THE MARINE DOC......HELP!!!!!!!

[riverebel]

I have a 60 hp mariner bigfoot.It starts fine & runs great for about 45min. At wide open throttle it slowly starts losing power until it dies out completely.If you throttle it quick it will go to full throttle ,but just to 4400 RPM when it usually runs at 5000.if you try to back off any at all it dies to 3000.Sometimes when it does it you can activate choke & it will pick up but not to full W.O.T.I replaced fuel line,vents ok,spark ok,new plugs installed,& it recently had a new stator put in.After it sets for a few hours its good to go again.yesterday after it started doing it,I took engine cover off & there was a little water in bottom of cowling.I removed the airbox & in the bores of carbs it was milky looking.Where could the water be coming from & could this cause the problem.The cowling seems to be watertight from the top. HELP...its driving me nuts. Thanks

 

[riverebel]

Re: THE MARINE DOC......HELP!!!!!!!

Would this be electrical if pushing the choke in when this happens helps some?

 

[The Marine Doctor]

Re: THE MARINE DOC......HELP!!!!!!!

It sounds like your carbs are Icing up. Yes I know you will say...its HOT out. But when its really hot we get a severe drop in temp when the carbs pull the air in this ices the carbs up and resticts the flow of fuel to the engine. Once you throttle down the ice melts and water is deposited inside the airbox...of couse making the milky substance.<br /><br />Now when you push the choke..you are suppling the fuel through a different means than directly through the venturi<---carb. The RMP will pick up.<br /><br />IF this is not the case.<br /><br />Use a larger fuel line to the engine. You could be vapour locking. Are you using a Remote tank? How long is the hose?<br /><br />Vapour lock happens when the fuel within the fuel componets boils. This happens because the fuel is in a vaccuum and has a lower boiling point.<br /><br />TMD

 

[riverebel]

Re: THE MARINE DOC......HELP!!!!!!!

Doc....it is a 30 gal. built in tank.The hose is approx. 5 foot.How would you keep the carbs from icing up this being the case.Also when it does this the hose seems to be real spongy & it wont pump up real firm. Should I use a longer hose ?or bigger diameter? <br /> Thanks

 

[jdkzrt]

Re: THE MARINE DOC......HELP!!!!!!!

Carbs can't ice up at 72'F or higher (and humidity has to be perfect for this to happen) <br />1) Are you positive your fuel tank vent is not kinked or restricted?<br />2) Rebuild your fuel pump.<br />3) make sure your carbs float bowl vent lines are not kinked or restricted.

 

[The Marine Doctor]

Re: THE MARINE DOC......HELP!!!!!!!

JDK...the info I have recieved about the icing was directly from Mercury.<br /><br />Not to start an arguement...but do you have evidence to back up the 72deg theory?<br /><br />I would like to know this so that I do not mislead anyone with false info and can pass it on to the factory.<br /><br />AS for the hose...shorter hose...larger diameter.<br /><br />TMD

 

[riverebel]

Re: THE MARINE DOC......HELP!!!!!!!

Doc....It has been in the high 80's to mid 90's with high humidity here in Kentucky.My tank is venting fine.I have a fuel pump kit to put in when I get to the lake tomorrow.Would the carbs ice up on the outside where it is noticable if I ran with the cowling off,or do they ice up on the inside?This motor has less than 100 hours & starts idles & run's so good before it does this.I just bought this pontoon 3 weeks ago, & when I test ran it ,I only went a little ways.The next day I took it on a long cruise & found this out. Thanks for all the help you can give me.<br /> Charlie

 

[The Marine Doctor]

Re: THE MARINE DOC......HELP!!!!!!!

The carbs will be icing up along the inside of the Venturi as well as the emulsion tubes. Restricting both air flow and fuel.<br /><br />For more info:<br /><br /> http://www.may.ukf.net/Tugs/Carb Icing.htm <br /><br />TMD

 

[riverebel]

Re: THE MARINE DOC......HELP!!!!!!!

What can you do to overcome the icing and or vapor locking

 

[The Marine Doctor]

Re: THE MARINE DOC......HELP!!!!!!!

Vapour lock can be overcome by pressurizing the fuel. Or have a shorter Vaccuum.<br /><br />As for icing up...Mercury has addressed this problem with some of its models by adding water channels within the carb. But this is designed to prevent icing during the winter.<br /><br />TMD

 

[jdkzrt]

Re: THE MARINE DOC......HELP!!!!!!!

TMD:<br />72'F. max icing temp is from flight training manual for Cessna 172. Generally, 40'F is the max. temp drop seen in carburated aircraft engines.Your chart shows some possible exceptions, but in reality these are very very rare. The condition that I was taught to be most concerned about was the temp/dewpoint spread especially when descending (low power and engine over cooling). This is a condition that is hard to duplicate in a boat. I guess it's possible, but I don't think you will see carb ice in an o/b when it's over 72'F and not foggy or raining.

 

[bobmac]

Re: THE MARINE DOC......HELP!!!!!!!

Did you ever correct this problem? I have a 1996 Mercury Bigfoot that does the same thing. Runs fine for a while, then won't run in the mid-range, but will run at high and low speeds. In the mid range it just dies. If you get past the mid range while either slowing down or speeding up you are okay, but if you pause it stalls. I'm at a loss. Any ideas?

 

[wilde1j]

Re: THE MARINE DOC......HELP!!!!!!!

JDK, I don't believe the 72°F ambient temperature deal at all. I was taxiing a 172 in Santa Fe, NM on a bright, clear, dry {15 to 20% RH} 80 to 85° F afternoon. It took proressively slightly more throttle to maintain the same taxi speed. When I got to the runup area and did the runup including application of carb heat, the engine ran rough as hell and almost stopped. It continued running and after a minute or so of carb heat {incoming air is heat via a heat exchanger}, it ran fine. I had an instructor in the plane and he suggested I should have applied carb heat during the taxi. The point is, you can have carb icing in conditions you would never suspect.<br /><br />On the OB subject, I've never encountered carb icing in any conditions, including high humidity and near freezing ambient temperature. But that's not to say it can't happen. Most outboards, when warmed up, have sufficient heat conducted to the carb throat to preclude icing. Some specific designs may be susceptible, however, especially at low power settings.

 

[BillP]

Re: THE MARINE DOC......HELP!!!!!!!

When temperature and dewpoint are within 3 points of each other carb icing conditions exist.<br /><br />N276SP

 

[P.V.]

Re: THE MARINE DOC......HELP!!!!!!!

I believe you have water entry into the engine. As you run WOT, you are drawing (sucking) more air via the cowling vents, in an attempt to provide proper fuel/air ratio for the engine. Any little water entry or leak (tell-tale hose??) will get drawn into the carbs, hense, the poor running, dropping of rpms and "misting" of the in-coming air!! (At least your motors inards are probably very clean at this point, but water does not lubricate very well, so don't continue to run it very long as you'll be rebuilding it after the rod bearings rust and fail!!) Run the motor without the hood on and see if you can (maybe difficult on a pontoon boat)see water leaking/gathering around spaces it should not be, IE: wiggle the hoses and check the ez things first!! Remember, that air flow will be different with the hood on vs. the hood off!! BTW, it ain't carb icing!!

 

[Merc805]

Re: THE MARINE DOC......HELP!!!!!!!

This post was put up 1 year ago.. The poster may not be here any more and TMD has his own forum.

 

[Flounderhook]

Re: THE MARINE DOC......HELP!!!!!!!

I had the same problem with a Mercury 60 hp Bigfoot, the water was coming in around the cover seal. The engine is on a 24ft pontoon and there is alot of spray when you are underway and the water was getting in around the seal. I replaced the seal and it did not lose power again. Something to try....

 

[P.V.]

Re: THE MARINE DOC......HELP!!!!!!!

Good point Cast! In an attempt to get promoted, ( need many, many, many more posts) I did not notice!