Boat was running great as of last weekend. Engine is a 2004 496 with 268 hours.

Yesterday as I left the ramp I could immediately notice something wrong as I tried to get on plane. Engine was shaking and didn't have the power to plane.
Back in the yard on the flusher it starts easy and it sounded fine at idle, just under 800 rpm. I couldn't hear any miss. Scanned the engine and had no faults beyond the usual ones. I still figured I might be missing a cylinder due to a bad plug or something so I tried disabling one cylinder at a time with the scan tool.
I could not tell any difference as I shut down each cylinder. No change in the sound and the rpm remained constant.

Shouldn't there be an obvious change when I shut down a cylinder or do I really need to be under load in the water for that test? Or is this telling me I really don't have a single cylinder misfire? I would have to think that with two cylinders not firing that engine would not idle very nicely, even on the flusher.

It's been a few seasons since I've changed the plugs so I plan to start there anyway.

Engine shaking/no power to plane off sounds more like a fuel issue to me. A missing or dead cylinder should not keep boat from getting on plane. The scantool misfire test really needs the engine under load to be noticeable.

I would check fuel pressure. 43psi KOEO. Also monitor while trying to plane off to compare. How old are fuel filters? Fuel?

Yeah I could plane no problem when my 496 had a plug wire come off. Just was down 300 RPM or so at WOT. When my booster pump failed I could not plane out the boat. Fuel pressure would drop to like 17 PSI as you throttled up to plane out the boat.

Thanks, sounds like good suggestions. Fuel is fresh and filters are new this season.

Not being under load alot of times bad running conditions do not show up that easily especially fuel pressure issues and besides with ECM 555 and pcm 555 one is limited to overspeed up to certain rpms running the engine UNLOADED.

Most of the time a 496 when the engine shakes it was caused by cyc (s) not firing HOWEVER (ignition related ) / injectors not working properly / next is bad / low compression

Spark test -- examine and or change all spark plugs and plug wires - compression test - fuel pressure testing on the water, Make sure all injectors are working correctly.

the first gen 1 ign coils can be an issue as well. Seen many of them go bad overtime. 496 fuel injectors SUFFER from contamination issues. More so with gen 3 cool fuel but even with your gen 2 cool fuel.

Thanks BUP. I'll start running down that list.

New plugs did the trick. I should have changed them sooner. The old plugs looked good but one of them was defective somehow.
Maybe now I can get a little boating done before fall. Thanks everyone.

I had the same problem with less than 60 hours on my 496 MagHO, new plug wires did the trick, one was defective.

90% chance it is ignition, even today..

In order to pin point the issues. file it for whomever