What the...? Robbin 350 pre-MCI, milky oil in carb?

[JPonLKN]

Ok guys, figured I would post this to see what ya'll think could cause this. I'll be doing some diagnostic dissassembly, but figured one of you may have run into this before and can point directly to the fix. Maybe not.

20220218_145907.jpg

From the photo, you can see there is some "milky" oil in the intake side of the carb. Not sure how oil got up to the carb unless its coming from the PCV? check valve? Too much crank case pressure?

I was driving home yesterday, and it almost completely lost power, barely stayed running enough to go down hill on the driveway to the house. Today, pulled the intake hose and found this mess.

I'm going to drain the oil out of the crank, pull and clean up the PCV assembly, hose, intake, carb, etc.

Any thoughts?

[JPonLKN]

Pulled the PCV hose, and pulse hose to the fuel pump. Nothing of interest there...a little thick milky residue at the connection end at the block, but dry the rest of the way.
Drained the oil and it was definitely a gas/oil mix. Half a gallon came out. I'm flushing the block with used oil just to get the gas/oil mix out. I'll probably fill it with used oil and flush it a couple of times to get as much out as possible. Tough, as the cart is lifted, so takes a pretty precarious lift system to get the block angled enough to get oil out at the filter and drain plug.

I guess next is to pull the carb and check the float and needle valve. It's been running a bit rich, but running, so shouldn't have filled the block with gas.

[slonomo]

Yep float probably stuck and fuel didn't shutoff. Could be a bad needle valve. I have had grains of sand stuck in the rubber plunger end that was causing flooding issues. It doesn't take much to prevent a seal.

[JPonLKN]

But how would the gas get in the oil? This thing runs on a pulse fuel pump, not a gravity feed, so when the motor is running, the gas gets burnt. When it isn't running, no gas moving through the system. Even if the needle valve didn't seal, or float wouldn't push it closed, there would be nothing to push gas through the intake and into the cylinder. It needs either the vacuum of the pulse, or the vacuum of the 4 cycle intake process while turning over to pull gas through the carb. I thought maybe the membrane on the fuel pump, but doesn't appear to be.

[Dflowe]

I believe this to be correct, but if the needle valve is bad as Solonomo suggest, the fuel that's in float bowl drips down into the cylinder (downdraft carb) and washes down the cylinder walls, past the rings, and into the oil. You do that a couple of times and you have milky oil. Plus, that makes your oil level go up and that increases crankcase pressure. That increased crankcase pressure is how you end up with an airbox full of oil/gas mixture.

That's my theory.

[Dabble Inn]

My first thought would be bad fuel pump diaphragm, but you say it looks ok. Maybe take a closer look, as you know sometimes they are hard to see. It would explain the how and why

The first pic it looks fine right? Second pic shows it but still hard to see till I removed it

[slonomo]

Originally Posted by Dabble Inn

My first thought would be bad fuel pump diaphragm, but you say it looks ok. Maybe take a closer look, as you know sometimes they are hard to see. It would explain the how and why

The first pic it looks fine right? Second pic shows it but still hard to see till I removed it

I had this happen on a 2 stroke ezgo before. But it literally "pumps" the fuel into the engine because of where the pump is mounted to the engine. I'm not sure if this would happen on a remote mounted fuel pump or not.

[slonomo]

Originally Posted by Dflowe

I believe this to be correct, but if the needle valve is bad as Solonomo suggest, the fuel that's in float bowl drips down into the cylinder (downdraft carb) and washes down the cylinder walls, past the rings, and into the oil. You do that a couple of times and you have milky oil. Plus, that makes your oil level go up and that increases crankcase pressure. That increased crankcase pressure is how you end up with an airbox full of oil/gas mixture.

That's my theory.

[JPonLKN]

Ok, so update on the cart. Changed the oil. Pulled the fuel pump. It is a Mikuni pump and doesn't come apart. Checked it with a vaccum pump...covering the vent, it will hold vaccum, so the membrane is intact and fuel isn't sucking past it. Also, no indication of fuel in the pulse hose, so I don't think it was the fuel pump.
I pulled the carb, needle valve seals the way it is supposed to. I can blow into the fuel inlet tube on the carb and not blow air past the closed needle valve, lift the float and opens the valve as it should, so I don't believe it is the float or needle valve.
I did observe that the idle screw was turned down leaving the carb in an open condition when the throttle closed. Turned it out all the way so throttle closes on spring action. I'm hoping maybe it was dumping fuel in the cylinder when off throttle. I'd expect back firing, but none that I recall. It did run rich though. Also, it doesn't get a lot of run time, not even 1/4 mile trips each way to my neighbor's house, so maybe not hot enough to ignite backfires. I'm gonna run it the way it is and see what happens, check the oil periodically. I also considered that the milkyness could be condensation in the crank case, but it doesn't explain oil in the carb intake. Anyone have any other thoughts, I'd be interested to hear them.

[slonomo]

In the mechanic world we used to call cars with milky oil "Short Trippers". Usually grandma's car had milky oil. If you take the oil fill cap off grandma's car, it usually had some goopy chocolate colored stuff under there. Yep, that's from condensation. Typically driving for 15 minutes or more will help to burn all that condensation off, but a short 1/4 mile ride will not. So, it just might be condensation.

Water is more likely to cause "milky oil" than fuel. Fuel is a solvent and will thin out the oil, sometimes changing it's color, but not usually like chocolate milk.

Did the oil level ever get too high? If the fuel was leaking down into the oil you would have a high oil level and if the amount of fuel was high enough, the oil will seep past the rings and start smoking. And a high oil level will force oil into the vacuum lines and pcv system.