First nitro motor. In a heli. No experienced help at hand.
Recipe for fried motor.
It's not your fuel, especially if you bought a new gallon, 15% nitro, 20% oil.
I don't know what you have for pitch curve, or even throttle curve at this point.
But for starters, how about a pitch curve of -2 degrees at low stick, +6 degrees at mid stick, and +10 at full stick.
Then start with a linear 0 to 100% throttle curve.
As for your carb...mechanically you'd want to set the throttle stop screw (this is on the body of the carb) such that when looking down the venturi, as you close the throttle barrel down, it stops just as the throttle barrel closes off the venturi. That would be your low throttle, low trim setting. This will allow you to kill the motor by going to low throttle, low trim. At this setting, you'll also be able to start the motor at low throttle mid trim, it will idle and not engage the rotor system clutch. At low throttle full up trim, you'll begin to spin up the head.
Full throttle -- the carb barrel is completely open, no servo binding or stalling. If everything else is OK, you'll be half way open at half throttle.
Picture above shows an OS 50 carb, particularly the low speed adjustment. The slotted screw at the right, just right of the intersection of the two lines, is the low speed adjustment. IT IS A CAM, not a screw. Its adjustment range is ONLY 1/4 of a turn left or right of factory setting. The slot should NEVER go past (left side of) line number 2.
The two lines intersect at the pivot point of the cam. If you have turned the cam too far, you will find that the pivot point of the cam is now on the right side of the slotted screw, not the left as shown.
Factory setting is pivot point on the left side of the cam as shown, and the slot lined up with line number 1 in the picture.
Clockwise leans out the low speed setting, counter clockwise makes it richer. The setting as shown above is just slightly lean from factory setting, and is probably about where you'll end up with the low speed setting.
Beware going too lean on the high speed needle. You may get away with it for a short time, but if you run too lean, you WILL roast your piston and ring. Cylinder maybe.
If you are hovering, and on the rich side, you will hear the motor speed changing back and forth from a lean four-stroke setting to a rich two-stroke setting. You may find the tail wants to twitch a little each time the motor crosses that boundary. Lean the top end out a few more clicks, the motor should begin to run evenly, but still rich.
Bring the motor to idle, let it sit at idle for about 30 seconds. Open the throttle suddenly, and listen to the motor. If it spits, gurgles, sputters, then picks up, your idle needs to be leaned out slightly.
If, at idle, after a long period, gradually slows down and stops, the idle is too rich.
If at idle the motor gradually speeds up and dies, the idle is too lean.
On the other hand, after sitting at idle, it just dies suddenly, or hesitates, then picks up smoothly, the idle is too lean. Back it off a hair.
The goal is to have a motor that will idle well, evenly, yet pick up instantly the moment you romp on throttle.
When getting the low speed set properly, at the beginning use the throttle trim to maintain idle speed, don't keep leaning out the low speed setting until you have the top end set close to where it will end up.
Second test -- hover the heli for about 30 seconds, land, and let idle for about 30 seconds. This should clear out excess fuel from the crankcase.
Pinch the fuel line just where it enters the carb fuel inlet nipple. The motor should run for about 3 - 5 seconds, then pick up speed. If you continue to pinch the line, it will speed up and die. If it runs much longer than 3 - 5 seconds, your idle is too rich. If it speeds up and begins to die in less than 3 - 5 seconds, the idle is too lean.
To set the top end, take off, hover briefly. Give full throttle/collective and climb vertically as long as you can (and still maintain control). Listen to the motor. If it begins to bog and slow down, your top end is too lean.
When you have the top end set close to where it needs to be, the heli will climb vertically, will climb with authority, and should accelerate as it does, the motor will continue to pull strong and not bog down.
If you are using a governor, disable it until you have the needles properly set.
And don't blame a new jug of fuel for tuning problems. Tuning a nitro motor is a learned skill/art. Be patient and learn. And you're best to run just a bit on the rich side. It makes for a healthy motor.
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