Weird DA85 problem

[gapple12]

Having a weird issue with my 85. The plane seems to fly great, but around mid throttle, if you point the nose up the RPMS drop quite a bit. If you point the nose down, they rev back up. In my experience this is the engine running lean, but I have the needles at 1 1/2 on both. I am running a Greves pipe with a 11" header and about 2 1/4 inch extension on the header. Also, inverted the engine revs up a little. Just weird to me........

The engine has all the updates recently from DA

Drew

[gapple12]

Oh, I also did a really good clean on the carb to see if that helps, and nothing.........

[MODE 1]

Getting air bubbles in it? Can you simulate it whilst holding the model and pointing it up n down like a glow?

I would check the tank out first and the plumbing for any split hoses.

[gapple12]

	Quote: Originally Posted by MODE 1
	Getting air bubbles in it? Can you simulate it whilst holding the model and pointing it up n down like a glow?

Well, its a 35%, so holding the plane pointed up would be difficult. Now, the air bubbles could be something. With the hatch off I can see the tank bubbling up a bit during throttle up.

Lines are all brand new.....no leaks

[MODE 1]

Thought you Texans were all big n strong LOL
Gas will foam anyway when the motors running. You cant see any bubbles in the lines going to the engine though?

Does it do the same thing during KE?

[kevinf2501]

Sounds like a fuel pickup problem. You may want to check and see if your clunk has come off or has come forward.

[JoeAirPort]

This could be in-cowl air turbulants that are caused by the change in pitch. Could be changing the mixture since the RPM's are changing. Do your hear the smoothing out of the sound as the RPMs increase (lean)? Does it pop and sputter or 4-cycle when the RPM's drop (rich)?

[MODE 1]

	Quote: Originally Posted by JoeAirPort
	This could be in-cowl air turbulants that are affected by the change in pitch. Could be changing the mixture since the RPM's are changing. Do your hear the smoothing out of the sounds as the RPMs increase (lean)? Does it pop and sputter or 4-cycle when the RPM's drop (rich)?

Easy fix then to solder a small brass fuel tube to the top plate of the carb and run a line into the fuse

[gapple12]

	Quote: Originally Posted by MODE 1
	Thought you Texans were all big n strong LOL Gas will foam anyway when the motors running. You cant see any bubbles in the lines going to the engine though? Does it do the same thing during KE?

Well, our airframes are STRONG!!!! I will have to run the motor again and see if there are bubbles in the line. I dont notice it in KE flight.

[MODE 1]

Get Mike to give ya a hand deffo sounds like air but doing what I suggested from Joes post will alleviate that issue.....I do that on mine (brass tube) as a default cuz the air in the cowl is all messed up during flight.

[JoeAirPort]

Could very well be the solution. If you think about it, when he points the nose up the air could be pressurizing the cowling which would make the metering diaphragm get pushed by air through the vent. This would call for more fuel and make it rich. Pointing down could get rid of the pressure and maybe even cause a slight negative which would pull on the diaphragm causing the metering diaphragm to get pulled and the mixture becomes lean. It's pretty hard to know what's going on in that cowling during all the flight conditions. You can easily put an 8-32 barb on the vent after tapping it out. Some red locktite, snug it down lightly. After it dries overnight take the dremmel and remove the excess threads off the inside to clear the diaphragm. Worth a try. It certainly doesn't hurt anything unless you do it wrong and plug up the fitting or pinch the fuel line going into the fuse. If that doesn't work you can slap a velocity stack on the air intake. That can clean up dirty air and make your mixture very consistant. It's also very easy if you have the space in back of the engine. It fits right through the hole in the firewall. If that doesn't do it I have no idea what else would cauce that RPM's changing...besides some obvious plumbing issues.

PS: Sending it back to DA is probably not a good idea since it's probably not the engine. I'd try the above things first.

[gapple12]

	Quote: Originally Posted by JoeAirPort
	Could very well be the solution. If you think about it, when he points the nose up the air could be pressurizing the cowling which would make the metering diaphragm get pushed by air through the vent. This would call for more fuel and make it rich. Pointing down could get rid of the pressure and maybe even cause a slight negative which would pull on the diaphragm causing the metering diaphragm to get pulled and the mixture becomes lean. It's pretty hard to know what's going on in that cowling during all the flight conditions. You can easily put an 8-32 barb on the vent after tapping it out. Some red locktite, snug it down lightly. After it dries overnight take the dremmel and remove the excess threads off the inside to clear the diaphragm. Worth a try. It certainly doesn't hurt anything unless you do it wrong and plug up the fitting or pinch the fuel line going into the fuse. If that doesn't work you can slap a velocity stack on the air intake. That can clean up dirty air and make your mixture very consistant. It's also very easy if you have the space in back of the engine. It fits right through the hole in the firewall. If that doesn't do it I have no idea what else would cauce that RPM's changing...besides some obvious plumbing issues. PS: Sending it back to DA is probably not a good idea since it's probably not the engine. I'd try the above things first.

Thanks Joe, I think your right. I will try that this weekend and see if it fixes the problem. So, take off the silver cover plate where the diaphragm is at and tap it for a small nipple. Then run the line into the fuse where the air flow isnt changing, right? Ive seen this before, but on mine I put the Gatorade bottle around the carb to catch the spit fuel from the motor box and figured this would cause a steady air flow to the carb.

Thanks for everyones help, its been bugging me, but the plane still flys great so Ive ignored it till now.

Drew

[gapple12]

Here is a pic of my tank set up.....

[JoeAirPort]

Yep. That's it. Very easy to do. I sure hope it isn't the Gatoraide bottle that's creating the weird air flow problems. Hard to tell though. If it's the vent getting hit with air turbulants your vent barb fix will cure it. Just an 8-32 barb and 1/8" tygon...and a little zip tie is all you need.

PS: I gotta try one of those soda bottle tanks. It's great that you can see through it.

[gapple12]

	Quote: Originally Posted by JoeAirPort
	Yep. That's it. Very easy to do. I sure hope it isn't the Gatoraide bottle that's creating the weird air flow problems. Hard to tell though. If it's the vent getting hit with air turbulants your vent barb fix will cure it. Just an 8-32 barb and 1/8" tygon...and a little zip tie is all you need. PS: I gotta try one of those soda bottle tanks. It's great that you can see through it.

Great, thanks Joe

I love being able to see inside my tank, big plus for me.