I think I have those from one of mine. I'll send it to you when I get out of the duck woods

When the 280's first came out I believe they were the first Mercury Racing outboards to have the break-in programming. It was interesting because if the engines weren't loaded enough the clock would not count down. Some people would have twenty to thirty hours on the engine and it wouldn't reach full rpm and customer would insist there was something wrong with the engines. This was a big problem on cats. Later learned to run really tall props, twin engine boat running on a single engine at a time, or towing another boat, during break in.
The programming was set not to start counting down until a minimum rpm and vacuum. The engines had to have a good load on them!
Your fresh rebuilds two points. You want to break them in with a good load on them, varying rpm and not going over 5500 rpm. Do it for a minimum of three hours. The long idling your talking about not only is bad but should not count at all on your break in times. Unless someone reprogrammed the break-in mode on your ECU's the ECU is going to be running regular power mode. Not richer mixture and not more oil so be careful.
There is a way to put the ECU's back into break-in mode with a DDT or the newer computer analyzers and if you have one or access to one it's highly recommended to do that.

[ATTACH=CONFIG]535810[/ATTACH][ATTACH=CONFIG]535811[/ATTACH]

Couldn't find anything else in the Operations Manual regarding what the timeframe or maximum RPM would be in Guardian Mode, other than what I posted.

My service manual has almost nothing that will help you, but this is the only page referring to any break-in. Does have some other interesting limits for Guardian.

Good post Onetime.

Ron, please tell us you reset the clock to break-in/ Guardian mode with your DDT

Absolutly. We had a computer on the them during start up. Bled the oil pump, He had the pump run for extra oil on initial start up. Also the motors were put in breakin mode which also is showing on my smartcraft system.
I have put my big props on too.
Thanks for the additional input. I have heard some of the same things Onetime has posted but not sure if that type of break in applies to the 300x.
I know the 300xs did not have that type of programming.

Cant wait to hear the good news about performance gains.

If you just hate the X's I might have a place to hang one of them, and it will be in good company.

Good Luck out there today.

With my 260s' I took 10+ hours ,varying rpm to break them in. On the other hand I have watched a friend window his block before even getting on plane. Just take your time, pun intended, and enjoy being on the water. I let a friend/engine builder run my 260s after one rebuild and he blew the motor the first time he pinned the throttles pushing it to hard. If a builder tells me he doesn't believe in a break-in period he doesn't know anything about expansion of metals in my opinion. Enjoy Ron.

Put another 3hrs on them today. Seas 2' with occasional 3' swell. Break in mode now clear according to Smartcraft displays. Still did not pin it as I want to put more time on them. I will say this thus far. They are definitely a step above my 300xs's. Fuel mileage dropped from 2mpg to 1.7mpg running in 3500-4500 rpm band.
Below is screen shot of today's run. I was 30" Dewald cleavers.

Also here are some videos taken from my phone. Will be using Garmin VRIB next time out for sure.
Sea Conditions
http://youtu.be/s3OIYim64sc

Idling
http://youtu.be/-Uxcvb-V-BE

Planing off http://youtu.be/mXe8MPgB1fs