Calvinman

Hi guys and gals

I acquired a MDS 48 that appears to have had little to no time on it. I ran it a bit in an aircraft (at idle it sounded like a dirt bike...ring...ding...ding....ding) It made gobs of power but it was a bit unreliable though so I did some research and found it had to be run in for an extended amount of time so I ran a gallon through it at a very rich setting. Now at anything over half throttle it quites. After reading a thread in the Beginners thread about air leaks I dug the motor out of storage and had a look at it.

I noticed that the crown of the piston seems to be copper colored....is this normal or have I peeled something. The compression still seems very good, but storage oil will make it seems tight so not sure.

Calvi

1QwkSport2.5r

The piston in my GMS .47 is the same way. There is a light copper colored band around the top of the piston. Feel the piston at the crown and see if there is any difference in size between the band and the rest of the skirt. The piston in my GMS has a small taper at the top of the crown. The band is very narrow. It could be a modification to ever so slightly open the ports a little sooner - possibly done after the liners were made.

blw

My Feedback: (3)

Calvi- those leaks can be devious. Treat fuel tubing as part of the leaking problem. The inner surface gets soft and gooey, and the tubing collapses in on itself. It's happened to me on my fuel can as well as on an engine. Brass tubing can develop cracks inside the rubber stopper on fuel tanks. The fuel tubing from the remote to the carb gets overlooked often. Crankcases can have hairline cracks.

Calvinman

I think I can see the taper in the copper colored band 1QwkSport, I am going to strip it down next week and have a closer look while I check for any source of air leaks.

Mr Cox

There is a relief at the very top inorder to avoid damages to the piston in a tight ABC engine. I doubt there is any copper there, it just takes on a darker colour from the oil and the high temperature.

1QwkSport2.5r

I'm not sure what it is on the surface, but in the case of my GMS, the same type of band around the piston crown doesn't come off. My GMS hadn't been run enough to build up any residue. FWIW, the bottom edge of the band on the GMS piston is dead straight all the way around the piston. Ill see if I can find the GMS and post a few pics of the piston.

Mr Cox

I don't have any of the MDS .48 myself, only a few of their smaller engines (.15, .17 and .18). I have never seen copper on an aluminium piston, and cannot see any purpose in that either, I could be wrong though...

Here is what the relief typically looks like in a new engine ( a REX .12 in this case):

jrf

My Feedback: (551)

Calvin:

Sounds like you need to replace the glow plug. I doubt that there is anything wrong with the piston. It is an ABC engine. That means that you can not break it in by running it rich. Once it is oiled up, you should set the needle valve so that it is a few clicks richer than top rpm and then fly it.

MDS engines are OK, but famous for having a terrible carburetor. You will need to put a Perry carb on it to make it run right.

Calvinman

I read that some had a better carb on them? Anyway, I will take some pics after I tear it down (hopefully in better focus). Maybe I'll find the source of the air leak(if there is any). What size or model Perry would fit the MDS jrf?

Calvi

1QwkSport2.5r

Pictured below is my GMS .47 engine. The band looks more like bronze - almost as if cast with the aluminum and then machined. I'm sure the piston in this engine is cast and machined, not machined from barstock.. If I get time to pull the engine apart, I'll look at it closer.

1QwkSport2.5r

I wouldn't bother with a Perry. I would put a SuperTigre carb on it in a heartbeat if it fits. Measure the neck of the carb where it goes into the engine if you could.

FWIW - MDS and GMS had similar problems with carb's. Some work out of the box, some need modification to work properly, and some just can't be fixed. The Metallurgy in the MDS is likely better than GMS - I have an MDS .40 piston and liner in a Fox .50 case that is still pretty tight even after over 10 gallons of fuel.

Mr Cox

The GMS .47 pictured above looks like it has a dykes ring on it...?

The smaller MDS engines have great twin-needle carbs with rubber seals both below and above the pinchbolt. I have even bought a few extra of those carbs to use on other engines.

The GMS ones did have a design problem when the remote needle was used.

1QwkSport2.5r

Not ringed. ABC. FWIW, GMS had problems with both carb's - needle on carb and the remote needles. Mine was obstructed on the inlet which I fixed, but there's still a problem with the idle needle valve (mine has he NV on the carb). Can't get a good idle and transition. One or the other, but not both. I think the idle needle is too long. It screams on top with a tuned muffler though.

jrf

My Feedback: (551)

Go to perrypumps.com and get the Medium carb that fits the carb mount of your engine. You do not need a pump and should not get the pump carb. If you can't measure the carb mounting hole, call Perry, they will probably know.

1QwkSport2.5r

There ARE better options than Perry carb's. ST carb's are far better and easier to set than those finicky (pesky) Perry carb's. I have some Perry carb's and I removed them to swap in other carb's. Much happier that way.

Making sure the MDS carb is in fact a problem first is the first thing to do. Don't go throwing parts at an engine unless you know they're bad. Keep in mind -> Perry carb will cost more than the engine is worth!

jrf

My Feedback: (551)

Italian made Super Tiger carbs are an excellent carb if you have an expert who can tune them for you. We had one here and lots of the local Super Tiger engines ran beautifully. Unfortunately our ST expert passed away and since then nobody can get one to run as nicely as the old ones used to. The Chinese made Super Tigers were never as good either. So if you can find an Italian made ST carb that will fit your engine and you have someone nearby who can tune it, that would be the way to go.

1Owk, you must have been using a Perry Pump carb without the pump. We have saved a lot of cranky MDS engines with Perry carbs.

So now the OP is confused and still hasn't gotten his MDS running right. Sorry about that Calvin. Either way though, you are going to have put some money into that "good buy" engine. Maybe a new ASP or Magnum would be a better idea.

Calvinman

No sweat jrf, I got the engine in a lot of 9 other engines, and I did have it running strongly, so hopefully it is just an air leak. If not no big deal, just hate to leave an engine with potential to sit and not get used.

1QwkSport2.5r

Setting SuperTigre carb's is easy....... IF YOU READ THE INSTRUCTIONS! That's the number one problem with people tuning ST carb's. They don't read and understand the instructions.

Incorrect. I could get a standard non-pump carb to work fine... Until one spec of dust gets into the carb and then you can't get a reliable setting until you tear apart the plastic carb taking extreme care not to damage the plastic carb body. No thanks. I'm not much a fan of the spring loaded needle valves that move on their own either..
I don't think the OP has spent enough time with it to get it tuned or determine if the carb is bad. I would absolutely be pulling a carb from another engine and trying it in the MDS - but only after spending some time tuning the MDS carb.

Calvin - did you measure the neck of the MDS carb by chance? Choke size (ID of carb)?

Calvinman

Using my ultra accurate(not) plastic slide caliper and not taking the carb off it looks like 16mm?

Calvinman

Ok, I took the carb off it is 14mm, does that sound right?

blw

My Feedback: (3)

That's a very good picture, Tim.

the pope

I remember reading that the machined carbs were the good ones and the cast alloy carbs not so much . This info might help u save some $ .

1QwkSport2.5r

Sounds about right. I'll check my parts box and see what carb's I can find that have the same neck size..

1QwkSport2.5r

Good old smartphone camera.. They say that smartphone camera technology is so good, Russian hackers can copy your fingerprint from a smartphone picture. They say NOT to hold up a peace sign when taking selfies - that's how they usually get your prints. Crazy the times we live in.

the pope

U dont need to worry about that Tim . They could just hack into the F.B.I. and get your prints that they have on file !