VF250 rough running

[racinginparadise]

Here in the sunny BVI's I happen to have the diagnostic disc and manual.

A good friend at a local dive company runs a large cat with two VF 250's.
One runs perfectly, the other can't acheive anything over 3000 revs and runs rough.
Their mechanic had been through the fuel system, no change.
No alarms either.

With the engine plugged in I see no fault codes at all.
I try the injector shut off test and ignition misfire tests.
In both cases cylinders 1,3 and 5 fail to respond to the test, ie, no change in engine sound.
Cylinders 2,4 and 6 respond and the engine runs really rough during the test.

We try the same thing on the other engine and it runs MUCH smoother with only a slight fluff as the tests are done but all cylinders respond normally.

Any ideas where to go next??

[boats.net]

Check spark and compression on all cylinders. You may not have spark on those cylinders, however it is very possible to have blown those cylinders. The spark and compression will narrow your search.