My MDS .40 on my trainer keeps stalling in the air. I can start the engine with a flick of my finger on the ground. When I hold it in the air it runs great. BUT: I've landed dead stick more than I have with the engine! It usually happens when I throttle down for aproach, but not always. On the ground when I throttle down and let it sit for more than 5 seconds and then throttle up it always hesitates like it will stall before it 2 cycles. I have adjusted the low speed needle everywhere, but the hesitation persists.

short OS #8 plugs
10% fuel

HELP..

Sounds like your low end is way too rich. But before your adjust the low end you need to have the high speed set properly. So set your high speed needle so your engine is 400 rpm down from peak at full throttle. Now drop to idle and use your radio trim to get the slowest reliable idle. Idle for 5-10 seconds and go to full throttle. If it quits you are too lean, if it spits out smoke and stubbles to high speed you are too rich. Kill the engine and adjust the low speed needle rich or lean, make 1/2 turn adjustments at first then as you get closer to a good setting make smaller adjustments.

You can usually get a good low speed starting point by setting the throttle to your idle position. Screw the low speed needle in GENTLY until it just touches closed. Blow into the fuel line and slowly open the low speed needle when you hear the air whistling out the carb stop turning the low speed needle.

What if it sputters even when the low speed is closed? I've adjusted top to bottom and I can't detect a difference.

The two needles interact with each other so if your high speed needle is set too rich you may not be able to lean the low end enough to get good transition. So double check your high end.

The problem may be that it is an MDS and if you read other posts you will find others with similar problems. Trying the things mentioned will probably make the plane flyable and with some run time the engine may just start to run ok.

I am curious as to why you are running a short plug in that engine, I would think a Fox Gold Post long or similar would be more suited.

I had a .46 MDS same problem you have. I think the problems is that the carburetor opening is to large (the venturi). What I think is happening is because of the large venturi when you set the main needle valve at full throttle to get the correct high speed mixture the low speed and idle fuel mixture is way to rich. Then to correct this overly rich mixture, the low speed needle has to be turned in to much to lean out the engine. This then upsets the high speed setting. The way I got around this to large carburetor was to set full throttle on the your radio so that the carburetor only opens a little over 2/3's on your .40 MDS engine now reset the high and low speed needle valves and see if this resolves or helps the problem with the dead sticks.

I know everybody wants to have their engine carburetor fully opened for max power but if the carburetor is to large for the engine this is the kind of problems to expect. This is the same type of problems full size car engines have if you try to use to large a carburetor. What is called drive ability or poor throttle response is the result.

As for MDS engines in general nobody in our club buys them anymore way to many problems. As for my .46MDS after many pulled hairs I used it for two seasons and bought a OS .46 I am in heaven what a great engine no tuning problems at all. I hate to say it but if you are just learning to fly you do not need the aggravation of and MDS engine I would invest in a OS or thunder tiger engine and enjoy your trainer much more!

Biker

Your response makes absolute sense. Looking at my venturi it seems cavernous compaired to my buddies thunder tiger 42 venturi. I am going to try the 2/3 setting. If that doesn't reep instant results I'm throwing a different engine in. Thanks again.

Well, I did try to reduce the "max open" on the venturi. Throttle response was improved, but it was not silky smooth like my everyone else's plane I've ever seen. Screw it. I pulled the MDS .40 and put the Thunder Tiger .46 pro that I was going to use in my Ultra stick in the plane.

MDS= Most Definitely Sucks
MDS=Might Dead Stick

Just mho

this are one of the most wonderfull engines around.
Powerfull and no porblems. I have 8 of these guys and no porblems at all.

Ive got seven MDS engines and wouldnt trade any of them. Got a 40 on a 3D Magic, will idle all day long and has plenty of power.Maybe Im just lucky, but anyone with those "most definitely sucks" MDS's can send them my way! Got a pair of 28's on a twinstar.

My MDS 40 flys great, but I did have a long break in period, almost 2 gallons sig (plenty of oil) fuel before it worked flawlessly. I don't think the plug your using is MDS recommended. I've used a number but found the RC fox long, with idle bar was always dependable and gave smooth transition. Also you might find that the 15% fuel is better. Myself and someone else at my field have used this combination. Give it a try and been very pleased with results.

2 gallons of fuel to break in a 40? Guess you gota wear them out before they run right. I need to get my mds out and run some more fuel thru it. Right now it is setting in the back corner of my cabinet with some old heavy rubber wheels that I was going to throw out.

I was helping a guy with one of those MDS things and the best way to keep it running was to just leave the glow ignitor on and fly. It helped him get through the break in period. My 2 cents says if it needs break in time than I have no time for it. Go O.S.

osfx 2 strokes or ys 4 strokes. Have not found any better.

Um...Most cases here probably your glow plugs too cold.
When new they work best with a hot glow plug.
Runs better when u leave glow ignitor on it definte sign your plugs too cold.

Finally made it to the field with the new Thunder Tiger. Used the same plug and tank from the MDS and the engine ran beautifully. I was able to pull off my first full loop and immelman turn (spelling ?) with the new motor. Thanks for everyone who replied.

Please sell it with the 50% discount because it is such a terrible running engine. I need to add to my MDS collection!

I've broken in at least 6 .40 MDS engines on trainer combos last summer and never had a single problem with any of them. One tank of fuel on the ground was all I needed for a reliable top end (400 rpm shy of max lean rpm) and half way decent idle that got better with each flight and subsequent leaning out of the low end.

MDS Secrets Revealed:

Cheap idle bar glow plug (Fox Idle bar works fine)
If no Idle bar, then OS 3 or OS 8 are ok.

5% nitro, maybe 10% when she is loosened up after 2 or more gallons.

Prop for speed, not thrust. 10x6 for the .40, nothing larger.

Lean out low end so when you pinch fuel line at idle, it will die after about a second. Try to keep that low end as lean as possible without affecting your transition.

High end, 400 rpm shy of max lean rpm. Pinch fuel line momentarily and you should see/hear/feel a significant speedup.

Last but not least, before installing an MDS engine, always remove backplate and flush it out with fuel to rinse any possible manufacturing residuals. Also ensure carb o-ring gaskets are properly installed per the manual. The rinse is something I do for all new engines to include OS to ensure no manufacturing debris is left in the engine.

The word from a motor manufacturing friend of mine is that the carbs on the MDS engines are bad and MDS knows it. I would put a different carb on the motor.

the MDS engines have terrible low end.
Trust me: Use a long reach idle bar plug.

I use a fox R/C long w/ idle bar

My MDS Idle all day long without any problems.
there is a problem with their Carbs and they replacing them all.
But even with the old Carbs I have no porblems at all.
I fly one plane since the 3rd season without any changes or even changing the glow plug. MDS 68.
Any questions ask me.

MDS engines are like women. Some are sweet and some just ain't right. It's all chance!

Like my Grandpa said, "Women aren't like other people" Neither is an MDS.

hehehehehe!