Ok guys, something is up.

When I take my 3.5 merc outboard on actual water (it's usually a large reservoir) it does this thing where after I run it for about 20 - 30 min or so, it suddenly quits.  Then, I wait about 5-10 minutes and I can run again.  This is VERY frustrating because the motor is essentially brand new and has done this since the first time I took it out.

So, I took it to a service shop and, of course, they couldn't duplicate the problem.  So, I decided to play with it at home by setting the motor in a (clean) trash can full of water....and *I* couldn't duplicate the problem.  But when I'm on the actual lake, it still happens from time to time.  Is it possible for the water to be dirty and the engine get clogged up somehow and cause it to quit?  I am just at wits end trying to figure this out.   

Again, the motor has NEVER quit on me when I have it running in water from a hose.  It's ONLY when I'm on the lake.

Any ideas you all may have...not matter how silly...are welcome!!

Thanks!

Sounds like a fuel delivery problem. I would start with the bulb. Sounds like it may be losing it's prime. Next time it happens check to see if the bulb has softened. Pump it back up and see if it will continue to run. If so, change it out. Also could be an electrical piece that may be slowly overheating. If it runs after a cooling down period that could be it also. There are a couple of starting points for you. Let us know what you find.

Good luck,

Jack

One other thing. It's a whole different story when you put a load on the motor. I'm betting that an electrical piece is slowly over heating or you have a fuel delivery problem. It may take a diagnostic machine to successfully track it down.

Good luck.

Jack

Air or water cooled?

definitely water cooled.  It's the standard 3.5 hp Mercury that you can buy at Academy.

Do you run it the same when on the lake and in the trash can?

Mke sure the tank is vented properly. Just an idea, it does happen.  Not sure this is the same symptoms.

Yes, I do run it similarly on the lake as I do on the garbage can.  The good thing is, this only happens after running at a high speed for quite a while (20 - 30 minutes).  I've never had problems during short 5 - 10 minute runs.  This makes me think that something is overheating.  This, combined with the fact that I've never been left stranded: it eventually WILL crank back up.

but what is overheating?  The oil level looks good, and there are VERY few electronics in this kind of motor. 

Maybe these motors just aren't meant to go that far on the lake, I don't know.

Thank you all for your ideas though, seriously.  I know this is a tough issue to deal with.

Well what a coincidence. Me and my dad also have a 3.5hp merc. We took it out and really ran it tonight for the first time and it is doing the exact same thing. This is only the second time we have used it. The only thing we could think is it is overheating somehow too.

Do you run the motor wide open while on the lake?  In the garbage can?  Maybe you aren't getting good water flow while on the lake.  Is the motor sitting low enough on your boat for the inlets to stay submerged?  Especially with waves and such.  My guess is you aren't duplicating things close enough to get the same results.  Try using a different gas tank and fuel line.  Just some thoughts.

I have a hunch, farmpond, that you're probably right: I'm probably not able to reproduce the exact conditions that I have on the lake in the garbage can.  The ONLY thing I can think of is that it DOES seem to have this problem more on DIRTY lakes (with murky, scummy, muddy water) than on clean ones.  Also, I should note that I HAVE made either 2 or 3 (it was last year) trips to the lake which did NOT include a breakdown... the "successful" trips were all to much cleaner water too...  Which is why, in my total ignorance about how these devices work, I wanted to think that somehow the water that the motor is circulating through itself may be so dirty it's causing a clog.. but I can't find much support for that theory on line.... grrrr. 

In any case, I cranked it up in the garbage can for the first time this season and it cranked on the FIRST pull and ran until I stopped it myself...so, basically, I think the motor is good... but I do think it is quite the pansy when it has to deal with dirty water.

I'll let you know how my first lake test goes.

Thanks again to all of you for your advice!

Fouled plug maybe?