Monday, November 28, 2016

Blown Motor

At the end of March, it was a nice sunny and warm Sunday afternoon. Those days are rare in Seattle in March, so I brought the bike outside; washed it, waxed it, and cleaned the engine. I was running the engine at idle speed to burn off the smoke from the engine shine spray I used, and was wiping down some parts at the same time. That's not a good idea, and I kept telling myself to be careful. Why? Because I have open timing belt covers, and only the sprockets are covered, partially.

Running the motor while wiping the engine down, is not much different than using a hair dryer in a bathtub with 6 inches of water in it. You're probably going to be fine, unless you slip and drop that hair dryer in the water. Well, I slipped, figuratively speaking. The crankshaft sprocket snagged the rag and sucked it in. To describe this better, Ducati engines are "Interference" motors. They have two timing belts, one for each cylinder. Both belts connect to the center sprocket, or the crankshaft. The timing has to be exactly perfect, I listed the steps below for an understanding of how this works:
  1. There are two cylinders in a 2 valve, L-Twin Ducati engine; the horizontal, or front cylinder, and a vertical, or rear cylinder.
  2. One piston goes up on the compression stroke (front cylinder), the rear (opposing) piston goes downward, on what is known as the combustion stroke. this sequence must happen 4 times for a complete rotation or revolution of the engine (hence a 4 stroke or 4 cycle engine).
  3. As these actions or sequences occur, valves open and close. You have intake, and exhaust valves. When the piston heads down on the combustion stroke, the intake valve opens and the combustion chamber (cylinder) draws air in from the throttle bodies. At the same time, the injectors spray the fuel into the combustion chamber. When the valve opens, its extending into the cylinder.
  4. When the piston comes back up on the compression stroke, the intake valve closes, and the exhaust valve opens. Engines vary, and you can have 2 valve and 4 valve engines, depending on who makes it, the displacement, any number of variables.
  5. In Ducati engines, they have a complicated, yet powerful valve system known as the "Desmodromic", or Desmo valve engine. On these engines, there are no valve springs, just mechanical levers. They are very efficient, and eliminate a phenomenon known as "valve float". This occurs when the engine reaches such high RPM, that the valve springs cannot open and contract fast enough to maintain speed combustion and compression cycles, and the valves remain open when they should be closed. This means loss of compression and loss of power.
  6. Because Ducati incorporates the Desmo valve system, there is no valve float that occurs. However, like some other engines, they are known as "interference" motors. If the piston firing and timing sequence becomes lopsided, you will almost certain strike an open valve with a piston head. This is catastrophic, and also known as a blown engine. Many cars use interference motors, Volkswagen is a good example.
This happened to me. When that rag got sucked in, one of the timing belts skipped a tooth, the engine jumped time, and both intake valves were bent. They were bent soooo miniscule however, you couldn't even tell. But it has to be precision, to maintain compression on an engine that can reach 8000rpm in first gear alone.

Anyway, I spent about $800 getting the intake valves replaced, new timing belts, and labor. I am lucky the engine was at idle speed, had this happened on the highway, the engine would have exploded and the rear wheel locked up. I could have gone over the handlebars.

Impossible to tell, but they are bent.
Bent intake valves; the one on top has been striking the piston head. 

The moment after I blew the motor. You cant see anything obvious.
The three red discs are the open belt covers; the only cover the sprockets. The
timing belts are exposed though. The one in the center is the crankshaft cover,
the rag got sucked in behind it.

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