Rebuilding GSM026's BME-115

[KrisW]

	Quote: Originally Posted by RTK
	Nice work there Kris, do you ever sleep Looks like the reed block off of a 110????? If so keep an eye out for signs of leaning at WOT.

Nothing wrong with that reed design, Ralph, and reeds do not make an engines mixture change. That is all inside the carburetor. People like to over-lean the top end needle of the 110's for some reason. The SDC-80 and stock reeds flowed well enough for 8000 rpm, with canisters or pipes, without leaning out. They should be fine in this application.

I remember when the 102's "leaned out" too. People kept twisting needles, looking for more RPM, instead of setting the engine up for smooth running, even if it cost 100-200 rpm. Peak rpm is NOT where it's at. Smooth running, and dependability, will keep your engine alive, and give you plenty of fuel at max rpm and throttle settings.

One thing I noticed on the SDC-80 on the 110. After a while the carburetor butterfly plate would start to over-center at WOT, giving a boost of 200+ rpm at the top, but generally leaning the mixture a bit at the same time. Re-setting the max opening to 90 degrees, instead of 95+, cost some top end rpm, but the engine ran a lot smoother at WOT. This may have been what was occuring to some people, causing great power but lean runs.

[RTK]

Yep, if the carb will deliver the proper air/fuel mixture through out the throttle range. I must have had a bad sdc-80, had to open up the high needle seat a tad to make the engine run rich enough at WOT

[KrisW]

	Quote: Originally Posted by RTK
	Yep, if the carb will deliver the proper air/fuel mixture through out the throttle range. I must have had a bad sdc-80, had to open up the high needle seat a tad to make the engine run rich enough at WOT

Would not be the first time, Ralph. How many SDC-80's you think Walbro builds every day? Gonna be about 1-2% that are not quite perfect.

I remember on my first BME-100, the pump hole in the reed valve plate was not there. This was the old 4-petal Chainsaw reed that Keith was using. I finally found the problem, and poked a hole in it, and the engine ran great from then on. NOT the best way to start a long term relationship with a company, if you ask me.

One thing that always bothered me about the SDC-80 . .EVERY one of them I had got a loose screw in the throttle plate. I finally gave up and started red loctiting the screws the moment I got the carburetor, otherwise the plate would start rattling around in there, and you could not idle the engine down. On top of that, it would beat the plate up so badly that it would change diameters, and be literally unusable from then on.

Even the parts from the "Big Name" producers need improvement for our applications, IMHO.

[dick hanson]

way back in time - when all cars ran carbureters -- there was a goofy dual carb setup offered by a discount house which was simply a Y adapter which enabled the aspiring hot rodder to put two carbs on the engine - whre only one had been before - not a great setup to say the least
but with two smaller carbs it worked.
maybe - two small carbs on the 115 would work - using a vacuum pot to open the secondaries - just like the old GM tri power setups.
I once made a special adapter using rock hard maple - just for test purposes - it was quick and easy and plenty strong- no good as a long term setup -but using a couple of walbros from say 40-50 size engines it might work. You could actually open the secondary carb with exhaust pressure -instead of vacuum to diaphragm-- GM used diaphragms (vacuum) on everything for years - -I got 3/4 psi on my tuned exhaust setups which came on as the pipe built pressure . In that setup -I was working with a 2300 tigre on ignition and alky - end result very smooth mid range and great top end power which never went lean (the 3/4 psi went to the fuel tank.)

[KrisW]

	Quote: Originally Posted by dick hanson
	way back in time - when all cars ran carbureters -- there was a goofy dual carb setup offered by a discount house which was simply a Y adapter which enabled the aspiring hot rodder to put two carbs on the engine - whre only one had been before - not a great setup to say the least but with two smaller carbs it worked. maybe - two small carbs on the 115 would work - using a vacuum pot to open the secondaries - just like the old GM tri power setups. I once made a special adapter using rock hard maple - just for test purposes - it was quick and easy and plenty strong- no good as a long term setup -but using a couple of walbros from say 40-50 size engines it might work. You could actually open the secondary carb with exhaust pressure -instead of vacuum to diaphragm-- GM used diaphragms (vacuum) on everything for years - -I got 3/4 psi on my tuned exhaust setups which came on as the pipe built pressure . In that setup -I was working with a 2300 tigre on ignition and alky - end result very smooth mid range and great top end power which never went lean (the 3/4 psi went to the fuel tank.)

Uh. . no.

[dick hanson]

ok-

[KrisW]

While disassembling the reed block for my 115 from the aluminum carb mounting plate, the actual reed block itself came off in TWO pieces. Careful examination of the broken area showed that the plastic was soaked with oil/gas inside the cracked area. This area is downstream from the reeds, so not a factor for vacuum leaks to the atmosphere, but the fact that it came off in two pieces means that in the past it was cracked while being handled and disassembled/reassembled, then someone used RTV to glue the reed block to the carb mounting plate and put everything back together.

This is a NO NO!!!!!

I'll degrease everything and used JB Weld to put the reed cage back together again.

Almost amazing what you find inside an engine these days. . . . . .

[KrisW]

Ya think MAYBE there's a mismatch between the size of the carburetor throat and the carb mounting plate and cross-section of the reed bank?

Maybe??

This is the first place where the mixture slows down, becomes stagnant, puddles gas, and then drops the gas back into suspension as the plane rolls over.

This is where we start.

[GSM026]

Kris,

Just curious - was the broken reed assembly on my old 115, or the one you had previously?

I had asked Keith about seperating them when I was contemplating installing a smaller carb (which would have required a different plate). Keith told me that he uses RTV to bond the reed assembly to the aluminum carb plate, and that you might not be able to seperate them without breaking the reed cage.

Greg

[KrisW]

	Quote: Originally Posted by GSM026
	Kris, Just curious - was the broken reed assembly on my old 115, or the one you had previously? I had asked Keith about seperating them when I was contemplating installing a smaller carb (which would have required a different plate). Keith told me that he uses RTV to bond the reed assembly to the aluminum carb plate, and that you might not be able to seperate them without breaking the reed cage. Greg

It was off the 115 you sent me, Greg, but I don't think anything you did resulted in it cracking.

Actually, this is not the first instance I have encountered with one of these blocks cracking. Others have related how the blocks crack at the corners, where the bolts go through. It's not that they fail but there seems to be a tendency for them to crack in stressed areas.

This break had gas-oil saturating the material along about 30-40% of the crack area, and when it came apart during disassembly, it did not suddenly snap, but part pulled off, then the other part was removed. About the only conclusion I can come to is that it was cracked for a while.

After re-examining where the crack was in relation to the reeds, it's evident that a small vacuum/pressure leak could have been occuring between the crankcase and carburetor, and by flowing through this small cracked area it would have rendered the engine hard to start, although it probably would have run fine. Given the engines reluctance to pull fuel, or pump when the prop was rocked back and forth, this small pressure loss point is probably the culprit. Any air leak in the crankcase is a source for a lack of fuel pump efficiency from the pump section of the carburetor.

I have no idea why it would crack in this area, other than possibly from the double gasketing allowing things to flex more than is normal, and stress this area of the block, resulting in it failing under vibration and heat cydling. Unless I had taken the block apart, I doubt I wouldhave ever found this problem.

[KrisW]

Allrighty. .

Things have been kind of quiet for the last couple of days as I've been working hard to develop my 2nd "fix" design for the 115.

This idea involves not only stuffing both the crankcase and the area between the reeds and carburetor, but basically "porting" the material to promote not only increased velocity, but to help aim the incoming air charge toward the cylinders and help it "turn the corner" instead of blasting straight into the crankcase and bounce off the top of the casing and rods/crank before hading toward the ports of the cylinders.

For anyone who remembers the old 3W Phenolic carburetor mounting blocks (that used to warp about every 30-40 flights), some people may have looked closely enough at the block to notice that the inlet hole fromthe carburetor was not a straight walled round hole, but rather was shaped in a venturi style, being smaller in diameter halfway through the hole than at the entrance and exit. The plate was only about 20mm thick, so the effect was slight, but it WAS noticeable.

I always liked that little touch of detail that 3W did to that phenolic spacer. To me it showed an understanding of airflow as it exits the carburetor and heads for the reeds, adn that there was a need to promote fuel atomization by passing the mixture through a venturi, which both pressures, then depressurizes, and increases, then decreases, the velocity of the mixture, helping to keep the fuel atomized and creating a sort of "one way" orifice that promoted flow toward the engine during the overlap period whenthe reeds were slammed shut.

To that end, the top plate I have designed uses not only stuffing to take up area that allows air to slow down, become turbulent, and would allow the fuel to drop out of the mixture at low speeds, but also I shaped the inlet to promote the same venturi effect as the 3W spacer plates. In the end about 35-40% of the overall area in between the carburetor and reeds is filled in, the throat of the venturi starts as the same diameter as the carburetor throttle plate, then decreases in diameter about 1.5mm, then changes shape and aims the mixture toward the reeds while at the same time maintaining velocity as best possible.

The final shape is a very smoothly contoured, and highly efficient channel that will promote better flow and velocity, while at the same time removing open areas and places where fuel can puddle, and help the incoming air charge hit the reeds in the most advantageous way possible.

Take a close look at the pics to see how it works. Bear in mind, there is a lot of shaping and sanding going on here. I'd LOVE for it to look all spiffy keen and pretty and shiny, but it ain't happening until the design proves itself and I need to make a plug for molding. If it works, however, the aluminum plate will be replaced with a dedicated one-piece molded plastic block, with all the proper contours in place.

Clearance betweent he reeds themselves, and the filled-in area is only about 1/16-3/32", leaving almost no place for fuel to puddle, or air to get trapped.

[KrisW]

And now some pics of the stuffing around the reeds themselves. Note the contouring and channels in certain areas topromote flow fromt eh sides, as well as the ends, of the reeds. Most reeds, even at full deflection, only flow mixture past about the last 30-35% of their length, so this is the most important area to concentrate on when establishing a bias for where the mixture is going to flow. But, when the reed opens, anyplace the air can be pulled from will be utilized, which is whi it is critical to leave channels where the mixture can get out the sides of the reeds, while at the same time minimizing the amount of open space around the reed block to prevent loss of velocity and remove places where fuel can puddle during low speed operation.

The stuffing actually extends into the crankcase area, and curves toward the cylinders, to promote the mixture "turning the corner" instead of going straight into the top of the casing. without the curved area, the mixture will be more liable to just bounce around the crankcase as it flows into the engine, instead of being directed toward each cylinder.

3W, in some of their more recent engines, has taken to casting vanes and runner paths inside the casings, to promote mixture distribution to the cylinders, , basically like desiging ports in an automotive engine. This is a step in that direction, but only in the reed valve area.