Its a little tight

A big part of my off season maintenance is a valve check/adjustment. Ducati engines are pretty unique in that they use a desmodromic valve actuation. Like many modern engines, the desmo system uses an overhead camshaft design. However, the most unique feature of the desmo system is that it DOESNT use springs to close the valves. Instead, a desmo engine uses an opposite, closer cam lobe. This closer cam lobe provides a very specific and controlled way of closing the valve. Benefits to this system include highly defined, and consistent engine breathing, along with the elimination of valve float. The desmodromic system has enabled Ducati twin cylinder engines to produce more horsepower and higher revs as compared to conventional twin cylinders.

One of the drawbacks to the desmo system is its relatively short adjustment periods. While most motorcycles require valve adjustments every 15k miles or more, Ducati suggests checking theirs every 6k. Since my 748 is a race bike that spends 99% of its life at redline or on its way there, I like to check/adjust mine every year.

To start, I set the crank to TDC on the horizontal cylinder. Then remove the cam belts to free up the cams. It is important that all measurements be taken at TDC when all valves are at rest. Next, the valve covers came of exposing the valvetrain. Now its time for measurements.

The first measurements are the opener gaps (or unloaded gap). This is the distance between the opening rocker and the valve. This gap must be within a specified distance or damage to the valves may occur. Not to mention, out of spec valves drastically reduce combustion effiency. Using feeler gauges, I measured all the opener gaps and recorded the numbers. Measuring the closer gap is a bit more tricky. I use a 8mm nut driver to push down on the edge of the closing rocker to engage the cam thus exposing the closer gap to the opener. Basically by pushing down on the closing rocker the opener gap increases by the amount of the closer gap. This is the loaded gap. Once I get a loaded gap, the unloaded gap is subtracted to get the precise closer gap.

The measurement from my motor (mm)

Vertical Cylinder

Intake Exhaust

L R L R

Open 0.12 0.10 0.17 0.15

Close 0.15 0.16 0.09 0.05

Horizontal Cylinder

Intake Exhaust

L R L R

Open 0.10 0.08 0.16 0.15

Close 0.17 0.17 0.09 0.07

Most of the gaps are within spec short of the exhaust closers which are a little on the tight side. Ideally, all the gaps should be about 0.10-0.15mm. So it seems a few shims are going to need replacement.