Walbro Carb diagraphm hole - what's it's "scientific" function?

[RMaia]

After I had a few problems with them my DA-50 dying when inverted, I decided to follow the tips and make the vent in the carb diaphragm hole (as well as re-tune the needles, which turned out to be way off...).

However, that got me wondering... What's the exact function of that hole? Reading up on how these carbs work, seems like the diaphragm is what controls how much fuel is fed into into carb itself, so the hole would allow the fuel pressure (under the diaphragm) to be equalized with the ambient pressure... Did I get that right?

[Al Lewis]

Yep! That's why the mod works. It allows it to read the ambient pressure in the fuse where there isn't any turbulence to cause false readings. BTW That may not make your problem go away. I know guys that did this to DA-50s and it didn't make a bit of difference. I believe it may be caused by their reed design. A few guys replaced the reed cages and reeds with those out of a DL-50 and they said that fixed the problem. Of course most DA owners just swear the problem doesn't exist.

[JoeAirPort]

So did it stop dying when inverted with your mod?

[bodywerks]

Well, I've only owned a few DA-50's and never had a 'dying inverted' problem. So I guess you could say I swear it does not exist...

[Al Lewis]

I have two friends at the field that both have DA-50s. One of them is Mike c. http://www.flyinggiants.com/forums/members/mike-c-.html here on FG. The first guy, Terry, sent his back to DA so many times and did so many mods he finally got tired of it and just tuned it rich so it wouldn't lean out too much and die inverted. The last I heard of Mike's he wouldn't even fly it inverted anymore. There is definitely an issue there on a lot of these engines, obviously as the guy that started this thread also has the same problem.

[classclown]

	Quote: Originally Posted by bodywerks
	Well, I've only owned a few DA-50's and never had a 'dying inverted' problem. So I guess you could say I swear it does not exist...

Yup, same here. I have never had any problems with any of my DA engines either, unless you count the time I planted a plane into a mud bank, and mashed the bearings. I ended up getting the engine back from DA, full upgrade, bearings, shipping and all, free of charge.

[Al Lewis]

Then of course there are these guys.

http://www.flyinggiants.com/forums/f...bles-help.html

http://www.flyinggiants.com/forums/f...-inverted.html

http://www.flyinggiants.com/forums/f...tml#post824907

[gapple12]

Well, I can say it was like night and day once I did this on my DA85. Yes, I had to re tune the engine, but damn if all problems are now gone. I had the RPM's dropping when inverted and elevator up and down would also change the engine. Very happy with my results. The question is, why dont they just supply them from the get go??

[Al Lewis]

Scott Ellingson is the only guy I knew of that installed the nipples on the metering plates on his Brillelli engines. He told me one night that I wouldn't believe how many guys tried to run their fuel line into the nipple! LOL Maybe that's why. I believe some of the 3W engines come with a similar mod already installed.

[gapple12]

	Quote: Originally Posted by Al Lewis
	Scott Ellingson is the only guy I knew of that installed the nipples on the metering plates on his Brillelli engines. He told me one night that I wouldn't believe how many guys tried to run their fuel line into the nipple! LOL Maybe that's why. I believe some of the 3W engines come with a similar mod already installed.

Yea, I bet your right.

[FlyingMadDog]

	Quote: Originally Posted by Al Lewis
	Scott Ellingson is the only guy I knew of that installed the nipples on the metering plates on his Brillelli engines. He told me one night that I wouldn't believe how many guys tried to run their fuel line into the nipple! LOL Maybe that's why. I believe some of the 3W engines come with a similar mod already installed.

The 3W's I've seen have a pressure nipple piped to the intake bell that they mount on the carb intake to get the pressure from the moving intake air (which is more constant than just leaving it in free air).

I actually made a static pressure tube (from a pitot tube) and used that on my Brillelli (idea stolen from P.E. Reivers) and may use that on all my planes from now on. Sold the plane recently so I don't have pics, but will be making another soon, I'll take pics when I do.

[WOODisGOOD]

	Quote: Originally Posted by RMaia
	After I had a few problems with them my DA-50 dying when inverted, I decided to follow the tips and make the vent in the carb diaphragm hole (as well as re-tune the needles, which turned out to be way off...). However, that got me wondering... What's the exact function of that hole? Reading up on how these carbs work, seems like the diaphragm is what controls how much fuel is fed into into carb itself, so the hole would allow the fuel pressure (under the diaphragm) to be equalized with the ambient pressure... Did I get that right?

Is there a cover that goes over the motorbox to prevent air from going into the fuselage?

[Al Lewis]

If he followed my thread the tube terminates inside of a pill bottle or film container so it is fully protected from any turbulence. Even without that though there usually isn't enough forced air turbulence inside of the fuse when compared to the cowl.

[gapple12]

	Quote: Originally Posted by RMaia
	After I had a few problems with them my DA-50 dying when inverted, I decided to follow the tips and make the vent in the carb diaphragm hole (as well as re-tune the needles, which turned out to be way off...). However, that got me wondering... What's the exact function of that hole? Reading up on how these carbs work, seems like the diaphragm is what controls how much fuel is fed into into carb itself, so the hole would allow the fuel pressure (under the diaphragm) to be equalized with the ambient pressure... Did I get that right?

In the pics on the first post, there looks to be a cover that goes on the motor box?? I'm assuming your running the cover on that, or that air would go right at that tube.

[closetguy]

well i guess i am one of the other guy's,i have one da50 and a da50r both engines can go inverted,flatspin,blender and never died yet,i think it is all in the needles setting?
also i have look at the dl reed design and i don't see how you could easily use it in a da,but who knows.