OS FS 52S confusing instructions HELP please

Assuming it's an exhaust pressurized tank, put your finger over the exhaust.

Steve

Put your finger over the end of the exhaust to prime the carb. There is no pump.

I sure wouldn't prime it with a starter though !!! The other guys are right, put your finger over the exhaust and pull the prop through 3 or 4 turns. If you use the starter, you can hydrolock the engine and bend a crank.

CJ

Many thanks for the advice gentlemen.

I hadn't actually intended to use exhaust pressure, as the motor is inside a round cowl in a Flair Puppeteer

and there's insufficient room for the OS muffler without spoiling the lines. I intend using the same simple exhaust arrangement as I did with the previous FS40 . I made a simple muffler with a brass tube leading from the exhaust header to an expansion chamber (much bigger brass tube with ends) and then an egress pipe; all silver soldered together and fits in the cowl neatly.

Do I need to fit a nipple to the expansion chamber to pressurise the tank do you think, or is there a simpler way? I had no trouble with fuel supply using the FS40 without exhaust pressurisation.

Mike

A lot depends on the position of the tank relative to the carb. If you can keep the tank level the same as the carb, you shouldn't have much trouble with fuel flow.

Take a look at the uniflow setup

You can actually use a vent line to the clunk instead of muffler pressure. That is another version I have seen. It works best on engines with small carbs and good fuel draw.

You could simply tap into the brass mainfold tube and solder in a 4-40 or

6-32 (3 mm???) brass pressure tap with most of the threads cut off to keep from restricting exhaust flow... It'll work fine. It shoudn't increase fuel flow; it will simply make it more reliable at lower RPM..

Bill

Or you could use an electric starter

This whole business about feeding fuel to the engine has caused me to re-examine fuel tanks, piping and fuel systems.

Firstly I agree that the Uniflow system makes good sense and in any case the application of positive pressure on the tank to assist fuel flow is well established

I keep coming back to basic tank theory. Firstly let?s consider some basic principles:

1. R/C model aircraft fly upright or inverted. The fuel changes location due to gravity so let?s agree the fuel feed tube needs to be a clunk type.

1. Every tank needs to admit air to replace the fuel as it is used.

2. Every tank needs to be refilled.

I have normally made RC tanks with three tubes. We?re agreed one is fed from the clunk and goes to the carburettor of the motor. Number two goes to the top of the tank and emerges at a higher point as a breather tube open to the atmosphere. Number three tube I?ve always put to a similar high point in the tank and use it only for filling, with it closed at all other times (at

the writer suggests it should point downward in the tank which I don?t follow).

Thus I end up with a set-up where I know the fuel tank is full when fuel leaks out of the breather tube. I think this arrangement is flawed, in that when flying upside down a partial vacuum will be formed in the tank due to the incoming air having to overcome the pressure of the depth of the fuel in the tank.

So I think you need two breather tubes, one to the top of the tank (pointing downwards after exiting the tank) and one to the bottom of the tank (pointing upwards after exiting the tank). This is exactly the arrangement that I associate with my control-line models 50+ years ago. I think there are two alternatives. Either don?t fly inverted or use exhaust pressure on the single ?breather? tube.

Since most of my tanks have a small opening compared to the tank diameter I can?t see an easy way to provide tubes in that tank pointing up and down. How do you put the whole lot in the tank (shades of a ship in a bottle)? Otherwise the tubes would need rotating in the bung after assembly, which may be difficult.

Comments welcome.

Mike

A few of the pages are " one size fits all ".These pages apply to most , if not all OS 4 stroke engines. , pump and non-pump version.

Because the OS .52 does not have a pump.

The same way it gets fuel when it's running. In a 4 cycle engine this is what happens :

As the piston travels upward for the first time the exhaust valve is open in this case , and the upward travel of the piston forces exhaust gases and unburnt fuel out the exhaust. Since we are just starting there are no gases present , just air. (this would actually be cycle

4) but we gotta start somewhere.While the exhaust valve is still open the piston starts it's travel downward and approx 180 degrees later, the intake valve opens. The outgoing air and downward travel of the piston creates a vacuum in the cylinder. The exhaust valve closes and the fuel is drawn in through the intake valve. (cycle 1) then the intake valve closes and the upward travel of the piston creates compression (cycle 2). Just before TDC ( top dead center) assuming you have the glo plug heated , the engine fires (cycle 3) and as the piston travels downward again approx 180 degrees, the exhaust valve opens again, the piston travels upward and forces the exhaust gases out the exhaust (cycle 4) ...then it starts all over again.

No problem. It will suck the fuel in when you hit it with the starter.

Hope this helps. I'm just not very good at conveying my thoughts on the keyboard. Hope I didn't get a cycle screwed up somewhere LOL

Ken Day

The Uniflow concept eliminates the difference between upright and inverted flight.

You can choke the engine by putting your finger over the end of the muffler. Works fine. Many engines stopped using a choke some time ago , not sure exactly when , and they start okay without choking when using an electric starter. They also start okay by flipping , which I do most of the time , by putting the finger over the muffler end. Most times , I didn't use the choke anyway because it was too hard to get the linkage to it in many of my engine installations.

I didn't finish what I planned to tell you in the other post. Told you I was no good at trying to explain something at the keyboard. I got carried away with the 4 cycle bit and forgot the rest. Sorry if I told you something you already knew , but I wasn't sure and I did get a little typing practice :-).

I really can't offer any rationale for the manual because the manual for my OS 70 Surpass II , which is also a non pumped engine , reads exactly like yours. The 70 Ultimate which IS a pumped engine also reads the same. Same starting procedure and so on. Even my 91 Surpass read the same. Refers to the pump even though there is none. The 70 Ultimate is supposed to have a different starting procedure. My 70 has not been out of the box yet so I'm not sure what it is.....and it's not in the manual. :-( I wish they would write a new manual for each engine instead of piecing together the way they do now.

It is confusing isn't it. On the bright side , I have found all the exploded views to be correct.

Ken Day

I have one key question after all this great assistance, for which many thanks: The question is do I need exhaust pressurisation in order for the 'finger over the exhaust" priming to work or will it still function with just a plain exhaust system (without the nipple and exhaust pressure off-take to the tank)? Mike

The fuel tank's vent line must be connected to the pressure nipple on the muffler for priming to work properly. You have to put it somewhere, so why not there?

Ed Cregger

Mike,

If you send me your personal email address, I will send a three-tube diagram for RC fuel tanks. I found it on an Pro web-site.