Low RPM, Vibration

[jamepc]

I have a 2001 Islandia, 240 Merc EFI. The RPM's won't go above 4900-5000 RPM's. I did a compression check and all were around 130 psi. Next, I would like to verify all cylinders are firing. What's the easiest way to do this? If 1 or more aren't running, how do I tell which ones? Any other ideas or suggestions regarding this problem are appreciated.
Also at idle to about 1500 RPM's the engine is extremely rough. Very shaky like a cam with a lot of overlap in a car. So if all the cylinders are firing, maybe something else is off balance like a bad impeller? Would this also slow the engine and boat down?
Thanks,
James

[holdnon72]

Could you use an inline spark tester to see if they are all firing? It's a little gadget that lights up when spark is present. One end of the tester hooks to the top of the sparkplug and the other end hooks up to the ignition coil boot or sparkplug wire.

[jamepc]

I wonder if those will give a false reading. My friends outboard had spark at all cylinders but it was a weak spark. Not enough to fire all cylinders. A new coil pack fixed the bad cylinder. Instead of verifying spark at all cylinders, is there an easy way to tell if all are firing?

Thanks for your help.

James

[jamepc]

Also, how should spark plugs look in this 2 stroke. They always have a black film of oil on them but that normal, correct? I don't smell raw fuel.

[TM1]

They won't give a false reading, you just have to be experienced in using them and knowing a good one from weak one. In a very basic way; brighter is hotter and vice versa. If you've either got a good ear or a good tach, you can do the rpm drop method; pull one plug wire and note the rpm drop, do the rest for the remaining cylinders. The one(s) that make very little to no change in rpm are obviously not contributing to the party

If the compression is up then that leaves us with fuel delivery, mfi injected I assume? If so then we can use the same test as for the spark test; pulll the electrical connector to each injector and note the rpm drop of each one...lowest reading is the problem cylinder.

I'm assuming that nothing is wrapped around the driveshaft causing the vibration?