Running a 750CFM 4150, and brand new MSD dist, (ready to run type)

The motor has developed a condition where it will stumble or bog under full throttle. doesn't always do it, but most of the time it does. If I ease into the throttle, it won't do it until full open. If I back off the throttle, it will run normal.

It got better after dumping the fuel/water seperator, there was some rusty water in it, but not much.

Thinking I need a new separator. It was new at beginning of season.

I am by no means a "motorhead", but if u had rust in the new seperator, I would check to see if you have filter screens in the carb where the fuel lines connect. If so, try changing those. I had a similar situation a while back.

Might be caused by the Ethenal in the fuel. Do a search on Ethenal and you'll find a thread about it. I was having a similar problem. Checked my plugs snd they were white. I was leaning out. Usually after a short idle my plugs were black. Changed the fuel water seperater and only had a chance to run at idle, but plugs were black again.

Check your fuel pressure, maybe your pump is going bad. How many hours on your pump and what kind of pump is it? If it just bogs when you hammer the throttle I would say it's your accelerator pump in your carb and you may need to rebuild it, but it it's stumbling when your already at full throttle your probably losing fuel pressure, either something is clogged or bad (separator or filter at the carb) or the the pump is worn out. If you have the clear return line from your pump to your carb, see if it has fuel in it. If it does then the pump is bad.

pump is new (non marine, don't tell the coasties ) I plan on changing that later after reading about insurance.

As far as the accel pumps, they are rebuilt as well, and squirt fine. Plus, this bog is not momentary, it will last until I change the throttle position.

The motor has to be coaxed out of the stumble, the first time it did it was so bad the boat would not plane, but black smoke was coming out of the pipes?? and I could not coax it past 3000rpm. I thought ignition, but everything seems fine. dumping the separator out helped immensly, but did not get rid of the problem

I tuned the motor on the E10 in case i ever have to run it on that stuff.

I'd say you've got more water in your fuel. If dumping the sep filter helped immensely, then there is your problem.

Water is heavier than fuel and is at the bottom of the tank. When you accelerate, the bow lifts and the water goes to the rear of the tank where the fuel pick up is.

Pump some fuel out and see if it seperates or has water in it. You could always run a line into a gas can and see if it runs better.

Check the screen in your Carb. I just wen through this process where the E10 cleaned the varnish out of my tank and caused brown gas. In other words, this brown gas actually contained very small dust like, brown particles that would pass the fuel/water filter and clog the carb screens. It idled fine but wouldn't get to high RPM's due to the clogged screens.

This was my issue. Hope it helps. I emptied my tank twice and still see brown debris. It is getting better though.

I have the same problem.
It developed after running ethanol for a few weeks.

I read on the baja website that ethanol was okay but not on boats earlier than 1990.. mine is an '89.. now i don't run ethanol in the boat anymore..