Engine vent line question
 

Never had this happen to me before yesterday, but, will an engine continue to run (albeit not very well) with the vent line disconnected from the muffler? I was flying yesterday and except for my first flight, the engine seemed to not be generating very much power. In fact, full throttle was like 1/2 throttle. Halfway through my 3rd flight, the engine began to waiver and run unevenly, almost dying a few times. I landed, attempted to re-fuel, and fuel started pouring out of the cowl. When I removed the cowl, the vent line was dangling, totally off the fuel tanks brass vent line hook up. The engine is s Saito 72. Can it still run without the vent line attached?

Yes an engine can keep running with a pressure line failure and exactly as you said often it runs poorly. The degree of effect at the loss of a pressure line can run almost from little effect up to complete and immediate stoppage. The variables are many: mixture setting and even fuel tank orientation and tank height/spray bar relationship and even ventruri size.

Some of the earliest Saito's .45's and some .50's had no pressure fittings for tanks and needed none. The loss of muffler or pressure line will almost always result in the loss of head pressure at the carburetor so the effect is normally poor running or worse.

John

I kind of dissagree with John. The 4 stroke engine requires muffler pressure to pressurize the fuel tank, therefor pushing fuel to the carb. The 4 stroke glow engine is not like a gas engine, which has a pump in the carb to supply the fuel.
Just hook up the vent line to the tank, it will solve your problem.

Hmm seems thats pretty much what I just said? The level of problems with the loss of muffler pressure depends entirely an all the factors I just listed that's a fact. Also keep in mind the earliest commercial Saitos (.45) came with no muffler pressure tap at all on the short stack. However if one looked at the size of carburetor venturi you would see that it was is choked down far smaller than similar displacement engines to provide more fuel draw.

John

Years ago none of the model engines used a pressure line. It was not until the late 60's that it became the normal set up. Most 2 strokes will still run reasonably well without a pressure line, it does take a change in needle valve setting however. I also have some early 4 strokes that did not use a pressure line and, all my newer ones will run without the pressure line connected, just not as well under all circumstances.

With the muffler not supplying pressure to the engine, the mixture will be different. If you adjust the needles before you fly with the pressure line not connected, you can make it run ok, maybe, but just ok. They are designed to run with muffler pressure and that is how they run the best.

And due to the reasons John stated some engine plane combos will run better then others without the pressure line. I have had planes that didn't know if the line was attached or not but the same engine in a different plane would hardly run at all if I forgot to put the pressure line onto the muffler nipple. For the most part they tend to run poorly without the pressure line.
I lost my favorite Kaos when the muffler header broke on the down line from a stall turn, I hit the throttle and all the engine did was sort of burble and wouldn't rev up for the pull out.