Ok I just decided to use an OS 120FS Surpass motor for my P51 and I will say right up fron I am not tto familiar with 4 strokes so please bear that in mind in any posts. I understand that there is usually one line for fuel to the engine and then another going from the exhaust system to the fuel tank for pressurisation of the rank now this is the normal setup for a two stroke. But on this motor it has an additional line going from the bottom of the crank case in addition to the one on the exhaust and the intake on the carb system. What does this extra line do and how is it setup. Is it an additional pressure line that goes to the tank or what? As I said not a great deal of experience with four strokes so let me have it.

Lubrication of the crankcase (crank, camshaft, bearings, conn rod etc) occurs by means of blow-by oil through the piston ring...so oil accumulates in the crankase, and it is being pushed to everywhere in the engine....the oil has to get out somewhere, and that´s why most of the 4 stroke engines have a nipple at the bottom of the camshaft or at the backplate...Some enignes like the one you have, recirculates this oil to the intake to make them run cleaner, since the amount of oil expelled may be annoying etc...

I do not recall another port near the fuel inlet so do I pump this back into the fuel tank

The crankcase vent on the old OS 120 Surpass engine with the silver valve cover is left open to the atmosphere

First, which of the four or five Surpass 120's do you have. They all plumb differently. Do you have a pump? Is there a regulator on the carb. A few more answers, and We can give you the right plumbing setup.

A small black pump and a small black regulator cover on the carb is a Surpass III W/ Pump. An aluminum pump with no regulator on the carb is the Surpass I. The aluminum pump with aluminum regulator is the Surpass II. The I and IIplumb about the same. The non pumped ones IE the 120E are like any other engine, two or four stroke. The Surpass III is a two line from the tank to pump, one vent and one supply. None of the pump engines us a line from the muffler to the tank, and all need a vented to atmosphere tank.

Don

My Surpass has no pump it does have a choke adjustment cover I guess which is controlled by a pin assembly. It simply opens and closes the cover over the mouth of the carb. The valve cover is silver so these are my assumptions it is an old Surpass motor and that the port on the crankcase should be left open or a line connected to it to direct the oil drain from from the engine.
Could as estradajae says the oil that is draining from the engine be routed back to the inlet line. If so is there a oneway assembly and a T connection in the main fuel line that will allow this.

The port from the the exhaust will go to the fuel tank for pressure and the other is an inlet for fuel to the motor.

Doing that would cause lots of problems and you may not be able to even run the engine =).

The vent line is routed back to the intake through a nipple in the intake pipe, not via a "T" on the fuel line...
putting it on the fuel line would cause hard pressure variations on the fuel, air bubbles etc...

Yeah that makes total sense I totally mis thought that one of course it would inject air into the fuel system and I could definitely see lots of problems thanks for opening my eyes.

Jeff. The crankcase vent will either dump to tube hanging out of the cowling, or a fitting on the intake manifold. Whichever setup your engine started with is the one you should use. There is some raw fuel that blows by into the crank case. On the engines that dump to outside air, it isn't an issue, but the ones the are vented to the intake, well it effects the mix of the engine and failure to keep the stock plumbing is going to result in a hard to tune engine.

This problem is highlighted by a couple 120 pumpers I've worked on in the last couple months. One is a Surpass IIant the other a Surpass III. both had pump issues that was dumping raw fuel from the pump into the crank case. This fuel was finding it's way up the vent to the intake manifold and really screwing up the carb mix, to the point neither engine was flyable anymore.

The crank case vent is a part of the design of the engine. Failure to follow the design is going to result in frustration on the users part.It may even result in damage to the internals of the engine. Use a really long tube on the vent or dump it into a canister can screw up the overall flow of fluids through the engines.These fluids arewhat lubricates the bottom of the engine. The externally vented ones need as short as possible vent tube. The internally vented ones, IEcrankcase to intake manifold also require a short vent line or the pressure in the crank will build and choke the engine.

Check the manual for your engine and follow it.