If the engine needs primed, via eyedropper or pulling the prop around with fingers on the carb, that's okay. If it runs okay and pulls the plane around reliably, it's all okay. When you stop the engine, there's no need for the fuel to stay in the fuel line. There's probably stil a few drops left in the carburetor.

So... does it keep running on the ground? Does it keep running at high RPM when you lift the plane and point the nose up? If so, you have no problem.

How's the plane otherwise?

Best wishes,
Dave Olson
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Old 05-20-2014, 06:23 AM
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+1 for Dave (Scar's) answer. assuming the engine runs well and has no other major issue, there's no need to fix anything.
if on the other hand the engine does NOT run well, the other symptoms (priming required for EVERY start, fuel runs quickly back to the tank)
indicates the tank may be too low.
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Old 05-20-2014, 10:38 AM
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Every engine I've had that had the fuel run back down the line after priming had an air leak in it. Change the fuel line going to the carb first, then the needle valve o ring. An engine that won't prime by coving the carb inlet but will by blocking the exhaust is a dead giveaway of a leak, since the former works be creating a vacuum and the latter works by creating pressure.
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Old 05-23-2014, 01:48 PM
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buckinabush
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Thanks for all your help guys. I have been very busy all week but I will try your suggestions this weekend and let you know what happens.
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Old 05-25-2014, 01:23 PM
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ok guys, I followed through on all your suggestions. I pulled out the fuel tank. changed all fuel lines, checked o rings, shortened fuel lines and now the engine won't even start by priming the carb. As I see it either I need a new engine but at this time, I am seriously considering going to an all electric air fleet. I have had it with the unending things that go wrong with gas engines.
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Old 05-25-2014, 07:49 PM
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This is a gas engine?
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Old 05-26-2014, 03:36 AM
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buckinabush
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sorry Stick 40, by gas I meant nitro. just plain rc fuel.
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Old 05-26-2014, 06:36 AM
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not a problem, it would be different issue with gas.

Its hard to come up with a solution, if all the facts are not known.
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Old 05-26-2014, 12:08 PM
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When I've had an ornery engine before, I have taken it off the plane and pressurized the engine to see where it leaked air. Screw the HSN mostly out and hold your finger over the carb and exhaust. You can dunk it in water if you want to pinpoint the leak.
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Old 06-01-2014, 12:31 PM
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how much pressure do you put on it???

sticks
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Old 06-02-2014, 06:01 AM
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I blow into the fuel line attacked to the carburetor with my mouth. If there is an air leak, you'll find it pretty quickly.
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Old 06-09-2014, 03:55 PM
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could be dealing with multiple issues. Is this a new engine? How is the compression in general? Are you getting good ignition/glow from the plug since it does not start at all? Tried priming the cylinder directly? Sorry if this is a very dumb question but the carb and exhaust lines are not crossed mismatched right?
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Old 06-12-2014, 06:39 PM
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How old is your glow fuel? As it ages it looses some "oomph" from water absorption and alcohol evaporation and you have to be over-rich to get it to kick.
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