Outdoor Power Equipment and Small Engines - Strange Problem

I contacted you before about a Briggs 13 OHV engine I have been racing. The problem is that oil keeps being blown into the breather via the crankcase vent tube. I was told to check the head gasket and the valve tappet clearance, both of which i did. I replaced the headgasket and even put new rings in it. The problem is that it still does it. The strange part is that i put a pop bottle as a catch can at the end of the vent tube to see how much actual oil was coming out and to keep it from going into the carb. But, not a drop will go into the bottle, but the second I put it back into the breather, it does it again. Also, with the bottle snuggly on the end of the tube, the engine won't turn over, but if i loosen it, it will just fine. I'm have no clue what to do. Any help is greatly appreciated. Thanks.

Since you are racing this engine, the air filter may need to be changed to a different style, especially if you are running the engine un-governed. The pleated paper element on these engines does not allow a good high airflow rate. When the engine runs at higher than spec RPMs, the air filter must allow more airflow than it was designed for. If the engine can't get enough air fast enough, it will pull a hard vacuum on the crankcase. When this happens, oil gets pulled into the combustion chamber from all different places. The crankcase vent is one of those places.

The reason I say it is caused by lack of airflow is because it doesn't blow oil out into the bottle. If it was blowby caused by bad rings or such, then it would come out the tube no matter what. Since it only does it when it is connected to the air filter housing, then the engine is sucking it out. Adapt to a larger filter, or a filter made for a higher airflow rate. Let us know how it goes!

Hello: lg_rebel

LG? = Legal Government...:)

There is a possibility the breather hole between the carb body and the engine head is blocked or plugged up. The breather hole allows air to pass between the crankcase and the cyclinder.

The hole could be blocked by the gasket or some other type of restriction or debris. Remove the carb and check the breather hole and the gasket positioning.

A gasket between the engine side covers, breather and or any place else can cause the crankcase to have an air leak. Check the baffle plate and or one way check valve in the breather.

The other resident small engine service and repair professionals may offer additional suggestions, advice & help.

Regards & Good Luck,
Tom_B
Small Engine Forum Moderator
"Accurate Power Equipment Company"
Small Engine Diagnostics Services and Repair.

I have never run a larger HP engine without a governor for any length of time but on the 5HP run at absurd RPM's we have to catch the PVC blowby in a catch tube. I think cheese is right on about the extreme draw on the crankcase and if its like the 5HP you may need to bypass the PVC.....Mike

Don't know to much about the racing engines , but it seems the engine may be building to much crankcase pressure . I've seen this happen , especially in two stroke's , where the engine blew gas mixture back out of the carb , or would not rev up . The cure was to clean the exhaust port and unplug the muffler . The engine needs the same volume of air flow going out as is coming in , as you know . If it can't put it out the exhaust , it will put it out somewhere else ,or not run at all . This may or may not be helpfull in your situation , since I doubt you would be running a muffler on this type engine . Some exhaust valve and port work might help . Or the lobe for the exhaust valve on the cam may be worn to not allow proper exhaust . It all goes back to theory on what you need for proper engine operation .