Rebuilding GSM026's BME-115

[KrisW]

And lastly .. well, it does all bolt back together. . . .

The only visible difference, externally, is the use of a gasket betweenthe reed cage and the carburetor mouting plate, instead of using silicone sealer to glue the two pieces together.

But, internally, things are a lot different, and hopefully the changes will make the engine do the same thing. . run a whole lot differently.

[Hammbone]

Looks good Kris, and makes a lot of sense. Look forward to seeing if it works.

Jim

[KrisW]

	Quote: Originally Posted by Hammbone
	Looks good Kris, and makes a lot of sense. Look forward to seeing if it works. Jim

Thanks Jim.

I'm going to spend some time this evening going over the various pieces, polishing things with 800 grit paper, and making sure all the sealing surfaces are flat and ready to go.

Sealing is going to be done with the stock gaskets, and I'll be gluing each gasket to one part of the two surfaces it is used on, to help keep things lined up and prevent 50% of the leak that can occur at each junction. This is so that I can take it all apart if I want. Getting the siliconed together carb plate and reed cage apart was a total PITA. I realy do not want to repeat that problem if I can avoid it.

I'll have the plane back together tonight, and may get off work early enough tomorrow so that I can get the engine run up and check everything over for any inadvertant problems, and perhaps get some tuning done on the carburetor. It should prove interesting to see if there are any large differences in needle settings. I'm thinking that there will be little to change, but that the efficiency of the engine at low rpm and low throttle settings will be much better.

Only one way to tell .. flip the prop and see how it runs.

[KrisW]

All righty, ran the engien this afternoon.

Couple of things bothering me. The carburetor will NOT draw fuel from the tank, with the choke on. . EVER. Really bothersome. .

I yanked the carburetor off and took a look at the pressure regulator. The spring tension seems VERY high, and it takes a lot of pressure to open the needle. to alleviate this, I cut two coils off the spring, and put everything back together. The pop-off pressure seems to be much nicer now, close to what I am used to from other carburetors I've worked with.

If the pop-off pressure being lower makes the engine tend to run rich, I can always stratch the spring a little bit to compensate, but I doubt I'll need to.

Whoever, at Walbro, thought it would be a good idea to use a 3-piece fuel pump diaphragm needs to have their heads examined. It may work, but man it's strange to see the reinforcing weave separate from the pump layer. this carburetorhad been rebuilt, using this strange setup, so I put the old piece back in (it was perfectly good).

Now, why am I doing this?

Well, the engine runs fine. . in fact the needles seem to be a bit less sensitive to mixture now. But, at idle the engine seems to slowly starve for fuel. Anything over about 1500 rpm and things are fine, but at low rpm it will drop a cylinder and then shut down.

Another symptom .after running for about 10 minutes the engine suddenly seemed to starve for fuel. After 3 successive primed starts (squirt fuel into the carburetor throat, choke and flip) it finally re-started, then acted like it was starving for fuel for about 20 seconds, barely taking the throttle, then suddenly started running perfectly, with great top end power and transition again, though the idle drop down seemed to still be there (though better now).

So. . this little problem reared it's head the first time I tried to fly the engine. .and I tore the carburetor apart at the field, checked everything over, found nothing amiss, put it all back together, and after about 30 seconds of running it ran perfectly again. So I'm thinking that the carb pump being in 3 separate pieces, along with the high pop-off pressure from the regulator, caused the engine to intermittently starve for fuel.

I can't find any evidence of fuel leaking out of any of the gaskets, so a crankcase leak is about the last problem I would expect.

I'll run the engine again tomorrow, to see how the lower pop-off pressure and one-piece pump section work out.

In the meantime. . .once the engien started running properly, the transition issue changed a bit. I'll explain in the next post.

[KrisW]

Some differences in the way the engine runs now.

Before, at about 4 clicks off idle, there was a bit of roughness. Now it seems smoother, though it's still a little richer than I'd like to see.

Transition from idle-1/2 throttle is as good as it's ever been. .

Transition from 1/4-top end seems slightly better, with no short rpm peaks that slowly work their way higher as the top end clears it's throat. Run it at 1/4 throttle for about 30 seconds, then nail the gas and it just shoots right to the top rpm number and hangs there. This, to me, shows that there is a lot less gas hanging around inside the crankcase, so the engine does not have to clear it's throat as much as it used to.

Transition from prolonged idle (at about 1500 rpm) to top end is great. . no sagging, no burbling, no false rpm peaks. . just zoom, from idle to top end.

One thing that may have been causing my idle issue is that the ignition battery was down to 4.7 volts. . a bit lower than I'd like to see. I have it charging right now, so tomorrow afternoon I'll be able to give the engine a real thrashing to see how the carburetion mods work out.

[rcign]

Walbro 101....Walbro carbs have no"pressure regulator"..Crankcase pressure is what it is, from the crankcase to the carb, nothing in the way to regulate anything....It's a pulse, not constant....It goes to a dead end cavity above the diaphragm..
Before you tell me how much I don't know about carbs, PLEASE call Luis Salas at Walbro to find out how they REALLY work....His number 989 872 7322...Maybe you can tell HIM how they work...

[KrisW]

	Quote: Originally Posted by rcign
	Walbro 101....Walbro carbs have no"pressure regulator"..Crankcase pressure is what it is, from the crankcase to the carb, nothing in the way to regulate anything....It's a pulse, not constant....It goes to a dead end cavity above the diaphragm.. Before you tell me how much I don't know about carbs, PLEASE call Luis Salas at Walbro to find out how they REALLY work....His number 989 872 7322...Maybe you can tell HIM how they work...

Alright, Ralph. . FLOW regulator. . happy now?

Everyone calls it a pressure regulator, though, and everyone THINKS it regulates pressure, not flow. Can't help it if the lexiconis incorrect. . .

And, yes, I know exactly how the carburetor functions.

[dick hanson]

wow --picky -- actually the device is a generic thing known as a DEMAND regulator- which FLOWS-based on the TOTAL pressure difference on the diaphragm -which may or may not be a soft membrane -some regs use a sliding disc.
so -really either term is just fine

It can be pneumatic -regulating air -or a liquid.
(what is a carbonator?)

[KrisW]

	Quote: Originally Posted by dick hanson
	wow --picky -- actually the device is a generic thing known as a DEMAND regulator- which FLOWS-based on the TOTAL pressure difference on the diaphragm -which may or may not be a soft membrane -some regs use a sliding disc. so -really either term is just fine It can be pneumatic -regulating air -or a liquid. (what is a carbonator?)

A Carbonator is one of Ralphs flatulence attacks because he refuses to drink Decaf . kind of explains his extremely intense, defensive, and "I've got backup from Walbro that proves I'm right" response in his post.

Hey, Ralph .. . DECAF!!!!!

[RTK]

You are all wrong, I tend to use the more scientific term.............................................. ......that thingamajig or whatchamacallit

[rcign]

Yup, that functions when atmospheric pressure pushes the diaphragm down to admit more fuel to the cavity under the other diaphragm, which empties when incoming air pulls fuel from the cavity...
Which is why engines go rich when the diaphragm is acted upon by more than atmospheric pressure...
Which explains the need to sometimes put a fitting on the cover to run a line into the dead air in the fuse..I know YOU know, but most newbies don't, so they come here to find out why their engines act funny in the air but run fine on the ground.....

[KrisW]

	Quote: Originally Posted by rcign
	Yup, that functions when atmospheric pressure pushes the diaphragm down to admit more fuel to the cavity under the other diaphragm, which empties when incoming air pulls fuel from the cavity... Which is why engines go rich when the diaphragm is acted upon by more than atmospheric pressure... Which explains the need to sometimes put a fitting on the cover to run a line into the dead air in the fuse..I know YOU know, but most newbies don't, so they come here to find out why their engines act funny in the air but run fine on the ground.....

Hmm. . you mean I am now "Professor Kris". . Instructor at RC Engine University???

COOL!!!!! okay, all you newbies .. .Class is dismissed, and you all need to go down the hallway to room WB101 where Associate Professor Ralph is giving a class in Walbro Carburetor Basics. . after you finish that class, you have to go to room ZDZ80, where Associate Professor Dicky is going to give a class on Exhaust Makes Your Engine Seem More Powerful. . . and then you have to go to room BME115 so that Associate Professor RalfK can give you a class in 3-needle thingamajigs .. THEN you may return here for lessons in how engines REALLY work.. . . .