theres no turbulance in cowel just pressure and air flow tubulance is increace and decrece in air pressure not air flow once u start a plane airflow is pritty constant only thing canges is airflow in manuvers air attack comes from different angles .

here i go again
a little infomation is more dangerous than no information everyone has a opinion i not going to fill up this forum with replies or attacts unless u know more than walbro mechanic and if u have not tried it u will not know the airflow will affect 5% off planes due to location and air restriction now to make the full 5 % your exit hole is too small and run the risk off overheating . the spring is 10 x easer than trying witchdoctors way if u have the problem off engine dieing on inverted low and go to full throttle and it burps and dies its need a harder spring but it sags and dies it need a softer spring .
i know there will be a forum about this forum but i had enough off false hopes if it fails u can kick my ass .

well if u are all sick of problems who has $200,000 to get walbro to make 100,000 carbis for one model unfortunataly until someone orders that quantity its not going to be viable to get one made for there type of engine and some mods will be needed to create a reliable motor there close but some arnt untill people relise this there will be debates for as long the sun go down.
i dont want to piss anyone off but its the cold hard truth .

srm99... I did read the manual you posted. Please take a look at the site Greater Southwest Aero Modelers and go to the Tech Site and look for Walbro Carb Tune Up & Illustrations.
Link http//tech.flygsw.org/walbro_tuneup.htm

I have been around gas engines since the 80's when a local guy first put a Quadra engine in a scale plane, his name is Dario Brisighella, owner of "US Quadra". He had worked on the same problems we are doing battle over. This is not a new thing nor is it just a DA problem. I only mention Dario because anything I say or do has come from his earlier work and reading his book “From the Firewall Forward� . The above link is the next best thing to Dario’s book. This is how I set up all my gas engines and I do not have any problems. At this point I am going to follow the Airplane experts. As for the Walbro manual, I did read it over and if you could read Dario’s book and the info from the link. It makes sense to me. I am using the tube and the box. Sorry I am NOT attacking srm99, and I do not understand why you think everyone is attacking you ideas. I am sorry if I choose to take the advice of guys who fly gassers over someone who races go carts or runs chain saws. I would also recommend talking with the manufacture directly if you are having problems with your engine. As far as my DA 50, it is the most consistent running engine I have seen. Sorry if that bugs you.

3dsky i have read this manual and it says the same as i am trying to say.
my engine is running great i dont have any problems my mate did now its fixed as i wrote before . what i am doing is trying to educate people that do not know and hopefully help i know its up to the indervidual if they want to listen or not .

Ok so I was bored today and decided to join the balsa-box club..I had a spare backplate so I did the mod..my engine runs good now but I figure it cant hurt to try it..maybe it will run even better

Great let us know how it runs. I am wondering if you will hear a difference. I just tested (extended inverted drop) my DP Edge with a DA 50 inverted at about 2000 RPM idle and had no change and no dead stick. So far I get solid RPM no matter what I am doing. So far the box has not caused a problem.

I also think I have a test setup to test the effects of +/- pressure on the diaphragm. I need to get a set of springs so I can measure the effects of +/- spring tension. I have a Digital pressure tester and should have pictures of the setup and data by next week.

I will report back..The only issue I have is when doing an inverted elevator, when I throttle up there is definitely some loading up but not enough to put out the plug..my idle is about 14-1500..the same thing happens when exiting a knife edge but its very minor..Ive never had or came close to a deadstick..Im wondering if the box will make any difference in regular flight?

I have an older CAP 21 that has nothing on the carb and it has RPM changes when I fly it but never had a dead stick (NO 3D), it is heavy and it is flown wide open. I just put up with it, it is like auto RPM in the down lines. I will try the box on that plane in a couple weeks (battery issues). Good Luck

Well Im happy to report another balsa-box success! Works excellent! The carb plate worked pretty good but there was still the occasional loading up while inverted but with the box its completely gone..no loading up at all inverted..in fact you cant tell ANY difference between upright and inverted flight..I tried to get it to load up by doing long inverted elevators and rolling from inverted to upright and it never missed a beat! If anyone is having those issues..the box works!!!

Ya gotta love the BOX Ouch, I just think I broke my arm patting myself on the back

Ok, srm99, let's talk about the popoff pressure deal. I have a FPE 4.2 that runs great except it will not transition. Once I get it opened up on the ground or in the air, it is great. I had to put a static line from the carb into the fuse to eliminate in air burble but that's solved. My problem is the engine will die if you don't "nurse" the throttle up after it's warmed up. I've backed out the low needle as much as 3 turns, that won't fix it! I put in a new carb kit with diaphrams, fuel metering needle, etc., no luck. It just won't give a rapid transition.

So, could this be a pop off problem?

Larry

You Da man!!!

Long Live the BOX!!!

It possibly could be a pop off problem,,

Does the low speed needle make any differance could the low speed ports in the carb housing be plugged? Could the base gasket on the carb be leaking causing an extra lean condition? If the base gasket leaked it wouldnt die when you backed all the way off the throttle.

If you can make it run perfect on the ground, but it gives you fits in the air, I would not mess with the pop off pressure.
There is an external force making it run poorly in the air, that being air pressure (+ or -).

well its up to u u have read what i have said before its no use arguing the point u can try everything else then when it dont help try my idea ?????[:-][:-][:-]

Not arguing at all.
The point I am making is if it will run on the ground perfect then it is not a pop off pressure problem. You are changing the pop off pressure to compensate for a pressure gradient problem, not a pop off pressure problem.

ok if fine on ground going rich in the air or leen there is only one thing that regulates that and that is the metering spring and that works off the dirafram and that works off the vacuum pulse now u cant cange the dirafam or the vac pulse so whats left the spring on the metering neadle unfortunatly walbro do not check what pop off pressures as they were desighn for a CHAINSAWso varing the pop off pressure will normaly solve 99% off the problems the other 1% is other problems like the neadle high and low not correctly seated

when u are on the ground the motor is under load when in the air the engine is unloading and will rev higher than on the ground and also there is less load on the engine thats why its so different

it sounds like its too rich on the low end and if your low speed needle not working it could be that the metering needle is stuck and when it warms up its freeing up. it could be stale fuel too .
what is your low speed neadle at now

What about when you are at a constant throttle setting (engine running fine in the air level) and you start to knife edge or roll inverted and your engine goes rich and starts to 4 stroking when it did not just before at the same throttle setting???? Not pop off pressure, adverse cowl pressure.
This is what I experienced and so have many other people. In this scenario you would have to raise the pop off and some of the 50's are hard enough to get rich on the high end anyway.

RTK
DO U HAVE THIS PROBLEM

AND IF U DO DO U HAVE A PHOTO OFF IT IN AND OUT OFF THE COWL

I did have that problem with a 3W rear carbed engine. When I first tried this experiment I just taped the BOX to the side of the fuse to try it and it worked. I could not find a non pressurized area in the air frame.
People have been trying this since I suggested it over a year ago here on RCU. There were a few threads before and I do have witnesses that can corroborate my claims.