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Magnum GP .65 surging

Old 01-27-2020, 09:52 AM
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Doog-meister
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Default Magnum GP .65 surging

Have a new Magnum GP .65 that I installed in a GP Stick about a year ago. The engine was NIB, lubed and stored for @ 20 years. no evidence of any corrosion. Several tanks of fuel to break in, and maybe 10 excellent flights.

Using Omega 10% fuel, APC 12x6 prop. The engine has decided to start surging in flight as if I am running the throttle up and down. have tried three or 4 different glow plugs of different brands - same result. Completely replaced the fuel tank and all fuel lines - no change. Tank seal has been checked and is good. Tank is as high in the fuse as can be done, and is mounted in foam rubber. Plenty of oily smoke from the exhaust, needle valve opened @ 1/2 turn from peak using a tach. Can't see any dirt or obstruction in the carb. Muffler is tight. Engine runs good on the ground. Glow plugs removed work fine in an OS 46. Same for the fuel.

Any suggestions?
Old 01-27-2020, 12:50 PM
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A. J. Clark
 
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With the needle 1/2 turn out from peek I would think that would be very rich . Rich enough that you wouldn't have a 2 cycle . So from that I would suggest there is a fuel obstruction or air leak some where. If it only happens in the air could be fuel foaming or over heating from not enough air flow to the engine.

edit - Also check the throttle servo or change it for another.

Last edited by A. J. Clark; 01-27-2020 at 01:10 PM.
Old 01-27-2020, 01:01 PM
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Doog-meister
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Tank and fuel lines have been leak-checked. Acted this way with the stock (wood-mounted tank) and the replaced tank surrounded by foam. New fuel lines. Still running 2-cycle. You are correct, one would expect this to be slobbery rich 4-cycle with the needle this far open. Engine is fully exposed, no cowl. Air temp in the mid 40's.
Old 01-27-2020, 03:30 PM
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A. J. Clark
 
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Could have some dried oil in the fuel passage of the carb after setting for 20 yrs . Take the needle valve out and spray cleaner in the carb . Might be good to disassemble the carb and clean it.
Old 01-27-2020, 04:02 PM
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will give that a try. What puzzles me is that I had some good flights and ground runs prior to the problem starting.
Old 01-28-2020, 07:56 AM
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Could be a leaky O ring holding the carb, or an air leak at the pinch bolt or where the needle valve screws in. The Magnums are bad for that. A bit of silicone dabbed on with a toothpick can solve a lot of problems on these motors. A fuel bulb pressurizing the fuel inlet with your finger over the big holes in the carb can show up some small leaks. Is the crankcase hot on the bottom? I had one where the crank was too tight of a fit. Your ten successful flights make me doubt this.
Old 01-28-2020, 09:28 PM
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Have you checked that the head screws and back-plate screws are tight?
Old 01-29-2020, 07:20 AM
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yes - all screws are tight.
Old 02-05-2020, 11:05 AM
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Have removed and disassembled the carb No visible obstructions. O-ring under the carb and both o-rings on the look good, as does the gasket under the stop screw. Soaking the carb parts in alcohol to remove and gummy stuff that may be in there. Going to get a new O-ring for the bottom of the carb just in case. No visible tears or gaps. but I've seen better.
Old 02-05-2020, 01:55 PM
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the Wasp
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Doog, tell me something. can you wiggle the main needle back & forth ? the Magnums are well known for loose main needed fit. an OS needle should fit tighter, but I am not sure what OS needle to use. maybe find a friend that has an OS and try his needle.

Jim
Old 02-05-2020, 02:12 PM
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No - the main needle is snug - no movement at all save for the normal adjustment.

I have an identical engine that I have been flying since the mid 1990's on another plane. Thinking of swapping the carbs and see what happens.
Old 02-05-2020, 02:26 PM
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OK.
with the engine running how about spraying WD around the back plate gasket, then around where the carb inters the case.
BTW, I have not had luck soaking carbs in alcohol, soaking them in hot antifreeze for 5 hours works great, just don't inhale around it

Jim
Old 02-05-2020, 05:14 PM
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A. J. Clark
 
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If you connect a piece of fuel line to the carb inlet and blow into it you can test if there is flow through the needle valve. Then connect the line to the vent line and do the same test.
Old 02-06-2020, 08:50 AM
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did both of those after soaking and cleaning out the carb and needle valve. no visible dirt or gum inside either carb or needle valve. Replaced O-ring between carb and crankcase. Put a bit of silicone under the needle valve retaining nut and under the carb barrel retaining screw. Hopefully we get some weather above freezing this weekend so I can do a test run. Hasn't been above 20 degrees since Monday.
Old 02-08-2020, 12:13 PM
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Hi!
How is the tank mounted? Center of tank should be in-line with center of carb when plane sits horisontal.

Old 02-10-2020, 07:37 AM
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original tank was mounted per the cutouts in the fuselage (ARF). I replaced the tank, and made an access hatch. 2nd tank is mounted exactly as you stated. performance is identical with either setup.
Old 04-28-2020, 11:47 AM
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ok - got the main LG mount back in the Stick after repairs from when they were ripped out due to a rough-field landing caused by the last in-flight shut-down. Spring weather here isn't often conducive to flying. Tis' the season of unpredictable weather and gusty strong winds. Have made two trips to the field to find 17-25 mph cross-winds. nope.

Took the Stik out to the field last weekend, and put it in the test stand. Took forever to get the engine started - 1 1/2 turns open on the main needle valve flooded it badly. Put the glow plug from my other Magnum GP .65 (which flew fine) in it, and finally got it started. Went thru the tuning process from the beginning, low end, high end, good transition. Ran at full throttle w/o problems for @ 5 minutes. Idle for 3 minutes. still good transition. shut down, put the glow plugs back into their original engine.

Start it back up, and nothing is good. re-tune again. I know a glow plug can make a difference, but this was dramatic. Engine tuned w/ a tach (have been doing it this way for 20+ years) open the main needle valve (3-4 clicks) until rpm drops 400 rpm from max. pinch the fuel line, and the engine dies. Opened main needle valve 4 more clicks to stop that from happening. adjust low end again to get good transition. after fooling around with adjustments, engine dying, re-starting, and annoying the other flyers for 2+ hours, took it up. 1 1/2 laps around the field, flying straight and level 3/4 throttle, engine dies. Fly the other plane also powered by a Magnum GP .65 and forget about this one for a while. That engine has always performed amazingly well, and starts with a single flip of the prop more often than not. I've been flying that engine since the early 1990's, and it has been in its current plane for about 12 years.

Take the Stik up again, and it dies this time after maybe 5 laps. same thing, 3/4 throttle straight and level. put the plane back into the run-up stand and fool with it some more. Go fly again. Flew nearly a whole tank of fuel, most of it at full throttle. A few loops and many rolls - no problem. Throttle back below half on the down-side of the loops. it took a lap or so to come back after one loop, but otherwise uneventful. Having not taken the time to check how long, I had been up, I decided to to do a couple of touch and go's and land maybe 3rd time around. Nice setup, over the numbers, 4 feet off the ground, and the engine quit. Three flights, three engine-off landings. Upon checking, the fuel supply line had no fuel in it, and the tank had very little fuel. I am thinking probably not enough muffler pressure at low (just a couple click above idle) to keep the remaining fuel going into the engine? With an approaching squall, and wind picking up past 20 mph, it was time to call it a day. Had to tip the plane nose-up to pump out what fuel was remaining - one ounce, if that. Probably less.

Hopefully, the problem has been solved. We will see. Sitting here working from home during the quarantine today, with clear blue skies, 72 degrees, and very light wind wishing I was not trying to be thinking about work.

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