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  1. #1

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    An MDS.61 that is impossible to tune

    I have an MDS.61 that will start,tune the high end, then dropping back to idle will not advance beyond 1/4 throttle.No amount of needle tweaking helps. I have even tried an on-board Glo-driver.The engine hand starts easily and has the best idle I have ever seen.When you can get full power it cetainly has plenty of grunt. So excellent idle or plenty of go, just not both at the same time.Would changing carby help? Or should I take a sledge hammer to it? I also have a MDS .25 I bought for $50 brand new. It is a little gem and I have no complaints with it. Can someone save MDS's tarnished reputation? My other question is this: Since 4strokes cost double the price of equivalent 2 strokes , what are their advantages? Specifically I.. I am debating between a Magnum XL- FS 120AR Four stroke.a Super Tiger G90 and an O.S 120 Ax. I have also seen an ASP 1.20 Four stroke advertised very cheaply but I have never heard anything about these engines.Any advice will be greatly appreciated. P.S . The engine is to power a 71 inch span P-51B from the David Vaughan plan

  2. #2

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    RE: An MDS.61 that is impossible to tune

    MDS engine.

    Remove the carb and check for metal flashing (left over metal after machining). Sometimes fuel nipples (screw in) are blocked by flashing in the carb body at the seat of the hole where the fuel inlet nipple is threaded in place. Check for particles blocking the fuel passages. Also ensure that the muffler pressure nipple is drilled all the way through, as it should be.

    Ensure that the carburetor is seated and leak free upon reinstallation. A little bead of silicone rubber glue around the base of the carb would prove beneficial.

    Check the backplate for a proper seal. Again, if in doubt, a small bead of silicone rubber around the engine's backplate opening couldn't hurt. Use the silicone sparingly. You don't want silicone extending into the crankcase and running afoul of the crankshaft while in rotation.

    Check that the cylinder head bolts are uniformly snug.

    MDS engines can be finicky regarding fuel nitro content (lower is better) and glow plug selection. Try different glow plugs if the problem persists. Choose a prop toward the low end of the selection range while breaking-in the engine. Yes, even ABC engines can need some break-in time.

    Don't take a hammer to it. It is still worth something to someone else who needs parts, or who is accustomed to MDS engines and their quirks.

    Good luck and let us know how you fare.


    Ed Cregger
    \"Practice makes prefect\"

    Saito Club Member #52

  3. #3

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    RE: An MDS.61 that is impossible to tune

    I have an MDS 78 that has similar issues, and I just got tired of dinking with it. Not quite as bad as yours, though . . . it would idle great, and run full throttle great, but if you tried to stay at mid throttle for any length of time, it would load up and die. High needle didn't have any more room to lean without going too lean on the high, and any leaner on the idle needle and the idle wasn't stable.

    So, I spent a few bucks, threw a Perry carb on it, and it's perfect on the test stand now . . . one of these days, I'll get it
    into a plane again . . .

    - Tim

  4. #4

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    RE: An MDS.61 that is impossible to tune

    That's what I was going to suggest - a different carb, but forgot to include it in my note. Still, I would check the stock carb to see if it is obstructed in some way internally, or leaking air.


    Ed Cregger
    \"Practice makes prefect\"

    Saito Club Member #52

  5. #5

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    RE: An MDS.61 that is impossible to tune

    My money's on an air-leak too.

    If the carby leaks then you'll probably have tuned the low-end needle so fat (to compensate for the air leak) that once you open the throttle a little it's effectively running *way* rich.

  6. #6

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    RE: An MDS.61 that is impossible to tune

    Thanks for the advice, i will try your suggestions asap

  7. #7

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    RE: An MDS.61 that is impossible to tune

    Hi!
    What fuel are you using? Try 5% nitro! If you use too much nitro the engine could not be set properly.It will be very sensitive to any needle setting.
    Have you set the low speed needle/Idle needle?
    Jan Karlsson - Supplier MVVS Products

  8. #8

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    RE: An MDS.61 that is impossible to tune

    In terms of fuel, I broke it in using 20% castor and 80% oil. I have tried different nitro percentages and nothing really makes a difference.I forgot to mention that this engine can only be stopped when it runs out of fuel.It is amazing to see the carby barrel fully shut and the damn thing still keeps going! from what the other comments say, I suspect that there is a massive air leak somewhere.My next course of action is to try a carby from a sweet running Thunder tiger .61 I have.I've used gasket paste to seal the backplate and have checked all bolts and looked for flashing. If the new carby finally cures the beast, then I will fork out for a Perry carb.

  9. #9
    Sport_Pilot's Avatar
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    RE: An MDS.61 that is impossible to tune

    In terms of fuel, I broke it in using 20% castor and 80% oil.
    Well there's your problem! You got to have at least some methanol!
    Glow Head Brotherhood #15

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    RE: An MDS.61 that is impossible to tune

    Well spotted! Obviously that should be methanol.Cheers

  11. #11
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    RE: An MDS.61 that is impossible to tune

    G'day If your engine keeps running with the carby completely closed, then it is getting air from somewhere and you almost certainly have a leak. One problem I have seen is carby barrels which are too small for the carby housing (badly machined). One temporary fix to prove the point is to put some grease on the carby barrel and give it a run to see if this improves things. Silicon grease will not wash away so easily and is a semi permanent fix for some. I suspect you are going to be buying another carby eventually. All the best. Michael from Oz
    \"I just had no control. Must be the radio.\" Club Saito #597 Kadet Brotherhood #66

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    RE: An MDS.61 that is impossible to tune

    The problems your having is the reason the MDS line was recalled and is no longer for sale in this country. I did some RD for Horizon and tried every card combo there was and could never get a fix that would work. Some have had good luck with them but most experienced the same thing your having. There are to many good engines on the market now so don't waste your time dinking with MDS. In my option they are nothing but a paper weight. I had to take back everyone that I sold here and I'm not the only store that did it. Dennis
    DadstoysRC. I fly what I sell
    CD Vette City Big Bird

  13. #13

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    RE: An MDS.61 that is impossible to tune


    ORIGINAL: DadsToysBG

    The problems your having is the reason the MDS line was recalled and is no longer for sale in this country. I did some RD for Horizon and tried every card combo there was and could never get a fix that would work.
    My MDS 61 went very well with none of these problems until its crankshaft smashed into a million pieces but it certainly idled and throttled just fine for an engine of its era.

    Here's a vid of it in action:

    http://nz.youtube.com/watch?v=ESpwvfm5oF8

  14. #14

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    RE: An MDS.61 that is impossible to tune

    I see plenty of replies to most your questions, accept the "4 stroke verses 2 Stroke" .
    I am not experienced enough to comment on Yey or Neys of these motors but will tell you I bought the Siato 4 stroke .71 for it's classic "Tone" and Luggish sounding nature for my Balsa Piper J3 Cub.

    I wasn't concerned about horse power and if I noticed anything at all it is that,
    IF YOU WANT POWER, low cost and low weight buy a 2 stroke, power for the dollar they beat all, My hyper 50 in my Raptor helicopter would smoke the .71 Siato, but I didn't want power this time around for the Piper, I wanted a good "Tone" large Prop and lower cruising rpm to give it a scale like look, feel and sound. As for what’s the "Better Engine"? I can't dog on 2 strokes because so many of them are running like a million dollars worth compared to yesteryears motors. My Hyper OS 50 starts right up with a slow running 18 dollar Wal-Mart electric drill. I f you don't care about Tone or are wanting a ripping high performance speed demon I would go with the 2 stroke for the lighter weight and more power at a lower cost the 2 strokes are notorious for. My .71 Siato has no more Horse power than the .46 2 strokes out here. But it sounds like a mini Harley under my Cowling. ONE more thing, I personally think a "Ringed" piston is better then a butt naked one regardless of how many strokes.
    [8D]

  15. #15
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    RE: An MDS.61 that is impossible to tune

    Sorry - I missed the question about four stroke vs two stroke so here's my two cents worth on that subject. Two strokes develop their peak power at higher revs than four strokes and generally, if you want a fast model, go for a two stroke. Four strokes are more torquey than four strokes - that is, they develop their peak torque at relatively low revs but will swing larger props at those revs. For scale models they also sound more like the "real thing" They also use less fuel and make - to my ears - a more pleasant sound (not that a Nelson 40 on its 7 x 7.25 prop does not sound good). I am also told that their throttle response at mid revs is better and so are popular in 3D applications. They are also happy to work in their mid range and near idle without protest for long periods once set up correctly. The sight and sound of a Laser 150 gently ticking over at a low idle for an extended period and then roaring to full power just has to be seen to be believed. The big Saitos and OSs will do this too when set up correctly. The Laser will happily do it with no nitro.

    But

    Four strokes are far more expensive, are far more complicated and as a result, require more maintenance. They have tappets to adjust (apart from RCV engines), they are usually ringed and will eventually need a new ring. They do, however, last a long time. I bought a Saito 45GK second hand, used it for a year, sold it to a bloke who used it for 10 years and then I bought it back and used it some more. It eventually needed new bearings which I fitted and I then gave it away to a young kid who is still using it some years later. When pulled apart after about 12 years use, it still showed the hone marks in the cylinder but even then did not need a new ring. It is now close to 20 years old and still running fine.

    As a result, most of my engines are four strokes. I have a couple of two strokes in trainers and in a stick but the rest are Saitos and OSs in scale and semi scale models and one lone ASP 61 (very cheap and very light) in a Goldberg Cub. For my kind of flying, it has to be four strokes.

    The two pictures are of my Saito 90 3 cylinder radial nestling in its Hanger 9 Sopwith Camel and a little Saito 30 mounted upright in a Phoenix Tiger Moth DH82a (to look a bit like the engine in a DH60 Cirrus Moth). The Tiger Moth has been flown several times recently and flies really will with the undersized engine and the Sopwith will get its outing once the August Westerly Winds die down here.

    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    \"I just had no control. Must be the radio.\" Club Saito #597 Kadet Brotherhood #66

  16. #16
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    RE: An MDS.61 that is impossible to tune

    Mike - I appreciate your input on 4s vs 2s. But gosh, what happened with those pictures? They are huge! When I click on the "enlarge" button they actually get smaller and easier to discern. The text lines seem to have elongated to fit the pix so the text is 10 screens wide. I copied it into my text editor to read it.

    Not complaining, mind you - just trying to understand what's going on. Can anyone help with this?

  17. #17

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    RE: An MDS.61 that is impossible to tune

    Thanks to everyone for their input.Regarding the 2 stroke vs 4 stroke debate,I've decided to go with the OS 1.20 Ax.It cost the same as a ASP 120 4 stroke,but it puts out according to the published figures 3 Hp ,swings a 17'' prop at between 9000-10 000 rpm, thats in the the same range as similar 4 strokes but with twice the grunt!!!! Soundwise it certainly won't be an annoying little screamer.The only other concern I had was with the great little mufflers that 4 strokes have.They would certainly fit into the confines of a P51's cowl better than a 2 stroke's bulky item. What ultimately sold me to the OS120 is the fact that you can swivel its muffler 90 degreed to make a Pitts style approximation.Can't wait to pick one up. Regarding that troubsome MDS, I immersed the muffler in a cup of fuel,closed the barrel all the way and blew through a fuel tube into the venturi. You should have seen the bubbles!!!! As a stopgap until I get another muffler I Smeared silicon onto the barrel and that should help stop the leaks. Don't think I'm fretting over this engine.I've had it ten years and solely for my personal satisfaction I want to see if it is possible to tune it.It is on a test stand, as I would'nt trust it in an aircraft.

  18. #18

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    RE: An MDS.61 that is impossible to tune

    I owned an MDS-same problem. Ditch the carb and replace with any good one. Another gentleman I knew had the same problem. He ditched the carb and replaced it with a Supertigre carb. Problem solved.

  19. #19

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    RE: An MDS.61 that is impossible to tune

    I have a MDS .40 engine that was just sitting in a box due to the same reasons you explained. It ran really erratic and I could never get it to run right. Then I read a thread here on RCU about MDS engines and the author knew his engines! Basically he said, "Place a small piece of fuel tubing over the high speed needle valve to prevent the carb from sucking air through the sides of the needle. Next, change the glow plug from a "normal" plug to an idle bar glow plug and use 10% or less nitro and you should have a great running engine!" I figured I'd dig out that engine and wipe the cob webs off of it to give it a try. I performed all the changes this week with mine and it's now turning 12,300 rpm's and it transitions right on the money. The best it has ever run without me fighting the needle valves all day long! Just make sure it's not getting any air leaks from the back plate or underneath the carb like the others have said. Just give it a try first instead of spending a whole lot of money on a new carb. One other thing, before you put the fuel tubing on the high speed needle valve...take out the needle and check the o-ring to make sure it's OK, isn't dried out and doesn't have any splits or cuts and it's in good condition. A bad o-ring will make your day a whole lot longer! Hope this helps and the cost is minimal.

  20. #20

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    RE: An MDS.61 that is impossible to tune

    Success!!!!! Of a sort... Today I annoyed the hell out of the neighbours by finally hitting full throttle.The engine really has some balls. The low end needle is unscrewed to it's limit and the engine coughs and stutters as the throttle is opened but before where it used to die it kept running.As there are no other mods to be made with the current setup next week I will get a new carby and hopefully it will behave itself well enough to be trusted in a plane.

  21. #21

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    RE: An MDS.61 that is impossible to tune

    Great video.It really hauls that trainer around. What carb are you using on that MDS? It doesn't look like the standard MDS unit.

  22. #22

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    RE: An MDS.61 that is impossible to tune

    Yep, that's the stock standard carby that came with it.

    I had to make a new barrel-retaining screw (the original broke) but apart from that, no changes.

  23. #23

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    RE: An MDS.61 that is impossible to tune

    Change the carb with a Super Tigre one or with a Perry. The engine is powerful but the carb is a mistake. I had the same problem on a .40 MDS.

  24. #24

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    RE: An MDS.61 that is impossible to tune

    If you want to see where an MDS anything is sucking air check the BEARINGS - you'll probably find the front one looks like Swiss cheese and is the source of all that extra air. Find a source for a high RPM SEALED bearing to replace it and watch your engine behave.

  25. #25
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    RE: An MDS.61 that is impossible to tune


    ORIGINAL: cenoyes

    If you want to see where an MDS anything is sucking air check the BEARINGS - you'll probably find the front one looks like Swiss cheese and is the source of all that extra air.
    Cenoyes,


    I repeatedly write this...

    Take a look at some of the better engines, from Fox and from Webra, for instance...
    ...Also, bushing engines that do not employ any front bearing...

    Do you see a sealed bearing in the front, or a seal? Of course you don't.

    ...Yet, they can all be adjusted to idle and to transition immaculately...


    How, you ask? Because simply put, sealing the engine is not a part of the front bearing's 'job description'...

    This is done by the crankshaft within the crankcase.
    The front-rotary-valve assembly rotates and determines, when air will be sucked in through the carburettor and when it will not.

    At the same time, this valve is what seals the engine against air leaking in and out (and fuel leaking out) from the front.


    Using a sealed bearing, to overcome a front rotary valve assembly machined out-of-tolerance, or simply worn-out, may solve the problem and allow the engine to run.
    But the 'Swiss cheese' bearing attribute, is not the reason for this...

    It just tells you the workmanship of the ΠœΠ”Π‘ company that manufactured this engine, is rather lame... It no longer does, I believe.
    Dar Zeelon - ISRAEL - ddzeelon@gmail.com
    MVVS - Jett - MS Composit - Rossi - Mejzlik


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