os 120 fs needs help - RCCanada - Canada Radio Controlled Hobby Forum
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Old 03-21-2005, 07:54 PM   #1
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os 120 fs needs help

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we are trying to start this os engine, and i won't fire at all, we notice when we where priming it , ( using the finger on the carb technic) it would draw fuel from the tank. but it would develop preasure that if you don't take off your finger you could could see the fuel being being blown back into the tank.
you can infact see bubbles blowing into the tank when the carb is at fully closed, the engine model is around late 80's but hardly been used.
hope one of you pros can give me a hand.

thaks rr
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Old 03-21-2005, 08:33 PM   #2
Jay D
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Your intake valve is either too tight, not seated, or at worse the cam timing could be of..... but most definately if your blowing air through the carb.... your intake valve is still open on the compression stroke
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Old 03-21-2005, 10:02 PM   #3
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Also check that the pressure nipple on the muffler is not clogged....sometimes they get carboned up, the tank can't breathe, and fuel flow is extremely difficult.

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Old 03-22-2005, 12:34 AM   #4
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Running any new engine on the rich side is good practice until broken in but I have found that most people run a new fourstroke too rich during the first few tankfulls of fuel. I do this myself! But I found that running too rich like this will cause the oil to not burn off fast enough and it carbons up particularily the exhaust valve and it doesn't close fully. I repaired my own engine and three others for friends the past season all with the exact same problem, the exhaust valve was not sealing tight due to carbon build up.

Also check to make sure the carb is mounted tight and check any "o" rings that may be in the carb for cracks etc that may be allowing air to leak in letting the engine lose its prime ( fuel running back into the tank ) If the engine has been sitting for a long period, as you say from the late 80's, the "O" rings may have dried completely and cracked or split. Something has got to be causing air to leak in allowing the fuel to drain back. Give it a good check over and tighten every thing up..glow plug, carb, cylinder mount screws, and the back plate!
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