Iam in the process of breaking in my Magnum 91 4s. Ihave run 3 tanks of 15% already. On the second tank the return line from the muffler to the fuel tank popped off at the muffler fitting. The engine seemed to be running well before the line popped off. After it came off the RPMincreased and it sounded like it was running with alot more power. I ran it up and down the power scale a few times for a minute or two before Ishut it down and reconnected the line.
Why would the engine appaer to be running better with no pressure from exhaust to ffuel tank? And am Ijust imagining it is running better? Could it be that because i am running it rich without the backpressure it leans out some? Thus increasing RPM?</p>

You guessed it.

You're running it rich and without the pressure it leaned itself out and got closer to where it should be

You may very well have a point about it leaning out. How rich are you running it?

You might want to check your pressure line into the tank if the exhaust pressure is strong enough to pop the line off. Must have some kind of restriction. But to answer your question, yes, the engine leaned out without the pressurized fuel to the carb.
dan

Four strokes are much better at fuel draw than the current two strokes with their big carb throats. So yes, a four stroke will run quite adequately with no tank pressure, but I treat tank pressure as a plus to the over all fuel system. For instance in a long upward climb the pressure will ensure better performance. YS is a different matter since it has its own pressure feed system. I am told Laser engines do not even have a pressure tap on the exhaust system for their engines.

I had a pressure line pop off the other day. My Magnum did not even miss a beat, never slowed down, kept right on going. In my case the very end of the tubing split. I replaced it anyway. I love the old lady, she just keeps on chugging along. Looking a little worse for wear but turns the prop with authority.

Cheers,

Chip

The inside diamter of my fuel line to the muffler is too large. It is some old stuff I had lying around. I knew it was too large but it will be a few days before I can get 3 ft of the right stuff. And I was dying to run this bad boy. I do have the right size feeding fuel to the carb. Just ran out of it.
I even had a plastic tyrap trying to hold it on and it still popped off.
As for how rich is it? Pretty rich....Iam scared of overheating during break in. Right now it is running at 9500 RPM. Ifigure Ishould get another 1000 after a few tanks.

cut about 3/16" of an inch of the too-large fuel line, stretch it out and put it over the outside of the line where it connects to the muffler. This might help tighten up the fit until you can get some smaller line by putting thicker walls and more pressure on the fitting.

What prop are you running. I was using a 13/6 and getting 10,500. To much, went to a 14/6 to stay below 10,000

If you're running an engine on the bench then muffler pressure isn't necessary even with a big bore 2 stroke carb although the needle settings will be way different to what you'll end up flying with using muffler pressure.

G'day
Yes, muffler pressure may not be necessary. And as WCMORRISON said above, Laser engines do not provide a muffler pressure tap on their mufflers. My two Lasers, a 70 and a 100, both run perfectly without muffler pressure. The 70 is mounted with the cylinder vertical (plug up) and its tank is quite low relative to the carby but this does not worry it at all. It is in a large trainer and is not likely to need to fly inverted. The 100 is mounted sideways in a Decathlon and is quite happy no matter which way the plane is pointing. I also don't use muffler pressure on my old Enya 46 four stroke and I have an older OS FS40 which is quite happy without muffler pressure.

That said, all my Saitos use muffler pressure and so do all my Chinese clones.

The Magnum is mounted on the Tiger (side mount) and I am running an APC 14X6 per Magnums break in instructions. 
Friday and sat are supposed to be really nice days here this week so I will get a few flights in with last years model and run the Magnum another time or two to try to get my idle just right.  Sounds like my top end is where it should be at 9500.