I have an MDS 38 running on YS 20/20 fuel swinging a 10/5 prop. It is mounted on a Magic 3D.
I broke it in on the ground for about 4 tanks and started flying. It runs great on any attitude, upright or inverted but as soon as I go for a loop (at about 1/2 to full throttle, the engine dies.
Where should I start troubleshooting this problem.
Any info is appreciated.
Thanks

Does it die if you bank and pull a hard turn? Could be that centrifugal force is slinging the fuel back into the tank.

Investigate the height of your fuel tank relative to your needle valve height? they should be the same.

Also make sure that after you peak out the high speed needle to maximum, that you richen it back up 3 or 4 clicks.

The MDS requires more break-in than some others. Keep the mixture a little richer for the first couple gallons.

Jim

I know it may sound silly, but check the clunk to fuel tank clearance. I had a plane that did about the same thing, after checking, double checking everything over and over, I finally decided to change the tank. Once I had it out of the plane, I noticed that the back of the tank was concave shaped. As the clunk would travel across the back of the tank, it would barely rub this area. I shortened the clunk line by 3/16" and put it back in the plane, no more fuel problem. The clunk was sucking up against the tank and starving the engine for fuel.

My thought to Dave......its starving for fuel becuase the clunk is held up against the tank. Shorten the fuel line for the clunk should cure it. Take Care, JT

I've seen this a few times, too.
I love Sullivan flex-tanks, but have gotten two of them that way.
The latest one not only has *that* problem, but the internal "bubble" for
the vent line is closed off, too. ARGH!

Shouldn't there be enough fuel on the line for the engine to burn during the loop?
This thing dies on the top of the loop. I did not see this on hard bank, only on loops.

Does it do it in tight loops, or large loops, or both? Have you tried an outside loop to see if it happens then?

You said it flies fine inverted... does that include extended inverted flight, or just short (50' or so) flights?
Just trying to help slim down the possibilities here

Try a continuous turn. It may look silly but it might help to figure out the problem. There is enough fuel in the line to feed the engine for a while but... Have you ever placed your finger over the top of your straw at a restaurant and lifted up? It carries the soda, water or whatever up with it until you let your finger up. Same thing happens with your fuel, you plug the end and nothing goes out the other end.

Stick a straw in a glass of water, put your finger over the end, then pull the straw out and suck on the other end. You can't get the water out unless you remove your finger from the other end. The engine would not get the fuel in the line due to the clunk sucking up against the tank, this stops the flow. It's easy to try, and won't cost anything. You would not get the same effect in a tight turn as the clunk falls down the side of the tank, where there is more room.

Hallo!
The problem is usually a too lean setting!
I have never heard of or experienced that the clunk could get "stuck" against the tank wall so no fuel gets through to the carb!!!
That the engine starves is because of too high G pressures!
Either make the turn /loop larger or open up the fuelneedle somewhat!
But first you have to have a good tank and tankinstallation.
The best tank available are the Japanese Tettra "bubbleless" fueltanks. We use them in pylonracing all the time.
Regards!
Jan K
Sweden

It's *usually* not so severe as that, and I've never seen it happen with a tank
that has a clunk with a cross groove cut in it. (as most do)
Some of the cheapo clunks don't, and can suck tankwall in that case.

The problem with the clunk hanging up usually affects the plane more in a roll, bank,
or an outside maneuver, when below half tank. The clunk gets hung up in the air.
But, not seeing his tank install, and having had it happen to me, it's a valid "check" item.

I agree that it's probably simply "too lean", for whatever reason.
I had this on a Kougar once. It only showed up in high speed loops or pullouts, and I
fixed it by looping extra fuel line in front of the engine, and going around back to the carb.

Thanks for all your suggestions.
2 things I will try this weekend:

1. Make the needle setting richer that it is now.
2. Add more fuel line between the carb and fuel tank.

I will post the result after.

Thanks