There has been a long standing "BME-115 tuning help" thread in the forums for some 5-6 months now. In the end, we had tons of posts, helpful suggestions, tuning ideas, speculations, and a horde of commentaries pro and con concerning the BME-115 and the troubles people have, or not had, withthe engine.

To date BME has put out several "Fixes" for the 115, including a different reed spacer block, 2-3 carburetors, replacement of the Falkon ignition with the proven RC Xcel ignition (avalable from C&H), and ideas thrown back and forth depending on your applicaiton and needs of the engine.

But, for all that tinkering and talking, GSM could never seem to get satisfaction from his engine. In the end, given the lack of power he was experiencing, I suggested a comparative compression test between the left and right cylinders. VOILA!!!! The right cylinder had substantially less compression than the left cylinder, which was probably the problem all along, but was never considered or tested for. How the compression, or lack of ring seal, got that way is NOT important. We are not here to point fingers, lay blame, or make scurrilous remarks about BME or GSM026. What we are here to do, now, is discuss the rebuild of the engine, and getting it back up to full power.

GSM and I did a bit of a horse trade. His 115 is now MY 115, and he has a proven and reliable 110 to take it's place. He may have gotten the better part of this deal, IMHO, because the 110 he has just destroys the Mejzlik 28-10 in the air, runs smooth as anything I've ever owned, and is just one great little engine. Comparing carbon buildup between the 115 and 110, the 110, with 150 or so flights on it, shows substantially less carbon, less piston/bore wear, and has phenomenal compression.

So, here I am. . .going through a "Top out" of the engine. Basically, this includes deburring of the ports, honing of the bad cylinder, replacement of both rings on that side, double checking deck clearances on both sides, decarbonizing both cylinders, and otherwise making sure that the parts on hand are as good as they can be before the prop gets flipped again.

Here's a pic of the engine, as I received it. A couple of things to note. The bolts for the carb block have the heads sticking up out of the plate, almost hitting the carburetor. Plans call for countersinking the plate, and using flat head steel screws in place of the bolts already installed. I'll have a few pics of how that turns out as well. Secondly, this engine uses the modified WB25 carburetor, and shorter reed block. the carburetor has been modified with a small brass tube on the high speed circuit, extending into the middle of the airflow instead of allowing the fuel to be pulled form an annular ring around the periphery of the carburetors throat. The 115 I already own does not have this modification tot he WB-25 carburetor, yet runs extremely smoothly and well. It will be fun to compare the two modifications, to see if there is any noticeable difference in performance and stability of mixture.

So, here's the pic, and it's time to get to work.

First off, some pictures I was sent of the right cylinder and piston.

Looking at the combustion chamber, note the thickness and roughness of the carbon buildup. Then look at the top of the piston, and you see the same condition.

A couple of things here to be seen. First, the top of the piston shows signs of detonation, with pitting of the carbon. then yous ee that the carbon in the combustion chamber is crackled, instead of smooth. Again, a sign of heating or detonation. Third, there is little changing of color on the sides of the piston, which shows that blowby was minimal. Fourth, a large amount of small scratches are on the sides of the bore and piston. A comparison of the other side of the engine reveals substantially less of these being present. Scratches like this are sings of either debris passing through the engine, or or hard bits of carbon scoring the piston and cylinder as they break off the piston crown or combustion chamber, and are pulled up and down the bore.

Given the heavy carbon buildup (compared to the other side of the engine), scoring, pitting of the piston crown, and lack of blowby, it's pretty much a given that this cylinder was running lean, getting hotter than the other cylinder, and experiencing incipient detonation that can cause a thermal runaway condition if let to go for too long.

The first three pics are of the bad clyinder, the next are of the good cylinder. The difference in appearance and carbon buildup is very evident.

Now, what the other side of the engine looked like. Note the lack of carbon in the combustion chamber, bare areas on the piston crown, and general good condition, lack of scratches, and lack of detonation/heating evidence on the parts. My apologies for the longer distance shots. YOu can blow the pics up to see the details, though.

After cleaning things up a bit, and making sure there were no other problems, it was time to break out the cylinder hone and get to work.

Honing was done using automatic transmission fluid for a lubricant, and running the hone at about 60-75 rpm while moving the tool in and out of the cylinder. The slow speed was necessary to help achieve a decent 30-45 degree crosshatch, which would have been impossible at higher speeds and using a hand drill, as was done in this case.

It only took 2-3 minutes to clean up the bore, and remove the majority of the scratches. Then the cylinder was cleaned with carburetor cleaner, then soap and water, then deburred around the edges of the ports to assure a slight chamfer existed on the top and bottom edges of all the ports, to prevent the rings from possibly grabbing the lip of a port on the way by.

After the deburring, the cylinder was again cleaned with carburetor cleaner, then soap and water, then oiled with Pennzoil 32:1 aircooled 2-stroke oil, and set aside, ready for reassembly.

The three pics show the crosshatch, and deburring of the ports. Note that the scratches in the bore seem to be almost eliminated by this procedure.

After the cylinder was honed and deburred, the piston was cleaned, and decarboned, and two brand new rings were installed.

It's really easy to install rings, merely "spiral" them around the piston crown and into their grooves. On most 2-strokes, anti-rotation pins are in place in the ring grooves to prevent the ends of the rings from spinning around and catching in a port. Be SURE to put the rings into the grooves so that the reliefs in the ends of the rings match where the pins are in the grooves.

Then simply oil the piston, compress the top ring, slide the cylinder onto the piston, compress the second ring (using a fingernail usually) and slide the cylinder all the way into place.

After that, all that remains is to make sure the cylinder is properly sealed against the crankcase, and to tighten the bolts to about. . ohh. . . THAT tight.

Repeat the procedure for the other side, and once again you have a complete engine, ready to test run.

Okay. . . . things are back together. . hows the compression?

By removing one sparkplug at a time, it's possible to get a subjective "feel" for compression as the engine is rotated across TDC. Each cylinder can be tested this way, to determine if there is a problem with it holding compression, or if there is excessive leakdown as the engine crosses TDC.

In this case, due to the new rings and honing of the right cylinder, I expected there to be a slight, albeit not really bad, difference in leakdown and absolute compression. The new rings are not seated yet, and the roughness of the honed bore allows pressure to escape.

True to form, the right was down. . but only slightly, on absolute feel for compression and for leakdown. To me this is great news, because with the old rings in place, the right cylinder had basically half the compression of the left cylinder, which was probably the majority of the power loss that was experienced.

To finish the toppingout of the engine, pieces of solder were slid into the tops of the cylinders, then crushed by running the pistons across TDC, to check for squish band-piston crown clearance inside the combustion chambers. Both sides came out fairly close, with .025" for the right side, and .023" for the left side. Minimum clearance is .020", so these are very close to the most compression possible, but still have enough clearance where the possibility of piston or cylinder damage from contact at TDC is literally impossible.

So, I have new rings, a good bore, cleaned combustion chambers, good clearances, and everything is lubed and ready to go. Last step is to countersink the bolt heads in the carburetor mounting plate.

So what is your theory as to why one side is so drastically different from the other? I would think if there was excessive carbon build-up from being overly rich it should be the same on both cylinders.

Kris,
Are you planning on running this one on stock mufflers or tuned pipes? Just for keeping things the same it would be interesting to see how it works out for you on mufflers. I think most of us having troubles were using stock mufflers and I feel full length tuned pipes really change the results.

Originally Posted by dbcaster

So what is your theory as to why one side is so drastically different from the other? I would think if there was excessive carbon build-up from being overly rich it should be the same on both cylinders.

I believe he suggested an overly lean condition on the bad side. I was curious on the carbon buildup situation myelf.

Okay, let's address the carbon buildup issue.

Carbon buildup is a SYMPTOM of the engine getting hot on one side, not the cause of it.

First off, the idea of "rich or lean" in a flat twin is probably a misnomer, or incorrect statement. You have one source of fuel and air, being distributed to both cylinders. By the time the fuel and air get through the reeds the fuel is basically vaporized and totally suspended in the air. Unless it drops out of this suspension on it's way to the cylinder, the mixture on one side is going to be the same ratio as that on the other side. It's kind of hard to have two different mixtures in one closed environment, wouldn't you agree?

Both sides were getting fuel-air mixture, albeit at slightly different amounts. But, once the fuel-air comes through the reeds, it's pretty homogenous and mixed up, and tends to be distributed through the engine this way, with equivalent air-fuel mixtures to both cylinders. But, one cylinder will get slightly more than the other, due to the way the mixture flows in and around the parts inside the engine, and how the crankcase and rods/pistons bat it around and make it flow into certain places, and not into other places.

For some reason, this seems to be the front cylinder on a staggered flat twin. More air-fuel means more pressure, more heat, more power. . and being slightly more prone to borderline detonation. And that is where things start to break down. More heat makes the cylinder APPEAR to be leaner, when it is actually not. I've done tests using dual Oxygen sensors on flat twins, and the sensors read basically the same for both cylinders throughout the rpm range, even though an infrared heat gun showed that the forward cylinder was running slightly hotter. This kind of debunks the "leaner versus richer cylinder" idea.

What is happenieng is a mixture distribution imbalance, making one cylinder have higher pressures, more heat buildup, and come closer to detonation than the other.

In this engines case, the Carbon buildup is because the oil in the mixture was broken down from the heat and pressure, and coked the surfaces as it was breaking down. This is not the normal soft carbon asociated with burning gasoline, but hard and cracked carbon of the type associated with oil breaking down, coating the surfaces, and being baked and destroyed to the point of becoming carbonized.

As can be seen by looking at the pics of the left cylinder and piston, there are washed areas, without carbon buildup. This shows that the oil was not being deteriorated or broken down as much, mainly due to less heating, less pressure, and less detonation super heating things for that tiny split second that literally blasts the oil into it's component parts. Soooo. . less carbon on the cooler running cylinder. . . more carbon, with pitting and cracking, on the hotter runing cylinder.

And. . once the carbon builds up, it holds heat, making the cylinder more prone to detonation and helping to aggravate the buildup of even more carbon and causing things to deteriorate very quickly. In the end, all the heat buildup makes the rings wash out, lose tension and seal, and NOW. .you have heat, but less power, you try to lean the mixture to compensate, and you start to run into a thermal runaway situation where it's impossible to keep the engine cool until you go back and repair the basic problem of stopping the engine from heating up on the one cylinder to begin with.

A lot of engine makers have taken to double gasketing the heat prone cylinder. This reduces compression slightly, which helps because????? Because there is more air-fuel going to that cylinder, so it NEEDS less static compression in order to run at the same temperature as it's less efficient counterpart on the other side of the engine. By reducing the compression, you are reducing the pressure, and overall heating of the cylinder, keeping the cylinder from going into a thermal runaway situation, and helping it to stay cooler, longer, and stay alive.

So, it's not lean or rich. . it's static cylinder pressure and efficiency, and how hot things get, that causes your carbon buildup, and imbalance of temperatures and carbon problems in a flat twin engine. In this case, the engine probably had inadequate cooling to start with, and things took their normal turn of activities, ending up with the right cylinder running hotter all the time, resulting in more carbon, less power, more detonation, and on and on and on until poof. . .it stopped making power.

It only takes ONE bad flight. . going too hot, running a little lean. . working the motor hard before it is broken in, or not using enough oil, to set things on this course. Now. Why did it happen in this case?? I have absolutely NO idea. But, what I DO know is, I'm going to run this engine, see what hapens, get some tach numbers, and then get it into an airplane if everything works properly, and enjoy flying it.

class is in session.....

Originally Posted by krayzc

class is in session.....

Mr. KrazyC .. . Please go stand in the hallway until it's time for change of classes. People with your attitude are not allowed in here. . .

Originally Posted by dbcaster

Kris,
Are you planning on running this one on stock mufflers or tuned pipes? Just for keeping things the same it would be interesting to see how it works out for you on mufflers. I think most of us having troubles were using stock mufflers and I feel full length tuned pipes really change the results.

I have a set of standard mufflers on the shelf. Thursday I'll have the engine on the test stand, getting some break-in time on it. I'll use the stacks for that, and see how the throttle response and mixture work out on a setup that is not a canister or tuned pipe exhaust.