Can any one help???
I have owned a few DA's before 50,15o and 100's but my latest one in my 2.6 extra 260 is displaying a trait that I used to get back in the day when I ran weed whackers. Basically the mixture is fluctuating during aerobatics. It will be fine going along a straight run up or vertical but as soon as it gets to knife edge or inverted it goes rich as hell. Also while in the prop hang it just wont settle to a sweet spot. It either surges or dies thus causing the plane to rise or fall. As I mentioned I used to get this on my Zenhoas but never a DA.
I have built a baffle around the card to try and balance the air around it but with little to no change.
Anyone else suffered this problem? IF so what do I need to do to sure it?
Thanks for your time.
Regards Al

If the engine gets richer, the pressure on the regulating membrane increases. Make a pressure reference line on the carb plate, and run it to a small (matchbox sized) closed balsa box inside the fuselage.

That should do it.

Hi Pe,
Thanks for the feed back.
So I guess its the same as I used to do with the Zenhoas??
A few questions please.
Do I have to solder a tube onto the carb plate over the small hole through the plate on the front? ( Dumb question I know I just want to be sure ) Also when I used to do this the hole used to be in the middle of the plate. On this Walbro carb which made it easy... On this walbro the hole is right between the raised area and the flat outer.. On a radius [:@]
Last thing.. Does it have to go to a matchbox size hole? I used to just attach a tube through the firewall so it could breath from inside the fuselage. If it has to be a box then how much of an inlet do I make for this box? Or none at all???
Sorry for the daft questions.. I have got lazy with the fit and forget DA's that I have had up till now.
Regards Al

The box needs to be a "dead air" box, so the inlet should be the same size as the tube that passes through it with a simple pinhole in the box somewhere to stabilize air pressure. A tube still has to be added to the diaphragm cover if it does not already have one. The hole location is irrelevant on the carb plate as long as it's not internally blocked by some part of a gasket or diaphragm flange.

Another method is to attach a brass tube to the front of the carb that has it's opening in line with the carb venturi. Run a line from the diaphragm cover to the tube on the face of the carb. There's some pictures of this arrangement on the Wild Hare site somewhere. It's similar to the manner that 3w uses for their engines.

The tube into the fuselage method provides inconsistent results.

Building a reversed scoop on the bottom of the cowl to provide positive hot air extraction helps eliminate pressures indide the cowl. Sometimes just cutting a large hole in the bottom of the cowl allows air to hit the firewall and make extraction of hot air not happen and build more pressure.
Your engines cooling will be improved also.

Cooling is not an issue ( Especially in the UK []) I did however make a baffle and increase the exit hole size in the cowling with no improvement.
I have just made a Ply box internal measurement is 3"x 2.5" I ran a tygon tube from the 2mm ( I/D) Brass tube that I soldered to the front plate of the carb. I made sure this was all clear. I have then ran Tygon tube from this brass tube through a tight hole in the firewall. This pokes into the middle of the ply box which is glued to the inside of the firewall. In this box I have drilled a 4mm hole in the face opposite that of the tygon tube. I am just about to put a small 2mm hole in another face of this box, then glue it onto the firewall over the tygon tube.

Sound about right ????
Also will I have to alter my mixture at all??

Thanks again for all your help guys.
Regards Al

All gas engines with diaphragm carb's can do this, not just DA. The pressure in the cowling affects the vent side of the diaphragm, changing the mixture. Getting that vent to be at a constant pressure is the goal. I don't know personally which one of these methods works best. The fuse can change pressures just like the colwing so that's not a guranteed solution. I like the balsa box but if that's in the cowling or fuse couldn't it still change pressures albeit a little more dampened? The fix that 3W uses makes the most sense to me. Since the carb's vacuum that pulls on the other side of that diaphragm draws air from the venturi outlet, it would make sense that you want the diaphragms vent side to sense that venturi inlet air pressure. That way the carb is pulling air and fuel from the same reference pressure (venturi inlet/carb air inlet).

Here you go Pat.

That's the one, and thanks.

A 2mm hole may end up a little large on that box. Make it where you can cover it over with a piece of tape to later make a smaller opening for trial and error purposes.

One last thing, have you adjusted your needles lately too?

The first box I made I used thin balsa and made it about 1" or so square. I only made one hole in it just large enough to fit the diaphragm line into tightly, nothing else. Balsa breaths pretty well.
If you are on the rich side you also "might" be experiencing some fuel puddling in the reed area, when you role etc. it all get dumped at once.

Pat, do you think Tom's mod would work going into a velocity stack?

mrbigg-I have a rear induction 3W with a stock velocity stack from 3W on it. It has a line going to the lip of the stack.

Yep

Apart from the tube entering the balsa box (not ply), there should be no other holes!!!
The balsa wood breathes enough to allow pressure equalisation.
Altering the mixture will be needed if the reference pressure changes. Just tune the carb like you should, and you will be fine.
How?
look here
http://www.prme.nl/forum/viewtopic.php?t=10

I spoke to a 3W user today, who still had carb issues with the line running into the carb stack. It seems not a cure-all then.

That's for sure. I had to make the balsa box to fix a 3W 28 a couple of years ago. When I was experimenting I let the line dangle out of the fuse. Bad idea, the vacuum created when it was pointing back wards would kill the engine about 1/2 way through the pattern. Only took me 2-3 dead sticks to figure that one out
Don't laugh Pat, you were there too

I haven't found a winner take all yet. Neither have others that have expended a lot of time and fluid engineering on the problem. The 3w method of routing the tube into an air intake seems the most consistent, but not always. Toms' method works with about the same percentage as the one from 3w. The balsa box is the cheapest, easiest way to try getting a handle on the problem if an air pressure deflector did not work. Start easy and cheap and work up is about the best one can do. It's a royal pita when methods don't work.

I like the way you think. I had this issue with a DA 50 and never resolved it. I tried many things...even replaced the carb with a brand new one. Not sure it was even in-cowl pressure/drafts issues.....I sold it. Using a ZDZ 50 NG in the same exact plane had no issues at all. It wasn't the cheapest or easiest but it sure worked well. It's so awesome to have a plane with no engine issues.

Now, if we had ALL the data, maybe we could analise the situation, or come nearer to the solution.
The ZDZ of course was not the solution, the carb also was different, though carb loation was about the same. Maybe switching carbs would have done it as well? If it would, what parameters inside the car were different, how were the needles set?
Questions ad infinitum, but somewhere in there is the answer for sure.

I got tired of the problem to be honest. I almost crashed a 30% Edge that I had over $2000 into (and really liked). 6 dead sticks. A few of those were near crashes. One should have been toast but the RC gods were with me.The ZDZ did solve it. Not a dead stick since, engine doesn't die in outside loops and other weird attitudes. I'm going on my third season with the plane (see Avatar). I think it was a good decision.

Joe, I toasted a brand new 86" QQ Yak with a DA 50. Inverted harrier on high idle at 20 feet, engine stumbled, then quit on throttle-up.

Whoa, same exact thing happened to me. But I was somehow able to roll it and land it with a relatively soft landing. LUCKY !!!!!!!!!! I knew I only had so many of those left. The 6th dead stick I said to myself "ok that's it, that is the last flight of this engine".

I'm sure somebody pe reivers could have fixed it but I was tired of it. I admit I gave up on it.

And it was the engine's fault?

Come on, be serious. There are so many variables that affect the engine's performance, in the hands of the person setting up the engine, tank and/or the plane...

I have hundreds of DA-50R flights on about 6 airframes, without an issue.

I would not fly the plane unless I feel the engine runs properly on the ground and I would not go down low and inverted unless I feel the engine is properly broken in and the transition is perfect...

So if it's a new DA with only 2 gallons of Ashless through it and the mid-range is still burbling a little, I will not go down low and inverted and or do anything that might put the plane/engine in jeopardy...

When you have a dead-stick, you bring it down and do not fly anymore until you figure out what caused the dead-stick (unless you run out of gas). This is simple common sense, and if the situation continues, you can send the engine to DA who will test it for you and tweak it for FREE.

Different engine on same exact air frame and the problems are all gone. I don't really care what the problem was...I just go flying now.