To Pressurize or not to pressurize ...
 

That is the question, is it nobler to suffer the headaches of the 3rd line running from the muffler to the tank or to plug it off and just deal with the typical 2 line setup.

What would be the advantages of a pressurized system?

if pressurize system is not used, would it be necessary to have a pump for larger four-stroke motors?

I have a new Enya 1.55 4-stoker that doesn't seem happy with a pressurized system ... and it is finicky to set the carb to run at hi RPM ... only seems like a 4 click window from rich to lean

Suggestions and feed back welcomed. Advice from anyone except my dad is appreciated .... :D Joking dad .... NOT!

To Pressurize or not to pressurize ...
 

I'm not sure I fully understand what you're trying to do but plugging the pressure line is not a good idea. The tank needs to have a vent so as the fuel is used up it doesn't create a vacuum in the tank. That will lean the engine out, cause some damage, and then the engine will flameout.

I'm going to answer you're question with another question...

Do you want to avoid the hastle of a 3rd line just so you can create tuning problems?

To Pressurize or not to pressurize ...
 

The current setup is a 3 line setup ... vent line, tank to carb line and muffler to tank line ...

fuel system is the same ... only the motor was changed ... enya 120 R to a Enya 1.55 R ... The 1.20 ran fine not pressurized ...

thus acting like a dual vented tank ... I would think ...

only plugged off the pressure fitting to the muffler.

Same for the 1.55 ... only the 1.55 acts like it starves for fuel at the High end ... when I do get it to run at high RPMs (full throttle) .... I have a very narrow adjustment range ... making it hard to tune for peak performance ... motor has 1 flight on it .. which almost was its last ... dead stick landings are not fun when initialized at 150 ft upline to hammerhead and motor dies before you reach the top! nice tailside though ... but not much room for error when ground is coming up quickly and you need to find the runway in a hurry ... I made it with a textbook landing ... but the cardiovascular system got a workout!

other than that, I ran 2 16 oz tankfulls in it prior to flight and I was only semi confident in the setting ... which did seem to be ok for the ground ... not in the air ... Motor seemed to want to run on the lean side of the setting but I choose to stay on the rich side because of the new motor sysdrome ... fuel used: 15% cool power.

motor also had a 16 oz tank ran through it on the bench ... which dad said he had it adjusted and running fine ... I told him that was on the bench and not in-cowl nor in the plane and did not count for more than a good starting point ...

this procided to a Father-Son moment! or a double senior moment ... :)

To Pressurize or not to pressurize ...
 

Try it without pressure, your engine may like you for it . Only engines with poor fuel draw need pressure.

To Pressurize or not to pressurize ...
 

With my tank positioned with the center of the tank about equal or a little less than in line with the spray bar on the carb....... I do not run pressure. I have a lot more movement on the needle valve for fine tuning. The engine runs fine inverted or not. The only time I use pressure is when I am flying my Sig SE and the tank is too low. I run the line that would be used as a pressure line out the bottom of the cowl as an overflow line. Several of us do this. What did people do when they had no muffler? They could not run pressure and the engine ran just fine. JMHO

To Pressurize or not to pressurize ...
 

On first flights with a new engine (assuming it passes the nose high test) I'll always try to use no muffler pressure. Some engines are better than others at this. But from the symptoms you've got with the 1.55 it sounds like it needs the pressure because the carb can't generate enough fuel draw. Any RC engine will run just fine on the bench with no muffler pressure although transition could be affected if the carb is designed around muffler pressure.

Oh Boy...!
 

The engine is no where near being broken-in and it will not deliver power without sagging until it is.

Enya engines are wonderful works of art, BUT, they require an extensive break-in if they are equipped with a piston ring.

Blubbery rich running for a while is the ticket. Forget about aerobatics and high performance flying until the ring is seated. Just hope that you haven't already ran it hot enough to lose the ring's tension. Good luck.