Had a great day Saturday, boat ran awesome. When I was idling around the ramp I noticed that the engine was idling rough, tach was bouncing 200-300rpm, and I smelled fuel. I looked back and there was a sheen on the water that's never been there before. Seems to me like I have a cylinder that isn't firing properly, but I'm not set on that because the boat ran like a top until that point. (Don't they always run great before they explode?) Loaded it Saturday night and won't be able to see it again until Weds so I'm looking to grab all the potential tools I'll need to diagnose the issue.

I'm thinking timing light on the plug wire, or pulling plug wires individually to see if I can locate a plug that isn't firing, and doing a compression check for a hung valve.

Obviously fuel is getting to the cylinder, so I don't think that's an issue.

Engine is a 502, the only modifications are the removal of the VST, it was replaced by an Aeromotive pump and Hardin reg. Not sure of the hours because the meter doesn't work, so I'll be looking for a scan tool. All the other gauges were reading normal.

Any help is appreciated.

Start simple. Maybe just a bad plug or wire or cap? Can hope. Those caps rust, mine was a mess. I have a 96 502.

You ideas sound spot on to me. Ignition, compression. Easy checks too.

Thanks Pismo, I'll pull the cap and have a look.

Spoke with the local builder and remembered something else, the smell of gasoline under the hatch. He suggested that an injector may be stuck open and the excess cylinder pressure caused fuel to leak by the seals.

Start with K.I.S.S.( Keep it simple stupid.)

Check fuel pressure at the rail making sure your elec pump isn't doing something weird.

Then I would start with a basic tune up. Plugs, wires, cap & rotor. It's good maintenance practise any ways.

Fuel pressure gauge says I'm good, Ill check the others.

90psi in cyl 5, 112 in cyl 6. The rest fell between 138-160.

What's my next course of action? Pull the heads?

I wonder how AFR's would do? Just stick em in, run through the summer, then add a cam and the rest.

What did the plugs look like from the cylinders with the lower compression?

It's possible that the injector/injectors stuck open washing the rings out with fuel.

That doesn't sound good at all. A leak-down compression test would be my next step if possible. If not, check manifold vacuum for pulsations and remove oil filler cap and see if it's huffing.
Maybe the injs got plugged-up. Eng went lean and detonation took out a few ring lands??

I was hoping for some burned valves. The spark plugs from the low psi cylinders definitely got hot. Not hot enough to burn them up, but they were nearly white.