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May 25, 2019, 09:23 AM
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MDS .48 experiences


Hey guys,

I recently picked up two MDS .48 pros in a trade and they both look like they have very low/no time on them. Now, I am aware of the controversy surrounding MDS engines and their supposed crappy carbs and cantankerous dispositions, but I've also heard that these can be exceptional powerhouses as well. Since these came with a lot that I traded for I figured what the heck and decided to keep and run them......it's been an adventure!

The first thing I did was to strip them down and replace the bearings due to the rough feel of them and I also cleaned the insides really well just in case there was any swarf or dirt that might have been present. When I reassembled them I took extra care to use some RTV to seal the draw bar and the carb to the crankcase as well. All looked fine and felt good, so I popped a new OS #8 in each one and set up the test stand to see how they run. I have read that they are high compression engines, so I had a half of a jug of homebrew 80/20 meth/castor fuel that I thought I would try them out on.

Results.... 6000' ASL, MA 11/5 prop, OS #8 plug, Home brew 80/20 fuel, 52* temperature, 22% humidity.

Engine #1 - Set throttle to half and it fired right up and I thought it was going to be an easy run getting it dialed in, but after letting it warm up for about 30 seconds with the ignitor on I advanced the throttle, leaned it out to a slightly rich two stroke and then brought it down to idle. It idles really well and I was happy with how it was going so far......and then! I opened the throttle up and it was fluctuating like crazy at top end, but would idle and transition just fine, but the power was almost non-existent. I couldn't get it to hit higher than 10,200 and it was surging and fluctuating, running very rough and quitting for no good reason from time to time. I thought that it could be the fuel, so I changed over to some cool power 10% with added castor to bring the oil up to 23%. That seemed to really help with the RPMs and it was running a little better now, but still rough and fluctuating RPMs. I pulled it off the stand suspecting an air leak somewhere and went through the whole system from tank to engine trying to see if I had an air leak. I replaced all the tank plumbing and new fuel lines to the engine as well, put it back on the stand and fired it up and nothing changed.....now I'm peeved!

After taking a fishing break and clearing my head I decided to try and figure this thing out one more time. This time I started trying different plugs to see if I had the wrong heat range for the engine. Tried an OS A3.... nope! Mccoy MC 59....nope! Etc, etc, etc....then I remembered reading something about someone needing an idle bar plug to make their .48 run well, so I figured why not and scrounged around in my stuff until I found 3 never been used K&B L with idle bar plugs. I put one of those in and flipped the prop over and VIOLA! It runs like a top! Idle is perfect down to 2700 and top end is right at 11,000 and transition is great as well. I ran 3 tanks through engine #1 and it is steady as rock. Happy camper now!

Engine #2 was given a Super Tigre carb off of a deceased .51 and sealed up really well. As a control I tried it with the homebrew fuel and OS #8 plug and got the same crappy running as #1. Switched to the Coolpower fuel with added castor and better, but still sucked and then put in the K&B idle bar plug and again....perfection! It is the plug all along!

These MDS engines must have porting that is directing the fuel right at the glow plug or something, because even the hot OS A3 plug wouldn't run 'em right, but the idle bar on the K&B plug seems to be shielding the glow plug element from direct spray. Now my problem is needing to find some more idle bar plugs....not very common any more.

I've never owned an MDS .48 before so I'm not sure, but I would have expected them to wind up better than 11,200 with a MA 11/5 prop. My ASPs, Magnum, and Thunder Tiger .46s do about 13,500 with this prop, so it feels kinda weak to me. I'm wondering if these are actually "no run time" engines and curious if they will loosen up and gain RPMs with another gallon through them? I can peak them to hit 11,300 at extreme lean, but do not want to run them that lean.
Anybody else have experiences with these MDS .48s?

Mike
Last edited by gmwahl; May 25, 2019 at 09:30 AM.
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May 25, 2019, 09:53 AM
Engine man
dmrcflyr2's Avatar
I have had experience with the following MDS engines, 40, 48, 68, 148. The next remarks are my opinions only. MDS engines of questionable/inconsistent quality. Sometimes you get a good one, sometimes not so good. The only MDS I own at this time is the big 148. I own it only because I was comparing large 2 strokes, and I do not plan on keeping it. I would never install another MDS engine in an airplane because there are just too many other much higher quality engines out there.
May 25, 2019, 10:32 AM
Registered User
earlwb's Avatar
Usually if you have a problem it is in the carburetor. Once you solve that problem the engines run fine. Now then one issue is compression, in Europe and Russia they typically use methanol only fuel without nitromethane in it. So problems happen if you try to use 10% to 20% nitro glow fuel in them. They may have corrected that issue later though. I have a few MDS engines and they all worked fine, albeit the carbs needed to be dealt with.
May 25, 2019, 11:03 AM
A man with a plan
Balsaworkbench's Avatar
If you believe your engines are low on power, maybe you should try some fuel with less oil. 23% is a lot of oil, and oil doesn't make power.
May 25, 2019, 01:32 PM
Registered User
Thread OP
Quote:
Originally Posted by Balsaworkbench
If you believe your engines are low on power, maybe you should try some fuel with less oil. 23% is a lot of oil, and oil doesn't make power.
Ya, I already thought about that. I'm going to mix a fresh batch of 5% nitro, 18% oil, and the balance methanol. I only added the oil to 23% in order to make the cool power less dangerous to engines by adding castor oil. We shall see how the new fuel works, but I haven't generally seen much of a performance drop with higher oil fuel.

Mike
May 25, 2019, 01:49 PM
A man with a plan
Balsaworkbench's Avatar
Back in the early 90s the local hobby store used to sell fuel that was labeled 11-11, meaning that it was 11% synthetic and 11% castor. I used to buy whatever was the cheapest, so I picked up a gallon because it was about 9 bucks. I think they kept it around for the Fox guys, which is why it was marked down. The Fox market wasn't very big. I suspect that the Fox guys also preferred all castor and zero synthetic.

Anyway, I soon found myself asking the guys at the hobby store to help me figure out why my engine wasn't running as well as it used to. They asked me what fuel I was using, and of course they thought it was way too much oil. They were right. I went back to Omega 5% and everything was fine. The funny thing was that I was running OS FP engines, which have bushings on the crankshaft and thus require extra castor oil according to the conventional wisdom. But they sure ran better on the Omega. I ended up pouring the remaining 11-11 a little bit at a time into the next half dozen jugs of fuel I bought to get rid of it.
Last edited by Balsaworkbench; May 25, 2019 at 03:47 PM.
May 25, 2019, 03:27 PM
Registered User
Thread OP
Quote:
Originally Posted by dmrcflyr2
I have had experience with the following MDS engines, 40, 48, 68, 148. The next remarks are my opinions only. MDS engines of questionable/inconsistent quality. Sometimes you get a good one, sometimes not so good. The only MDS I own at this time is the big 148. I own it only because I was comparing large 2 strokes, and I do not plan on keeping it. I would never install another MDS engine in an airplane because there are just too many other much higher quality engines out there.
If these engines stay reliable through the next gallon or so of fuel then I will probably put them on something, but like you, I am leery of them to some extent. They DO have a true ABC piston/liner and will probably take a lot longer than an OS to wear in all the way, so maybe the performance will improve with more running in.

The carbs on these are the C2 second generation carbs, but I still don't trust them until I get some time on them. That's why I put the Supertigre carb on one and did a soup up job on the other. Right now these engines and carbs are running flawlessly and are equal to the best engines I have when they are idling and through transition so for the time being I will give them the benefit of the doubt.

Mike
May 25, 2019, 03:48 PM
Registered User
I TOBOR's Avatar
The .48 through 2.18 MDS are fine engines, if the carb has MDS cast on the front they don't need another carb.
May 25, 2019, 04:02 PM
GloBroz PowerLab
1QwkSport2.5r's Avatar
I have several MDS engines. They’re nice runners. One is even a factory made 3.5cc marine diesel.
May 25, 2019, 04:53 PM
Registered User
fiery's Avatar
I agree with I TOBOR.

My MDS .48 was picked up NIB for a song.

I was pleasantly surprised. Great tolerances and made from really high quality materials. Piston fit is very tight, as is usual Russian practice. Will require real running in, not an OS two tankful’s then fly.

C2 Carb is just fine.

I like it. Not for a raw beginner though. Also, I used a Fireball yellow long reach idle bar plug, so I got lucky as it had no stumbling issues or loping at top end.
May 25, 2019, 05:32 PM
Registered User
Thread OP
Quote:
Originally Posted by fiery
I agree with I TOBOR.

My MDS .48 was picked up NIB for a song.

I was pleasantly surprised. Great tolerances and made from really high quality materials. Piston fit is very tight, as is usual Russian practice. Will require real running in, not an OS two tankful’s then fly.

C2 Carb is just fine.

I like it. Not for a raw beginner though. Also, I used a Fireball yellow long reach idle bar plug, so I got lucky as it had no stumbling issues or loping at top end.
You guys are making me a bit happier about getting these engines now. Both my .48 carbs say MDS on the front, so I knew they were the C2 carbs. I've been running glow engines for almost 40 years, so these carbs weren't tricky at all for me and I'm used to the 1/16 turn is too much type of needle in other engines I've owned.

What prop and RPMs are you using and getting on your .48 after it was run in?

Mike
May 25, 2019, 06:55 PM
GloBroz PowerLab
1QwkSport2.5r's Avatar
A decent 7.5cc engine should run 13,500-14,000 peak on a 10x6 using modest fuel. A good one will run 15,000+. That’s just general across the 7.5cc size range. I have many .45 size engines and some turn a 10x6 at 14,000 and some will go almost 17,000 on the same prop. Depends on the muffler and size of carb, typically.
May 25, 2019, 07:44 PM
Registered User
My MDS .40 got 12,800 on a 10-6 MA. with 10%. About the same as an LA or FP but less than the other BB ones. I don't know the history of it. It seemed ok on the test stand. My MDS .15s seemed to run pretty good. Except for the sand in the castings ruining the screws, the quality was ok on important things.
May 25, 2019, 09:15 PM
GloBroz PowerLab
1QwkSport2.5r's Avatar
My FrankenFox .50 that has MDS .40 ABC guts in it runs pretty good. About 12k on a 10x6 with some obstructed ports. That piston and liner had over 10 gallons through it before I got them. Not much for pinch, but the seal is good and it’ll run on 25% nitro.

I put an Enya .19 carb on my MDS .21 diesel because the MDS carb was missing parts. It runs, but being water cooled, I gotta find a way to keep it cool while getting the carb and compression “roughed in” so I can put it in the boat and not have it quit on me.

I’ll say that the MDS .21 Aero engine has a different bore and stroke than the .21 diesel. The diesel is more advanced timing-wise, but the bore is smaller IIRC. I bought the .21 Aero NIB to swap the pistons, but they’re not interchangeable.
May 26, 2019, 06:21 AM
Registered User
I TOBOR's Avatar
A Fox Miracle plug and 5% nitro content will go a long way toward running an MDS reliably. An Idle Bar plug is next best.


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