I am flying a 40 size Ugly Stick Arf. I was runing an MDS 58 2-Stroke but had a very bad engine vibration. Ipulled that engine off and installed a New 52, new prop. Ran several tanks of fuel thru engine during breakin on my engine stand and no
vibration problems at all. Mounted engine on airplane, and the damn thing wants to crawl all over the ground. Don't understand why, prop is balanced.

Craig

If the prop "IS" balanced using a prop balancer and the engine bearings are assumed to be good in the new engine, then you may have a loose firewall or loose engine mount. Since the issue is present with two engines and props, it is most likely airplane related.

If it's the beam type of mounts they may not be straight, you could have one at an angle too. I would do as ABD said and check the plane itself. I'm a little in shock to hear someone is using an MDS engine. They quit importing those a lot of years ago. I have several of them under the bench.

I still have no idea why. Prop and Spinner balanced. After first flight lost 1 muffler screw. Something stupid with the airframe. It's an ARFso I didnt screw up the building that bad. Anyone have experience with airframe vibrations?

I have built a plane that had a sheeted stringer fuse, very long fuse. The plane didn't vibrate at all anyplace except the tail feathers, nothing to do about it, just the nature of the beast. Have you mounted your engine on a stand or other air frame? Was the engine ever in a crash? Maybe a slight bend in the rod?? Without seeing the plane it's hard to come up with any answer, all we can do is think of problems it could be. MDS engines were/are not the cream of the crop. Horizon gave up selling them for a number of reasons. That's why I have a few under the bench. I won't get into the problems I had with them but I don't even give them a thought when I am looking for an engine to use.

It does seem that most of the vibes are in the tail feathers. This is the second engine on the airplane. Then engine on now is brand new. It ran on the bench great (No Vibrations) with 2 different props.

Thanks

What does that tell you??? Did the plane vibrate with the other engine?? Either the plane or engine is trying to tell you something, it's up to you to figure it out. If the problem was there with both engines what does that tell you? Vibration will show up in a plane at the butt end on most all planes, some just more then others. If it didn't show up with the other engine that would tell you there is an engine problem. Unless the other engine is an MDS. New in box means you wouldn't think the engine has a problem, that just isn't a fact. MDS is a Russian brand that Horizon tried to import. They had crap for carbs and tried to make a better one, it didn't help. The castings were bad and the case would break right at the cylinder where that big muffler mounted. They are an engine that runs better with FAI fuel or 5% nitro, something that wasn't advertised, most testers used 15% and sang the praises of the engine. Problems showed up right away but users figured things out but not fast enough. They idle for crap until broken in, that is done on the bench or they tend to shake your air frame to pieces, it also requires a gallon or more of fuel to break one in. These engines had so many problems I don't think Horizon sold them for even two years. At swap meets I see the .58s sell for about $5.00 or $10.00 NIB. The lower Nitro may help you, it did seem to solve a few of the problems. I had one .68 that ran almost OK. Then it no longer drew fuel, even after installing a new carb?? At $99.00 they weren't worth fooling with so I tossed mine under the bench and forgot them. When you get one to run well they are very strong engines. Try breaking it in completely on the bench then mount it on a plane. Takes a bunch of time and fuel but you may be one of the lucky ones and have a good one on your hands?? Many Dead Sticks.

Sounds to me like it is pre- igniting. With a glow engine the compression ratio, fuel nitro content and heat range of the glow plug are what dictates the "Timing" By using too high a nitro content and/or too hot a plug in a euro built engine can cause pre-ignition. Running fuel with high moisture content will cause the engine to run hotter and can be a cause of pre-ignition.



If I were in your shoes I would put in a medium heat plug ( OS #8 ), fresh 10% fuel and a 10X7 prop. If it still vibrates alot, you can try a second plug washer to reduce the compression some. One other trick that sometimes helps is to put the engine at top dead center and put an un balanced prop on it with the heavy blade exactly 180 degrees opposite of the head. This can fix an engine that has too little crankshaft counterbalance.

The MDS engines are high compression and designed for zero nitro, FAI fuel, alcohol and oil only fuel. The OS #8 is/was the plug of choice but they ran better with very low nitro, 5% worked pretty well. Some of the other guys started mixing there own FAI fuel when we were trying to figure these engines out. Me being a lazy ass just bought some Power Master 5% and got them running OK. People that want to bother can do a search here on RCU and read up on what others did to try to get them running. The engines were here in about the mid 90s? You can get the exact dates from Horizon just by phoning them. Russia's pay back for the Wall!!!