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Old Apr 1st 2019, 08:57 AM   #1
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I Ride: 1993 Ducati 900 Superlight
Question Mysterious Gas-in-Crankcase Issue

First, it's good to be back here after a hiatus of a few years. This forum has always been a wealth of knowledge that I have truly appreciated. And I need to tap into that knowledge once again. Let me apologize in advance for the somewhat lengthy explanation below. But I want to provide as much relevant info as possible.

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.
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Old Apr 2nd 2019, 03:50 AM   #2
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did the carb kit replace the float needle and seat?

does the bike have a fuel shut off(fuel petcock)?


. my guess is it either didn't replace the float needle or something went wrong with the carb rebuild. possible the parts were faulty or human error.

in the carb bike days I'd see dozens of crankcases full of gasoline especially in the spring . most riders in that time would know the procedure for putting a bike away for the winter - since you're experienced I'd assume you already about how terrible the fuel is on these older motorcycles.

sorry to say unless you can trace a problem to the fuel tank feeding the carb I'm guessing you'll have to do the carb again . and of course replace the float needle & seat if it has not been done.
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Old Apr 2nd 2019, 09:23 AM   #3
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I Ride: 1993 Ducati 900 Superlight
Macattack: Thank you for the response. Yes, I rebuilt both carbs entirely including new floats & needles (Ducati part #13420201A). As I recall, I also replaced the fuel filter, inspected the gas tank & the fuel petcock. In addition, I cleaned & reoiled the round K&N filter on the end of the crankcase breather hose. Although I felt that I was meticulous during the carb rebuild (including cleaning all orifices with carb cleaner & drying with compressed air), I suppose that human error is always a possibility.

Admittedly in the past 9 month since the carb rebuild I have only run the bike for about 130 miles. However, every few weeks I will run the engine in the garage until the engine gets up to temperature. It seems unlikely that varnish would build up around the float needles in that time frame.
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