Rebuilding GSM026's BME-115

[KrisW]

WEll . . .I have GOOD news . and I have BAD news . .. .

Good news: The engine runs great, fantastic power, hauls around a 28.5 lbs 35% Yak like it's nothing, goes out of sight vertical and never misses a beat (I'm serious. .2500+ foot vertical line, till I could not hear the engine any more), starts easily, runs smoothly, transitions great, hovers at 1/3 throttle, rips the 28-10 Mejzlik at 1/2 throttle, and idles easily at less than 900 rpm.

Bad news: The dreaded roll-over burble is still there. At anything below 1/3 throttle, say 3 clicks off idle, going KE or inverted results in the engine sagging and stumbling, then slowly running smothly again. It only happens from upright, not from inverted, and here's the reason why:

Low speed engine operation causes low speed intake air velocity, and since the high end fuel pasage is almostnot used at really low throttle settings, the idle mixture is what the engine is running on for the most part. Low speed air, a large carburetor, a large reed bank, lots of open area right above the reeds,and what you get is the fuel falling out of suspension and puddling inside the crankcase. When you roll KE or inverted the fuyel drops into the airstream, causing excessive fuel to the cylinders, and making it stumble.

I tried leaning the low end till it was almost impossible to transition off idle . .I mean yiou had to move the stick s l o w l y to get past half throttle, then everything was fine. I then took off and flew around. Only with this extremely lean low end mixture would the plane roll over without burbling. I played with the low needle for quite a while, moving it this >< much each time I touched it, but the moment I got to where there was any sort of decent transition off idle, the roll-over stumble came back into play.

Soo. . . . .what to do? Next post will explain why the engine was set up this way, then we'll discuss how to fix the problem.

[KrisW]

Okay, what sells engines to 95% of modelers out there. . . RPM! ! ! !

Compared to the DA-100 and 3W 106, the BME is 200+ rpm stronger with the Mejzlik 28-10. That's it. MORE POWER arh arh arh. That's why we have this HUGE reed bank and carburetor on a 115 cc engine.

Back when the 110 first came out, I got one of the first production engines from BME. With a tuned canister setup it would turn over 7000 rpm with the Mejzlik 28-10, over 7500 with the Menz-s 26-10, and 8000+ with the Mejzlik 26-10!!! We are talking about STUPID power, hold your ears, pity the dogs within a 5 mile radius and rip the prop on the ground power. We're talking about pull a 40% plane vertical with a 1' takeoff roll here. Just silly power, from a 4 lbs powerplant.

So, why was the 110 put aside and the 115 brought into being?

According to Keith, the 110's SDC-80 carburetor would sometimes run out of gas, and result in lean runs. I don't know which SDC-80 HE was talking aobut, but I never encountered that sort of problem, and I ran these motors as hard, if not harder, than anyone else out there.

Then, the reeds were supposed to be limiting the top end power. . .HUH?!?!?!?!?

Like I said. . 8000 rpm with a Mejzlik 26-10 .. and the motor was limited by the carburetor and reeds .. . . . I don't think so.

But, I digress.

So, the ONLY reason why the BME-115 is designed the way it is, is so that you have a nice large flat spot to mount a HUGE reed bank, and larger carburetor. That's it. Reason numero uno. . bigger reeds and carb, and more top end rpm.

Look guys, this is the way it is. The 115 is over-reed sized, over carbureted, and has too much crankcase void area at the bottom of the reed bank to prevent fuel puddling and run smoothly in the lower rpm/throttle range.

That's it! Everyone who has fought the battle with this engine since it came out comes to the same conclusion (I told Keith it had fuel puddling the first time I flew mine). Full length pipes help. .a LOT. So do properly tuned canisters. But for the average Joe Modeler out there, running stacks for exhaust diverters, the low rpm flow is too low to make things work properly, and you will be running into exhaust reversion on top of it (due to the terrible flow characteristics of the stack type diverters), and anything below about 3000 rpm and 1/3 throttle is going to be hit or miss on the mixture.

NOW. . how to fix this problem??

See the next post.

[KrisW]

FIXING THE PROBLEM.

Anyone got a 15 lbs sledge hammer????

JUST KIDDING!!!

Okay, we need more flow VELOCITY, not mass, in order to help keep the fuel in suspension. We also need to provide a better signal to the idle circuit of the carburetor to help it atomize the fuel better in the first place.

Then, we need to eliminate all the areas where fuel can puddle.

Two avenues to fix the velocity problem. Go to a smaller reed and carburetor, or stuff the crankcase and reed area very tightly, with properly shaped fill-in material, so that it promotes velocity, atomization of the fuel, better power pulse transmission to the carburetor, and removes any place where the gas can fall out of suspension and puddle inside the crankcase.

Going to the smaller carb and reeds involves lot's of re-engineering, design, money, and time.

Properly stuffing the reeds and casing involves making a prototype piece(s), testing it, and then making molds.

Once the areas where fuel can drop out of suspension are eliminated, AND the flow velocity at low rpm is increased, AND the signal to the carburetor idle circuit is enhanced, all of these problems we have been experiencing will be GONE!!!!

I start work on a mass-produceable set of parts tomorrow. I'll keep everyone apprised on my progress.

[Flyinrazrback]

You should just start a BME repair shop so folks have at least one place to send their engines to get looked at.

[KrisW]

	Quote: Originally Posted by Flyinrazrback
	You should just start a BME repair shop so folks have at least one place to send their engines to get looked at.

That's in the works as well, but there are considerations of licensing, warranty, and things like that which need to be addressed. The situation is kind of fluid at the moment. I should know more in the next 2 weeks or so.

[ChrisR]

This is one great thread and I don't or never have owned a BME. Lots of great information here and was an enjoyable read.

Thanks a lot!!

[RTK]

Kris--I was able to make the dreaded burble almost completely disappear by leaning the top not the bottom and richening the bottom.
It also seems that the high speed starts feeding at a very low throttle setting. But.......you are lean on the top when set up this way.
The fix Keith came up with was to extend the high speed circuit into the venturi, this slows the feeding from the high speed circuit prematurely. Thus leading to the burbling. (PUDDLING) I have a stock 3W with a till carb that is set up just like that with the tube extending into the venturi.
With a 3 needle carb you can accomplish the same thing too by leaning just the mid range, decreasing excessive fuel that will puddling but will leave you rich enough at WOT with the third needle.
NOT saying that the things you mentioned don't also come into play.

[KrisW]

	Quote: Originally Posted by RTK
	Kris--I was able to make the dreaded burble almost completely disappear by leaning the top not the bottom and richening the low. It also seems that the high speed starts feeding at a very low throttle setting. But.......you are lean on the top when setting up this way. The fix Keith came up with was to extend the high speed circuit into the venturi, this slows the feeding from the high speed circuit prematurely leading to the burbling. (PUDDLING) I have a stock 3W with a till carb that is set up just like that. With a 3 needle carb you can accomplish the same thing too. NOT saying that the things you mentioned don't also come into play.

I have the modified carburetor, Ralph, with the tube into the vernturi. A lot of people don't have this modification, though. In fact, this carb seems to run worse than the one on my other 115. I'll play with the top end tomorrow (it was getting too dark to see the needles) but I doubt I'll be able to get it all out.

Thing is, with the bottom end leaned down it ran VERY smoothly, without the burbing a lot of people see at 1-2 clicks off idle. Even the idle became extremely smooth, witht he throttle close, pointing to the low end not mixing gas properly due tot he low velocity (IMHO). Another thing that points to the low end is that even at only 1 click off idle, where the top end needle was not going to be working at all, the burbling and roll-over problem were still present.

In the end, though, no one should have to go to ths much trouble to get an engien to run properly. A permanent fix, that addresses all the problems at once, is what is needed.

In the end, it's not just the carburetor, but everything having to do with the inlet side of the engine, that needs to be looked into and worked over.

[RTK]

I can't argue that Kris.
But with the 3 needle carb I would lean the HIGH (which acts on the low-mid range) and ALL burbling went away no matter how rich or lean I set the low. That kinda told me the high (mid now, but still in the same location) was the culprit and feed too much fuel at just a hair off of idle. I never touched the high needle because it was plenty rich.

[KrisW]

	Quote: Originally Posted by RTK
	I can't argue that Kris. But with the 3 needle carb I would lean the HIGH (which acts on the low-mid range) and ALL burbling went away no matter how rich or lean I set the low. That kinda told me the high (mid now, but still in the same location) was the culprit and feed too much fuel at just a hair off of idle. I never touched the high needle because it was plenty rich.

heh. . not arguing at all, Ralph, just forwarding the running characteristics that I saw.

It's a problem . .it NEEDS Fixing. . . .Since I have NO LIFE, I guess I cn spare some time to solve it.

[RTK]

Make the fix easy for dummies like me Kris Thanks for all the effort and time you have spent

[KrisW]

	Quote: Originally Posted by RTK
	Make the fix easy for dummies like me Kris Thanks for all the effort and time you have spent

Well. . you're no dummy, Ralph. I should make you figure it out for yourself. . . . .