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.