!Gasoline Filling Crankcase!

I first posted this inquiry in the Ducati Supersport subforum, but felt that it would be more appropriate here. My apologies if I have bent the forum rules.

Several months ago I fired up my 1993 Ducati 900 Superlight after sitting for a couple of months. With the choke on, I cranked the engine for about 10 - 15 seconds. All of a sudden, a mixture of gas & oil blew out of the crankcase breather like a volcanic eruption. In the 23 years that I have owned this bike (and with only 15K total miles to date), I have never had this issue. It was obvious that the "eruption" was the result of gasoline filling the crankcase & the pressure created by cranking the engine.

I concluded that the culprit might be either stuck/leaking Mikuni carb floats or gas leaking past the pistons rings. I assumed that with on 15K miles on the engine, this cause was more likely to be a carb problem.

So, I ordered complete carb rebuild kits (plus new floats) for the Moto Corso in Portland, OR. I was meticulous about disassembling the carbs, cleaning everything thoroughly & carefully reassembling.

A test ride indicated that the problem had seemingly been fixed. Over the next 10 months, I admittedly had ridden the bike infrequently. Last week I went for an hour ride & the bike ran fine. Once in the garage, I noticed some oil had sprayed out of the crankcase breather onto the rear frame. Upon inspection, I noticed that the oil level filled the entire sight glass. And to my frustration, when I drained the crankcase, the smell of gasoline in the oil was unmistakable.

I briefly (for about 2 minutes) ran new oil through the engine; drained it & repeated the same thing to assure that any residual gas was hopefully out of the engine.

So, I am now at your mercy. I would truly appreciate your thoughts & recommendations on how gas is finding its way into the crankcase. Thank you in advance for any help you can provide.

I’ve only been on this forum for about ten minutes but I’ve read your post and maybe I can help. Sorry to say I’m pretty sure your carbs are still the issue. Im not familiar with your exact carbs internals, actually I’ve never worked on a Ducati so I may way off, but if they’re the same as every other carb I’d check the needle valves and floats. The needle valve may be getting hung up if theyre dirty or corroded, or sometimes the rubber nipple gets a flat spot and doesn’t seal the way it should or maybe the float itself is defective. Some metal floats develop a pin hole and will fill with fuel so they aren’t buoyant enough to push needle valve closed completely. Good luck

I did a complete carb rebuild including new floats & float needles. I was meticulous about cleaning all the orifices with carb cleaner & drying with compressed air. Nevertheless, I suppose that error on my part is a possibility; although I would bet against it.

The other possibility might be a stuck float needle as a result of the bike not being run much. In the past 9 months, I ran the bike for about 130 miles. However, every few weeks I would run the engine up to temperature in the garage. But this has never been an issue in the past. So I am still scratching my head.

Sounds kinda dumb but I never use carb cleaner on carburetors, non-clorinated brake-cleaner is strong enough and won't deform rubber or plastic parts like carb cleaner does, it does more damage than good. Again not being familiar with your carbs I'm not sure what was replaced but the needle valve goes through a sleeve or seat that is sometimes replaceable, sometimes it's not. I clean these with something more than chemicals, like compound on a q-tip to polish and makes smooth.

I only use carb cleaner after all gaskets & rubber parts have been removed. However, your suggestion of brake cleaner is a good idea since I will like be only the floats & needles. The carbs on my Superlight are the stock Mikunis.

On a related note, it has been suggested that to avoid a stuck float needle from allowing gas to dump into the crankcase if the bike is ridden infrequently, that I install a more accessible fuel petcock (the stock petcock is located up underneath the fuel tank). This way I can turn off the fuel supply when the bike is not being ridden. Does this make sense? If so, does anyone have any recommendations on an aftermarket fuel petcock? Thank you for your help on this issue.

As the OP, I just wanted to offer a follow-up. I spoke with a Ducati tech at Moto Corsa in Portland, OR & explained my issue of gas in the crankcase. He corroborated the opinion that the culprit was likely the float needles. So another disassembly of the carbs will be in order with careful inspection of the float needles/seat, as well as the tapered sleeves that they fit into. He also suggested the installation of an inline fuel petcock. This way I can shut off the fuel supply at the end of a ride. I will also do another inspection of the fuel tank & vent line. The fuel filter was replaced about 130 miles ago.

So there you have it. Fingers crossed.

UPDATE: I finally disassembled the float bowls. Everything looked perfect with no signs of dirt or wear. Since the float assemblies that I replaced are plastic, there obviously was not corrosion nor pitting. The rubber seat on the float needles were in perfect condition. So I am at a loss as to the cause of gas in the crankcase. Nevertheless, I will install an inline fuel petcock. Sure would have been nice to have found an obvious cause.