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When does gas and fire not equal ignition?

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When does gas and fire not equal ignition?

Old 07-29-2014, 05:49 AM
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geebob
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Default When does gas and fire not equal ignition?

I got my old OMP 80" edge out of the mothballs the other day. Its got a MVVS 35 that I bought used years ago. Charged and checked all the batteries and fueled it up. Started right up and would run about 15 seconds and then die. I figured the walbro carb would need to be rebuilt and sure enough the little screen was clogged almost solid. So I rebuilt the carb put it back on the plane. Choke it with my thumb and its sucking gas like a trooper. I flipped and flipped till the sweat was dripping down my nose and I started to remember why I was spending all my time flying my little blue foam electrics. I figured I have gas so I must not have fire. I pulled the spark plug and grounded it to the top of the engine head and flipped the prop. Not much of a spark so off to Tractor Supply for a new champion dj8j sparkplug to replace the aging brisk uf17 that was in the engine. Grounded the plug to the head and flipped the prop..blue sparks a plenty so I re-install the spark plug and take the whole rig outside confident the problem was solved. After flipping the prop until my right arm felt like it was gonna fall off I give up in disgust. I know I have gas..I know I have spark..how come my little engine refuses to run? Could it be a timing issue? Could the moon not be in the proper phase? Are the stars not in correct alignment? Am I destined to only fly glow and electric? Anyone have any ideas?

Thanks for listening , I'll sulk back to my basement in defeat and watch the mvvs soak in brake cleaner and await your replies
Old 07-29-2014, 07:47 AM
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a70eliminator
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Have you tried to prime the carburetor, after having it apart the diaphragm pump may be dry, it can't pump air therefore wont suck fuel.
Also
[h=2]When does gas and fire not equal ignition? Simple Answer: When it happens at the wrong time.[/h]

Last edited by a70eliminator; 07-29-2014 at 10:05 AM.
Old 07-29-2014, 10:51 AM
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Did you make sure that the fuel jets are not clogged in the carburetor? That can stop it from running.
It sounds like the engine wasn't getting any fuel to get it run. Also if the carb was rebuilt and cleaned, it may need to be primed before it starts to work. Quite often the carb won't draw fuel when new or rebuilt, it needs a little help to get the pump and regulator primed before fuel can get to the engine.
Old 07-29-2014, 11:56 AM
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Take the plug out and do two things. One, see if it is wet. Two, with the plug hole downwards, turn the engine over a few times and see if any fuel comes out of the plug hole. If it does, give it a spin with an electric starter to clear out the flood, and dry the plug well. (Acetone works well to clean the plug, then blow off the acetone and let it air dry.)

If the answer to both those questions was no, then you are probably not getting fuel all the way into the engine. Put about a dozen drops of fuel into the plug hole, and re-install the plug. Then it will start and run long enough to re-energize you into wanting to trouble shoot why it is not getting fuel.

Lastly, if you are running a 4.8 volt pack for the ignition, try a 6 volt pack. Eliminate all switches, etc. and hook the battery directly to the ignition.

AND.... Yes we do not do electrics!! God forbid!!

AV8TOR

Last edited by av8tor1977; 07-29-2014 at 12:41 PM.
Old 07-30-2014, 06:59 AM
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geebob
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I've taken the motor off the plane so I'll try turning it over and seeing if gas is indeed coming into the motor. I know it was pumping fuel after the rebuild as I could choke it by placing my thumb over the air intake and within 3 or flips of the prop gas was running out of the carb. Thanks for the suggestions.
Old 07-30-2014, 09:24 AM
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A flaky ignition sensor can drive you nuts. It can even make you think you have a fuel problem. Been there, done that....

Post 36 here: http://www.rcuniverse.com/forum/gas-...rmation-2.html has some great info on testing the ignition, though a sensor that has gone "flaky" might fool you into thinking it was ok by just happening to work properly while you are testing it. We need to know if the engine is getting wet inside. It sounds like it probably is, but we need to know for sure. If so, then you are likely looking at an ignition problem. I have had a weak ignition system drive me nuts by acting somewhat like yours is doing. Bill from CH Ignitions advised me to try a 6 volt battery on it, and sure enough it worked perfectly then. Note that it showed spark just fine when testing it just like yours does, but it would start and run for 10 seconds or so and then quit. After doing this for about 20 times it would finally take off and keep running. With a 6 volt pack however, it would start and run perfectly hot or cold. Bill said that every once and a while a weak ignition would do and act like that, and often using 6 volts would increase the spark and allow proper starting. Of course, after that I didn't trust the ignition and therefore replaced it. As I mentioned, a sensor that is starting to fail can also cause similar problems. Keep after it; it is just a matter of trouble shooting and eliminating possibilities until you find the problem.

Good luck with it and keep us posted,
AV8TOR

Last edited by av8tor1977; 07-30-2014 at 09:30 AM.
Old 07-30-2014, 10:07 AM
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chuckk2
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Besides Spark, Fuel, and Air. the fuel/air mixture must be right, or the engine will not run.
Old 07-30-2014, 05:52 PM
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Spark, fuel, air, compression, all at the right time and amounts, and it will run. For two strokes, you have to remember also both primary and secondary compression. (The crankcase has to seal.)

AV8TOR
Old 07-31-2014, 05:40 AM
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My first question. How old is your gas?

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