4 stroke cuts at end of outside loop

I have a 120 4 stroke mounted inverted in a Cyclone 3D
Keeps cutting out at the end (top) of an outside loop (bunt) but otherwise
runs fine.
Doesn't cut with any other aerobatic manoeuvres
I've checked the plumbing, clunk & tank. All seems OK
What could be the problem ?
Thanks
Reply to
fred
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Hum, I'll take a swipe at that - You've got a 4 stroke mounted inverted that runs fine with the model upright (engine inverted) but then when you flip 'er over it cuts out. Hum, could it be that it's set up too lean but runs fine an upright model/inverted and the inverted engine is compensating for a too lean needle setting. I'd set the needle with the model inverted so that it's 300 rpm or so less than full bore and see what that does. Then it may be a little rich flying upright and then just right at the top of that loop. Prolly can't have it perfect in both attitudes without a pump.
fred wrote:
Reply to
2fast
It will fly OK inverted, its just the outside loop that causes the trouble. Might get a perry pump for it, and also the fuel lines could be shortened, they are needlessly too long.
Reply to
fred
Bottom end mixture set too lean.The engine will always go slightly lean when the nose is pitched down in a bunt (outside loop).If you were to keep the throttle half open when executing this manouver I bet it keeps running ok.If you richen the main mixture any amount to compensate for this you will lose too much power in level flight.Richening the bottom end mixture should cure the problem but make sure all fuel lines are ok and pressure test the tank.A fuel pump is not neccesary unless the tank is a long way from the engine.
Reply to
Paul
bottom end to lean? I would have thought you are going to be at somewhere near full throttle at the end of a bunt
Reply to
Kevin
Doesn't it depend which way you do the bunt ? If you start low then roll to inverted and bunt from there then you will finish at low throttle. If you start high and bunt from upright level flight then of course you do end up at full power.
Maybe the OP can tell us which way he does his bunts ?
Reply to
Boo
from the top, flying level, push the nose down.........plane is inverted at lowest point engine doesnt want to get up to full power on the climb out, cuts at the top.
Reply to
fred
Maybe the fuel tank's too high in the fuselage or there's a lean spot in middle of the throttle range ?
Reply to
Boo
Is it a Laser or a Weston engine where the carb is directly behind the top of the cylinder head ? If so then you may need to take special action to ensure the tank mean level is centered on the carb. Usually, engines have the carb mounted nearer to the crank axis and if a plane has been designed with this in mind then the tank position may be wrong.
As an aside, does anyone know why these two engfines have that peculier layout ? Quite apart from the tank position issue there is the problem of ensuring enough room for the engine to breathe between it and the firewall.
Reply to
Boo
I always thought a loop started from inverted was called an inverted loop rather than a bunt
Reply to
Kevin
@Boo: Putting the carb directly behind the jug shortens the intake manifold and helps prevent carburetor icing. Used to run into that with one of the first Saito 80's.
@ Fred: Richen up the low end 1/16 of a turn.
Reply to
Six_O'Clock_High
Thanks for all your comments.
Dug out the instructions and the factory default position for the low end idle is: Unscrew (ccw, richen) low idle until it stops, then screw back in 6 turns.
Mine seem to be screwed in a LOT more than that, which could account for the lean setting.
Got the tank back in, the plane is ready to fly with the cowl off for easy needle access.
Reply to
fred
Fred --
It sounds like the tank might be mounted too high in relation to the carb CL. In this scenario, you would have to run the engine on the lean side to get normal ops; when you go inverted and put some negative G's on it, the engine would really go lean causing your sag.
Cheers -- Lyman
"On so-called global warming or climate change, let us not scare ourselves with catastrophic forecasts, or use them to defend and promote irrational interventions in human lives."
Reply to
Lyman Slack

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