loss of power after 5 mins

[matth121]

Hey guys, got an 81 merc 25. Ive been working on it since I got it and its getting better and better. Most recently replaced the stator and saw a big jump in performance. What happens now has me confused though. when I 1st start it, i get half power, max speed 10mph not even on plane. If I shut it off and wait 20 mins, then start it up, i'll get full power for about 5, maybe 10 minutes. It'll get up to 18.5mph which is plenty for my boat. After the 5 or 10 minutes, power slowly drops off until it's back to half, running at 10mph. Shut it off, wait, start it, full powers back again, but only for the same time period. During all of this, the primer bulb remains hard. Has me baffled honestly. Ive been told to check compression after it loses power which i'll do today, but would appreciate any thoughts you guys might have. Thanks

 

[ONERCBOATER]

Re: loss of power after 5 mins

1st thought is overheating.... thinking check thermostat... and pee stream.
other thought is fuel pump/carb problem..... or coil getting hot ... when it runs good check spark on all cyl.... when it poops out check spark on all cyl again... you may find it that way.

Sean

 

[matth121]

Re: loss of power after 5 mins

it performs exactly the same on 1st startup as it does after it loses power, so im not sure it has anything to do with heat. Just replaced the coils today as I had ordered them and had them on hand, but it was working like this with the previous coils, never thought to check to see if they were hot. I'll know for sure soon if the coils were to blame. Good pee stream, not sure I even have a thermostat. Fuel pump and carb were my best guesses too, but if it was starved for fuel, wouldnt the primer bulb go soft? Thanks for your thoughts
Matt

 

[Texasmark]

Re: loss of power after 5 mins

Hard squeeze bulb confuses what I am about to say. Air not getting into fuel tank...or the squeeze bulb could be bad.

Go to an auto parts store and get a can of Sea Foam aerosol, or get a can of regular and put it in a squeeze/atomizer bottle.

Get on the water and remove the cowl and anything else so that you can look directly into the carb input. Crank it up and get going as usual. When it acts up spray the Sea Foam directly into the carb throat. If you have instant recovery, you have a fuel problem and I doubt it is carb related.....that kind of problem would not come and go as you mention. Otherwise go check your spark.....Sea Foam is primarily for 2 cycle OB's and contains oil which starting fluids don't normally.

Mark

 

[matth121]

Re: loss of power after 5 mins

my local napa only has the regular can, but i can put it in a spray bottle...i didnt know it would run on it. vents open, bulb isnt new, but i really dont think thats the issue. spark is good in test tank anyways. Thanks

 

[ONERCBOATER]

Re: loss of power after 5 mins

this may sound far off... try a dif fuel tank and hose/primer bulb.... I am leaning toward fuel starvation now too....
also... just for kicks and giggles.... make sure there are no kinks in old line.... and check fuel filter and fuel filter at fuel pump.... small possibility of a sticky float...... sea foam cant hurt.... if your hose is old... def replace it.

 

[matth121]

Re: loss of power after 5 mins

fuel tank is brand new, well, 3 months old anyway. Line and bulb are from the original tank so could be replaced i guess. No kinks in the lines, in-line filter is new, havent actually gotten inside the fule pump yet. Think i'm going to try seafoam. Theres no ethanol fuel here where I am in Canada, so the common ethanol problems shouldnt exist. Even still, i use Startron enzyme treatment, hopefully to get rid of any gum in carb. Keep the thoughts coming please.
Matt

 

[matth121]

Re: loss of power after 5 mins

Does anyone think this may be a switchbox problem? What are the symptoms of a failing switchbox?

 

[ONERCBOATER]

Re: loss of power after 5 mins

well, i know already asked one time....or at least i think... but have to triple check... the vent on the tank is open right? cause i have had this one mess me up more than once rather embarrassing i might add.... especially when your poking at everything else and going "WTH" you might take a squirt bottle of 50:1 mix with ya.... and run her with the top off enough to be able to squirt fuel directly into the carb(s) as it falters... if this fixes it momentarily then you know for certain it is fuel related and can narrow your search. Another thought would be to take a inductance timing light with you.... you can put it on any of the spark plug wires you like. this will show you if the spark is being interupted while underway when it falters... and will allow you to check all the cylinders but one at a time. Allowing you eliminate electrical issues. For you to both would require either a competent person to man the helm for you... or a competent person to do the checks. At the end you would be able to say... i squirted fuel and it helped... or not, and/or I saw that I had no spark on cylinder#? might help us narrow things down for ya.

Sean

 

[matth121]

Re: loss of power after 5 mins

i will be trying the premix or seafoam sprayed into the carb, next chance i get- the vent is definitely open, i'm sad to say that thats the original problem that led me to this site, lol. I have a tiny tach, i can put it on either plug, it gives me rpm, but i guess it might not be quick enough to show a misfire, but so far, spark looks good. Just happened to read this though-
http://forums.iboats.com/showthread.php?t=103073

not a merc, but i figure it might be worth looking into. I just dont know if I have a thermostat. Trying to figure that out now. I'll keep you posted on the results

 

[matth121]

Re: loss of power after 5 mins

no thermostat apparently, but did finally bite the bullet and pulled the carb. New fuel pump kit, all new lines on motor, still using old tank line and bulb but they look fine. Dipped and cleaned carb, really didnt look bad but who knows, completly blown out with compressed air, all passages are clean. Started after 4 or 5 pulls from a dry carb. Adjusted idle in gear in a tank, set a little rich. Sounds great, can't wait to test it. Cracked it for a second in the tank, sounded really good for the few seconds before it emptied my tank. I'll keep you all posted, but heres to hoping my amateur outboard mechanic days are over, at least for this season.
Matt

 

[matth121]

Re: loss of power after 5 mins

wooo hoooo. Guess thats that. Between the new coils, fuel pump rebuild and carb cleaning- its fixed. Just ran 3 gallons thru er at WOT. No drop offs in power. Got some fine tuning to do to max out the power but this is like a new motor to me. Thanks to everyone for your advice and comments.
Matt