Y'all, I am new to the 'big' engine arena (over 0.1") and have just bench tested a 0.40"

I elected to use the exhaust from the muffler back to the tank and noticed a great deal of black burnt oil being fed into my clean tank of fuel!

It suddenly struck me - why do we do this? Am I doing something wrong? Should I be using some kinda air filter for this application.

I have been testing an OS Max-H 40

TIA

Shobosh

Yes, if you are getting black oil, something is wrong. Maybe as simple as the muffler not being tight. If the pressure nipple on the muffler is on top of the muffler, you shouldn't be getting oil into the fuel tank any way. I woud suggest not running your engine again until you figure out why you are getting black oil.

Agree, there's something wrong but with any luck it might be something very simple. If there's black oil going down the pressure line you should be seeing it out the muffler as well. The usual cause as Jim suggested is if the muffler isn't tight enough on the engine and any tiny movement will give a lot of black from the aluminium being fretted off. But it's also possible (assuming it's not a new engine) that the muffler has old caked on carbon inside it that's gradually getting washed out.

Thanks guys. It sure is an old engine. I will give it a good clean and tighten up the muffler connection.

Much appreciated.

Shobosh

There shouldn't be any more air (Black or not) going into the tank than there is fuel coming out to the carb. If you have more than two outlets from the tank, make sure that the extra one is plugged up.
Steve