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Plane moves when 91 Magnum 4-stroke at Idle

I have read this forum and thank you for your knowledge. When I read the section about setting the low speed idle it says to "just set the airplane on the ground and set the idle so the airplane is on the verge of moving.

My problem is that I can't get it to idle low enough to just sit still.

Any suggestions?

I have a .91 four stroke magnum.


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Some questions...
by: Matt


Sorry it took so long to respond. Been a busy day at work.

How does the engine run while flying? Any dead stick landings or any type of hesitations while in the air?

Do you have a tachometer? Is the engine running so fast at idle that your plane gets away from you are is it just barely putting forward?

How does it act when you try to lower the low speed? Does it die abruptly or does it studder or hesitate before dying?

How is your engine mounted on your airplane? Is your engine mounted right side up? Inverted? Sideways?

Make sure she's not running too rich on the high end, or two lean for that matter. Richen the high end until the RPM's max out on our tach, then back it off just little bit. If the high end is out of wack, this can cause problems on the low end.

The first and probably easiest thing to try (if you haven't already) is replacing the glow plug. You wouldn't believe how often a worn out or faulty plug ends up being the solution to a days worth of frustration...

Or maybe trying a hotter plug?

Another quick and easy check is to eliminate bad fuel as the source of your problem. See if someone will let you borrow a tank of their fuel to see if that helps.

I trust you have checked all of our fuel tubing. The clunk is in the bottom of the tank etc. Check the elevation of your fuel tank, the center line of the tank should be pretty close to the elevation of your high speed needle.

by: Jake

Sorry I didn't Mention that I just bought the plane. Also it is mounted sideways. So I ended up setting the low end valve back to the factory setting and that seemed to fix the idling problem but now when I run it for a while trying to tune the high end I make progress but then all of a sudden it will die. After that point I can't start it again for another half hour or so. (Any ideas there). Also I'm going to invest in a decent tachometer to help. While tuning the high end, how long should i wait between increments turning the valve?

Its running too hot so it is probably too lean.
by: Mike from Oz

G'day I spend the morning on one of these engines. A friend gave it to another friend.

A bit of background you may already know. The carby is a twin needle model. The low speed needle governs the mixture up to about 85% of full throttle. After that, the high speed needle takes over. If the LSN is too lean, the HSN will never be able to make the engine run rich at high revs. If set like this, they can be impossible to start at less than full throttle.

1. They like a bit of castor oil in their fuel (about 2 to 5%). Without it, they do tend to run hot.

2. Always try to tune starting with the engine too rich so I would start by opening the low speed needle (LSN) by about one full turn. It will run like this but it will be rich.

3. Run the engine at full throttle. Tune the HSN to maximum revs (peak lean setting).

4. Slow the motor as slow as it will idle then tune the LSN for best idle. This may still be a high idle. Generally the revs will increase as you do the adjustment and if so, you are on the right track.

5. Now go to full throttle and repeat step 3.

6. Now repeat step 4.

7. Keep repeating 3 and 4 until you get the best idle consistent with good transition from low to high throttle. Better to be slightly rich on the LSN.

You will have to re-adjust the idle setting several times as you do this.

If it still won't idle, you have a problem with -

Fuel - water in it?

Bad Glow Plug?

Poor tank positioning?

My recommended fuel for ASP/Magnum four stroke engines is 10% nitro, 5% castor oil, 15% synthetic oil and 70% methanol.

Hope this helps.

Mike in Oz

new problem
by: Jake

Thank you for your comments.

So i just realized that my motor is a FS series Surpass, not the magnum. So latest update is that I think I have the low end tuned right (after an idle of about 30 seconds I throw the stick to full throttle and it responds normally...i think).

My problem now is that I don't notice a difference in RPM's from about half throttle to full throttle. In fact, sometimes I will notice a decrease in RPM's as I push the throttle to full.

Any ideas there? Could that be the low end valve or would that be an issue with the upper end?

Top end is too rich
by: Mike from Oz

Hello again.

An OS Surpass 91 is the engine from which the Magnum/ASP engines were clones so any comments about the one also refer to the other.

You did not say what sort of plane you have this in. You also did not say what prop.

But first to the fact that the engine slows past a certain point as you increase the throttle. If you remember, I said that the LSN controls the bottom approximately 85% of the throttle travel and the HSN the top !5% or so. It sounds as if you have the HSN slightly too rich so that as control is handed from the LSN to the HSN the engine goes from being about right to being too rich and so does not go any faster.

If you cannot get the engine to idle low enough you have two possibilities -

Either you have not tuned it properly or the engine is too large for the model. You can reduce the thrust by using a prop with less pitch and this sometimes helps with overpowered models.

You may also have a mechanical problem in the throttle linkage so that the throttle is not closing sufficiently. Most engines idle with the throttle opening large enough to put a pin in the hole. (About 0.5mm) It is worth checking that the throttle is closing sufficiently.

Hope this helps.

Mike in Oz

Plane model
by: Jake

Sorry it is a matt chapman 580. I am running a 13x8 on it right now but I will probably change it to a 13x6. Thanks for your comments they are really helpful. I will keep you posted on what happens.

idel problem
by: Anonymous

hi ihave a magnum91 ,never hat a drop of glofuel.i ren that motor ,and hi speed and low speed was exelant.i made one flye ,and went hom.five days later i try to fly ,icant get no idel,every time iget to low the motor stops.the setting from the factori is 4 and a quarter turn.witch way you turn the idel?

ASP 91 4strk idle
by: Cockpit2

Hello fellow rc modellers,I read with hi interest your comments about idling etc.All good with the glowplug you refer but what about using a sparkplug with its electronic ignition?

I'll be thankfull having an answer.

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