strange engine behavior

This engine problem is a new one on me.
Magnum 61 2 stroke.
Runs fine on the ground. Runs fine being held nose-up. Transitions
well from idle to full throttle. It stumbles and sometimes dies coming
back from full throttle to idle. If you ease it down in 2 seconds
you're fine, chop it and it's dead.
Dies shortly after lifting off the ground if done at full throttle.
Can get up in the air and fly at half throttle.
Now the even stranger part, the engine won't turn off in the air. I
tried to kill it coming in on a landing and it kept going. I
considered that maybe the air was spinning the prop and thus the engine
(12x6 prop). Really really doubted it, even my 46 with a 12x4 doesn't
do that, but it crossed my mind. After it starts to roll out I give it
gas and it comes back to life.
I was suspecting maybe an air leak but I can't find it. Might put a
new o-ring around the carb if I can track one down. Could it be
leaking around the high speed needle??
There is a wide range of adjustment on the high speed, it doesn't seem
to ever go to the "rich 4 stroke" that I'm used to for break-in, but
when it gets lean it gets lean fast and dies (has the remote needle,
those are nice for adjusting but I have to constantly remind myself to
wait for the adjustment to "catch up" to the carb from the needle).
Since it idles fine I haven't messed with the low speed needle (and
it's a real pain to get to).
The engine was used on a used plane I bought. I don't know much of the
history of it but the guy was an acrobatic flier and had some health
problems and had to give up the spot.
I am open to any suggestions....
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Let that be a warning to you acrobatic fliers. It's bad for your health.
It sounds to me as if your high speed is just too lean. I had a Saito 91 that behaved exactly the same way. The high end adjustment had a very wide band of tolerable settings, or so I thought. It was at least half a turn or maybe even closer to a whole turn. But I found out that the leaner end of that wide band of adjustment was in fact too lean because although it would run fine on the ground it would die on the takeoff run or shortly after lifting off. The answer was to simply roll the needle out farther than I thought it ought to be, and then everything was OK.
Reply to
Robert Reynolds
Yup. Harking back to my old diesel days the increase in RPM in the air - especially on a large fine pitch prop - leans the engine out a LOT..
And a large fine pitched prop would keep the engine spinning..BUT I am surprised the glo plug stays hot enough to re-ignite..
I'd think s complete strip down of the carb and check all seals wold be a start..then get the main mixture right by going super rich..then its down to fiddling with airbleeds and throttle barrels to get the air shut off properly to stop the motor.
Reply to
The Natural Philosopher
Try a new glo plug. Marginal glo plugs will cause some of these symptoms. Particularly running fine on the ground and dying in the air as well as a wide band on the needle on the ground.
Reply to
Sounds lean on the high speed needle and a seperate problem, air leak in the verturi/barrel mating. mk
Reply to
I've seen where the needle valve threads wear due to engine vibration, causing an air leak around the needle valve itself. A simple way to track down air leaks is to pack the suspected area with a heavy grease or Vasoline.....really gob it on. Any air leak will be temporarily sealed and your problem should go away. At least then you'll know where to start fixing/replacing.
Reply to
Tom Minger
Thanks for the tips. I hadn't considered a new glow plug, but that'd be the easiest to check first. I guess then I'll go richer than I think I should, and also expand the hunt for an air leak.
Thanks, Steve
Reply to
Even though the low speed needle is a pain to get to, I would try to richen it a little and see if that changes things.
Reply to
Bob Bauer
Its a Magnum. Had a Magnum .40 that never did run right no matter who tuned it or how it was tuned. Traded it in on a GMS. Only thing good about the magnum is that I got a lot of experience repairing the planes I had the misfortune to bolt it on. Have an excellent running Magnum 91FS; would buy another without hesitation. Would never use another magnum 2 stroke for anything but target practice.
Reply to
Fubar of The HillPeople
As someone else said ,it's a Magnum . The Magnum 2 strokes have had problems with air leakage around the carburator barrel and the carburator base for quite some time. Try checking that. Just a guess as it could be many things.
Reply to
Ken Day
I don't think that this is true any longer. Although checking it, as you suggest, is surely wise to do.
My latest ASP engines (same as Magnum), are amazingly clean and well built - even if over priced. Why do I consider them over priced? Because they are slowly catching up with OS prices, which are way too high.
Ed Cregger
Reply to
Ed Cregger
Since yours is a used engine that has ?? many hours of running it is possible it is a victim of engine varnish as discussed in a recent issue (this summer 2006) of Model Airplane News by David Gierke. The varnish problem causes the engine to run well until it warms up enough to turn the varnish into a tar like substance that gums up the engine and slows it down with a loss of power. In the article David describes the proper way to disassemble, clean and re-assemble a 2 cycle engine. One of the guys in our club had this problem with a K&B 60 which he bought used. As soon as it got into the air is started slowing down and ran sick. I emailed him the article and it solved the problem.
Reply to
Thank you, I will see if I can dig up a copy.
I pulle the engine from the plane and put it in a test stand and got it running well. Adjusted a screw and nut on the carb that I'm not sure what they do (I think it's a throttle lever stop...) and it really helped. Not sure exactly what it did, it doesn't look like it changed anything, but it worked. I also pulled the carb and wiped it down and cleaned the o-ring and put it back snugly in the engine. I plan to run it some more in the stand and then stick it back in the plane and see how it does.
Thanks everyone for the tips, I did a few of them which may be contributing to my new success...
Reply to
Can someone post the article from MAN (I think from May 2006).
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