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Engine basics => Engine set up tips => Topic started by: dave siegler on June 01, 2018, 09:15:15 PM
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Formerly dead reliable FP 40. Starts but after a minute of rich running goes dead lean an surges all over the place.
Wont stay running,
It has about 5 gallons though it then put in a box for a few years.
This inst a runaway, I know what that is about or just a lean needle. Not overly hot.
It seems like fuel flow problem.
Have flushed out needle, replace lines, changed fuel, added a filter , 2 different new tanks leak checked tanks.
Changed plugs, does it with the glow lead on or off.
Is this an air leak?
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Dave, check the o-ring under the venturi. It might be squashed out and allowing air in. On engines with some time on them, I have seen the through hole in the venturi become very wallowed out due to the o-ring not being compressed enough and vibration just eats up the hole. I have found some o-rings that are a bit thicker and I use a padded clamp to lightly squeeze it down and then insert the needle. I presume you have removed the needle completely and cleaned the spraybar thoroughly?
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One of the renowned here had a NV clogged with castor as ' He Hadnt flown it recently ' .
Said bits broke off / lean/rich/lean/ etc etc as the debri in the spray bar itself disolved in flight .
Sounds Similar ! . :-\
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Had a reliable fp40 go weird. Needle valve was reverse flushed. Fuel appeared to flow freely through the tiny exit hole. When disassembled a thin wire was run through the spray bar. A fleck of red painted came out. Engine ran fine after that.
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I had an O S 46 A that was doing the same when running half/half oil 5-10 % Nitro, tried other tanks, all the other stuff, then I ran it a couple times on 22% all synthetic fuel and it straightened up.
Needless to say I didn't exceed 9500-9800 R P M ..
This is a suggestion only, your mileage may vary, so beware...
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Cleaned the whole fuel system, there is some kind of tank problem. I see air in the lines
The tank is a brand new wide wide wedge, and it holds pressure nicely.
Tired both uni-flow and standard vent. It isn't vibrating very much and there is a foam pad under the tank.
The airplane had a 25 on it that I flew for years with a hays plastic tank, swapped in the 40 for a little more power, it would not run.
Thought it was the tank.
Swapped out the Hays tank for the new wedge tank.
Will pull the motor this week and run it on a stand.
very strange.
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If you know there's a problem in the fuel system, why take the engine out? Or do you just wish to verify that it works before going on?
I want to reiterate what Kennith said about the O-ring -- these get hard over time, and seem to shrink. The worst ones will just crumble off of the venturi. They're a standard metric size, so if things still fit correctly use that -- otherwise use an inch size (metric O-rings about that size have a 1mm diameter ring, while inch-sized use 1/16). I would check and replace as a matter of course. I keep a bunch of O-rings on hand for just that purpose.
If it doesn't work on the stand after replacing the O-ring, check the crankcase seal. There's no reason it should go bad if you haven't been messing with the bottom end of the engine -- but there's a first time for everything.