Engine dies when going from idle, straight to full throttle

[GregF]

I have over 3000 hours on mine with minimal maintenance. Changing oil is really about all you have to do and the oil is cheaper than 2 stroke oil, you use less of it and I don't have to smell it burning
Plugs are like car plugs, they last for years. It starts at the first bump of the key, every time, can idle for hours without sneeezing stumbling or smoking and I never worry that it is going to stall just when I need it to run, like when you are maneuvering around a crowded marina.

[GXPWeasel]

Check what kind of fuel pump your motor has, but if it's a vacuum or pressure operated unit (run off of crankcase pressure or intake vacuum) I'd check the diaphram seperating the pump from the crank/intake. One small hole is all it takes to introduce extra fuel in the engine.

How did the plugs look? Black soot on them is carbon from running rich. Light gray is a proper mixture.

Plugs did look a little bit black, but not bad at all. The only blackness was on the actual electrode, and not on the other metal parts. There was no gray though. I don't have the slightest idea what type of fuel pump the engine has. It's an '05 Mercury ELPTO 90 hp 2 stroke, maybe that can narrow it down.?.?

I will probably be replacing all of the fuel lines / bulb in august anyways, since I know they have never been replaced.

[GregF]

It has a vacuum operated fuel pump. It will be the 2.5" square doodad with the fuel hoses going to it. The kit should be about $10-15 but verify by serial number

http://www.boats.net/parts/search/Merc2 ... parts.html

[belercous]

If the ceramic around the electrode wasn't black, the engine doesn't sound like its running rich. Your problem of the engine not wanting to start after it sits a bit is a sign of running rich or getting flooded. I've seen fuel leak past the diaphram on vacuum operated pumps and cause flooding/overly rich mixture problems.

When an engine "stumbles" when going from an idle to W.O.T. that is a sign of a lean mixture. If the engine revs up but then loses power for a bit, that is a sign of an air leak in the fuel pump inlet line (the primer bulb is one culprit, but anywhere upstream of the pump to the tank is a possiblity.)

When our fuel inlet line (from inlet of engine to fuel pump) was going bad we noticed a bit of fuel dripping from the engine after squeezing the bulb a few times. Also, the bulb would not stay firm (because gas would leak from the line and not hold pressure to the carbs). If your primer bulb does not stay firm after a couple of squeezes, taking the cowling off the engine and sqeezing the primer bulb is a good test for a leaking hose. Look for fuel to drip, it shows up pretty easliy in the water.

You do have a blue spark on each of the plugs? Yellow isn't good enough, all plugs need to spark blue.
Does the engine run smooth at an idle? Sometimes a cylinder won't run good at an idle but smooths out at higher rpms. This could be either an electical problem or one or more of the carbs has a plugged idle system port/jet (Usually this is caused by fuel being left in the engine which will leave a residue that will restrict flow. These deposits can often be run-out in time. Sea-Foam will speed the process.)

If after checking these things (and all other suggestions previously mentioned) please reply with a list of everything you've done (to refresh us, the thread is getting long) and I'll keep trying to help you solve the problem. I really can't see how nothing suggested so far hasn't fixed the problem.
This shouldn't be anything that needs a shop to fix. 90% of fixing most problems is troubleshooting. We can fix this, but you may wind up taking all the carbs off & tearing them apart to clean them. We'll try to save that for last.

Just to make sure, the engine is warmed up when this happens? Gotta ask.

[GXPWeasel]

Sure thing belercous.

I haven't yet done anything with the fuel system, and I haven't had time to even check the stuff, as I can't keep the boat at home (damn neighbors call the cops if I park it in the street ) and I was packing for this weekends camping trip. I will be very observant this weekend about it, and get back. I did drop some seafoam in the tank last weekend, and ran 1/4 of the tank through. I will add the remiander of the bottle in there this evening, and run probably at least a tank of gas through over the next 2 days.

She runs great at idle, but again, I haven't checked any fuel lines. I will also have my BIL drive the boat, while I check the fuel bulb and system for pressure loss or leaks this weekend. I hope I can get this diagnosed over the course of the next few days, as we will probably put 20 hours or more on the boat this weekend.

Thanks again for your replies, and after the weekends over, I'll report back if fixed or not, or if it's getting better (due to seafoam getting rid of the deposits).