mercury 150 year 2000 son took it out today,starts good,idle good, opens it up,runs good for 10 15 seconds then cuts back to half speed,will run all day like this but wont run wide open,whats my problem?

Clogged fuel filter?

Fuel lines deteriorated?

Going into Guardian mode? Does the warning horn beep when the key is turned to on?

This was my solution. And I had the same symptoms as you stated.

^^^This^^^ 99% of the time

we can check out for you

+1 to this I had the same issue, looked at the fuel line and it was the cheap EPA hosing, new line instantly solved the issue

UPDATE: first thing I did was replace the fuel filter. I cut the old one open and found it was probably 75% clogged with sand. See pic. Checked fuel lines and those looked fine. Haven’t been able to get to the lake yet to test it but I’m pretty confident that was the problem. Next time out I’m also going to burn off all the fuel so I can check out the fuel tank and see if it has a lot more sand in it and clean it if it does. I’ll probably also replace the new fuel filter after I run the current tank of gas out to make sure there is no more sand.

Thats not sand. It the varnish that builds up over years of use with ethanol free fuel. Once ethanol fuel is introduced the varnish breaks up and clogs the fuel system. Was a big problem 15-20 years ago when E10 came out.

Varnish was my first thought also but after looking at it closer it is very uniformly granular like sand. Wouldn't varnish be more flaky? Or do the flakes of varnish get beat around and form a sandy texture?

They slowly dissolve and look like large granular sand just like in your photo. Don't know what rig you have but if you can remove the fuel tank thats the easiest route to clean it. Another option is buy a couple of fuel filters and keep the tank full with E10 and change filters as needed. It will go away after a few tanks of fuel

You should invest in a more substantial aftermarket fuel filter with the large spin-on filters. I'd be frightened that crud would get past that small filter and trash the motor.

It's called an inline water separator filter. Will clog up pretty fast with that stuff. But will catch it before it gets to the motor fuel filter.

After you get it all cleaned up, you might want to consider using ethanol free fuel and StaBil 360 Marine fuel treatment. I use both of those and also Ring Free in my Yamaha outboard. So far so good and the Yamaha Fuel Systems suck.

Wayne

The problem began by using ethanol free fuel. It’s varnish. But, you’re not gonna go wrong with either fuel as long as the fuel system is up to date. The problem is exacerbated by using e10 AFTER years of ethanol free fuel. The ethanol crystallizes the varnish and suspends it into the fuel allowing it to move into the fuel lines and filters.