Mercruiser 4.3 Stalling at higher RPMS

Update: Was at the lake yesterday. I replaced the fuel hose between the tank and fuel filter (this is the rubber section, the rest from fuel filter --> fuelpump --> carburetor is stainless steel). I also checked the carburetor inlet filter, it was clean. So with the new fuel line I went out for about 20min and several WOT runs and no issues. Came in, boat sat for an hour and when back out, same thing, no issues. I won't be convinced this fixed it until I can go out for a longer run 1hr+, but it was getting late and there is no one around this time a year, so I didn't want to have an issue and get stuck out alone. As others have mentioned I checked the coil, after both runs it was warm, but not hot at all.

I took the old hose and cut it apart, than seeming to be more flexible than the old one, it didn't really look any different - not like it was coming apart inside. I guess if this was really the problem, it doesn't take much to disturb the fuel flow - ie a permiated hose causing the system to suck air at high rpms, and I guess once the system get's air in it it's downhill from there?
 
The sucking air is a result of a loose hose clamp or cracked hose. Lets hope replacing the hoses also got you to tighten the clamps so It would stop sucking air.
 
Update 2/3/14: So I think this problem is solved, I've run the boat a couple of hours with no issues. So either the old hose was the culprit or maybe it was not tightened and was somehow sucking air at high rpms.

So now I have a new problem - if you recall from reading the thread this all started with a gummed up carburetor, presumably from running ethanol. The boat runs great now, except at cold start - it seems to start up very rich, lots of black smoke and smell, obviously a rich fuel mixture. In checking the choke, when you set it by a full pump of the throttle, it closes completely, the engine fires right up, and the choke kicks open 1/8" according to the spec. At this point the engine runs, but very rich and smokes a lot, after about a minute, the chokes opens up and it runs fine. I have the choke (electronic choke)adjusted all the way to lean (least choke). I am wondering if the float is maybe adjusted to high (ie allowing the bowl to stay too full). I feel like I adjusted it correctly when I rebuilt the carburetor, there were settings for the float level and drop. The carburetor has to come off to adjust the float, there is not glass or adjustment from the outside. It just seems if the float adjustment was that far off it would at the least have a rich flooding condition at idle, which it does not.
 
Sounds like you have a Q-jet. A problem that happens is leaking fuel from the well plugs that will drip fuel straight into the engine. It's the solder plugs in the tubes when you turn the carb upside down. Use JB weld to seal up the tubes. Scrape or sand to give better adhesion to the solder.
 
Charlie - it's a 2bbl mercarb.
 
So I pulled the carburetor off last week - I must have missed something when I adjusted the float because it was way off. Basically it was adjusted too high (keeping too much fuel in the bowl). Last time I ran it, in addition to the very rich startup was also running rich at idle - not loading up, but some black smoke and a sheen on the water. So I re-adjusted it - a tedious thing trying to bend the little tabs on the float - but now it is spot on. Finally got a chance to run it today - it fired right up and seems to run great.

At this point I am calling it fixed - for those that might not have read this whole thread, this whole issue was caused by ethanol. Even though the fuel lines on this boat are Ethanol rated, they still deteriated from inside filling the fuel filter with black bits of the hose that looked like pepper sprinkled in the fuel. The fix was rebuilding the carburetor (mainly cleaning) and replacing the fuel lines and filters. When I started using ethanol last summer I didn't thing there would be an issue - boat is new enough that it should not have a problem. Maybe it would not have other than the 10yr old fuel line and maybe I would not have a problem going forward with ethanol, but who knows.
 
Glad you got her fixed. Hope you'll have a carefree summer.
 
That's great , Bill. Thanks for posting the happy follow-up reports.
 
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